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Sr-Nd-Hf-Pb isotope and trace element evidence for the origin of alkalic basalts in the Garibaldi Belt,… Mullen, Emily K.; Weis, Dominique Aug 28, 2013

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Sr-Nd-Hf-Pb isotope and trace element evidencefor the origin of alkalic basalts in the GaribaldiBelt, northern Cascade arcEmily K. Mullen and Dominique WeisPacific Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences,University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, V6T 1Z4, Canada(emullen@eos.ubc.ca)[1] In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier,and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures arenegligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceaniccrust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display traceelement signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation ratherthan primary input from the subducted slab. Primary BSM basalts represent 3?8% melts that segregated fromenriched garnet lherzolite at significantly higher temperatures and pressures (70?105 km) than calc-alkalineCascade arc basalts. The BSM mantle source is significantly more incompatible element-enriched than thedepleted mantle tapped by calc-alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct fromcalc-alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NWNorthAmerica. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent withupwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, whichforms the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate. A gap orattenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low-degreedecompressionmelting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.Components: 17,992 words, 14 figures, 5 tables.Keywords: alkali basalt ; Garibaldi Belt ; cascade arc; Sr-Nd-Pb-Hf isotopes; trace elements.Index Terms: 1040 Radiogenic isotope geochemistry: Geochemistry; 1037 Magma genesis and partial melting: Geo-chemistry; 1031 Subduction zone processes: Geochemistry; 1033 Intra-plate processes: Geochemistry; 1065 Major andtrace element geochemistry: Geochemistry; 3613 Subduction zone processes: Mineralogy and Petrology; 3615 Intra-plateprocesses: Mineralogy and Petrology; 3619 Magma genesis and partial melting: Mineralogy and Petrology; 3060 Subduc-tion zone processes: Marine Geology and Geophysics; 8170 Subduction zone processes: Tectonophysics; 8413 Subductionzone processes: Volcanology; 8415 Intra-plate processes: Volcanology.Received 10 January 2013; Revised 29 May 2013; Accepted 30 May 2013; Published 28 August 2013.Mullen, E. K., and D. Weis (2013), Sr-Nd-Hf-Pb isotope and trace element evidence for the origin of alkalic basalts in theGaribaldi Belt, northern Cascade arc, Geochem. Geophys. Geosyst., 14, 3126?3155, doi:10.1002/ggge.20191.1. Introduction[2] The role of mantle heterogeneity in generatingthe compositional diversity observed in arc magmasis difficult to decipher because it can be masked bycontributions from the subducting slab. There is ageneral consensus that mantle melting in arcs occursin response to input of a hydrous component derived? 2013. American Geophysical Union. All Rights Reserved. 3126ArticleVolume 14, Number 828 August 2013doi: 10.1002/ggge.20191ISSN: 1525-2027from the subducting slab that generates hydrousminerals in the mantle, lowers the mantle solidustemperature, and produces melts by dehydrationmelting or ??flux?? melting [e.g., Kushiro, 1987;Grove et al., 2002]. As a consequence, basaltserupted in subduction zone settings are predomi-nantly aluminous, subalkaline, and bear a slab-derived trace element ??arc signature?? that includesenrichments in large ion lithophile elements (LILEs)and depletions in high field strength elements(HFSEs) [e.g., Gill, 1981; Pearce and Peate, 1995].The Cascade arc, which extends 1300 km fromnorthern California to southwestern British Colum-bia, is an example of an arc in which calc-alkalinelavas dominate [Schmidt et al., 2008; Hildreth,2007; Bacon et al., 1997; Conrey et al., 1997].However, the northernmost segment of the arc is un-usual in that mafic lavas are predominantly alkalic.Alkali olivine basalt and hawaiite occur at the vol-canic fields of Mt. Meager, Salal Glacier, andBridge River Cones (hereinafter referred to as theBSM volcanic centers) [Green and Sinha, 2005].[3] Alkalic lavas are uncommon in arc settings,particularly along the main arc axis, and are attrib-uted to a variety of phenomena that include tearsin the subducting plate, back-arc extension, slabwindows, entrainment of mantle hotspots, accretedenriched mantle, and intraarc rifting [e.g., Naka-mura et al., 1989; Abratis and W?rner, 2001; Fer-rari et al., 2001; Marquez et al., 1999; Turnerand Hawkesworth, 1998; Pearce and Stern, 2006;Thorkelson and Taylor, 1989; Skulski et al., 1991;Righter and Carmichael, 1992; Hughes, 1990;Pearce and Peate, 1995, and references therein].The BSM volcanic centers are located 110 kmabove the subducting plate, broadly similar toother Cascade arc volcanoes [McCrory et al.,2004], suggesting a potential connection to arcmagmatic processes. Elucidating the petrogenesisof the BSM basalts may provide valuable insightsinto mantle and slab processes under the dimin-ished subduction regime at the northern boundaryof the Cascade arc [Harry and Green, 1999].[4] The Cascade magmatic arc has been activesince 40 Ma and is a consequence of subductionof the Juan de Fuca oceanic plate beneath NorthAmerica [Hildreth, 2007]. In northwestern Wash-ington, an abrupt change in the orientation of thearc axis mirrors a bend in the continental margin,subdividing the arc into two major segments (Fig-ure 1a). The High Cascades segment (Mt. Rainierto Lassen Peak) is separated from the GaribaldiVolcanic Belt (GVB) (Glacier Peak to Silver-throne) by a 120 km gap in modern volcanism[Green and Harry, 1999]. Throughout much of theHigh Cascades, the subducting plate is 10 Myrold at the trench with ocean floor isochrons gener-ally parallel to the continental margin [Wilson,2002]. In the GVB, however, isochrons are obliqueto the plate margin and the slab age at the trenchdecreases northward to 6 Ma outboard of theBSM volcanic centers (Figure 1a), making it oneof the youngest and hottest subduction settings inthe world [Syracuse et al., 2010; Harry andGreen, 1999]. Subducted water is expected to belost at shallow depths from a hot slab, leading toreduced hydration of the subarc mantle wedge[Green and Harry, 1999]. A diminished subduc-tion regime may account for why the GVB, incomparison to the High Cascades, has a narrowerwidth, lower magma production rates, and magma-tism mainly restricted to the major volcanic cen-ters [Harry and Green, 1999].[5] In this paper, we assess three hypotheses forthe origin of the BSM alkalic basalts. In the firstmodel, the BSM alkalic basalts are essentiallylower melt fraction ??equivalents?? of more typicalCascade arc calc-alkaline basalts [Green andHarry, 1999; Green and Sinha, 2005]. Reducedhydration of the subarc mantle wedge may reduceits capacity for flux melting, resulting in lowermelt fractions that are enriched in alkali elementsbut display minimal arc signature. A similar modelhas been proposed to account for basalts elsewherein the Cascade arc that are geochemically indistin-guishable from intraplate basalts [Reiners et al.,2000]. Intraplate-type lavas dominate the back-arcSimcoe volcanic field east of Mt. Adams and areinterspersed with other basalt types in a swathextending west 150 km from Simcoe to Portlandthat has been referred to as the Cascades-Columbia transect [Leeman et al., 1990, 2005;Hildreth, 2007; Jicha et al., 2008; Conrey et al.,1997; Bacon et al., 1997]. A few other examplesoccur north of Mt. Rainier [Reiners et al., 2000]and in north-central Oregon [Conrey et al., 1997].[6] Second, a subducted plate boundary or fracturezone may trigger mantle upwelling, inducingdecompression melting that generates low-degree,alkali-rich melts. These processes have been pro-posed for other arcs, including the Mexican arc andLesser Antilles [e.g., Righter et al., 1995; DeLonget al., 1975; Pearce, 2005], and a version of thismodel is mentioned by Lawrence et al. [1984] in thecontext of the Salal Glacier alkalic basalts. The sub-ducted boundary between the Explorer and Juan deFuca plates intersects the BSM volcanic centers andhas been implicated in the origin of the WellsMULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913127Gray-Clearwater volcanic field (3.5 Ma?7.6 ka[Hickson and Souther, 1984]) and Chilcotin Groupbasalts (32?0.8 Ma [Mathews, 1989]) that arelocated in the GVB back-arc region [Madsen et al.,2006; Sluggett, 2008]. Near the triple junctionamong the Explorer, Pacific, and North Americaplates, the Quaternary alkalic seamounts of the TuzoWilson volcanic field are attributed to a ??leakytransform?? in an oceanic setting [Allan et al., 1993].[7] Third, the BSM basalts may reflect one of theother mechanisms proposed to account forMiocene-Holocene intraplate volcanic centers thatoccur in a broad swath extending from southernBritish Columbia to Alaska. The east-west trend-ing Anahim volcanic belt (14.5 Ma?7.2 ka [Bevier,1989]), located immediately north of the GVB,may be related to a hotspot [Bevier, 1989; Char-land et al., 1995], an edge effect of the Juan deFigure 1. (a) Map of the Cascade arc and its tectonic setting. The extents of the Garibaldi volcanic belt andHigh Cascades segments of the arc are indicated with pink arrows. Volcanic and plutonic rocks are shown inyellow and orange shading, respectively. Black triangles denote composite volcanoes. (b) The study area isenclosed by a small bold rectangle and is enlarged. Igneous rock distributions are compiled from Lawrence etal. [1984], Monger [1989], DuBray et al. [2006], Green et al. [1988], Wheeler and McFeely [1991]. Oceanicplate configurations are from Braunmiller and Nabelek [2002], Audet et al. [2008], and Wilson [2002]. Col-ored lines on the oceanic plates are isochrons; accompanying numbers indicate the age of oceanic crust in Ma(from Wilson [2002]). Pseudofaults are shown as thin gray lines. Four heavy gray arrows on the Juan de Fucaand Explorer plates are convergence vectors (mm/yr) obtained from McCrory et al. [2004], Riddihough andHyndman [1991], and Braunmiller and Nabelek [2002] for a reference frame fixed relative to North America.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913128Fuca plate [Stacey, 1974], or ridge subduction[Farrar and Dixon, 1992]. Farther north, theextensive northern Cordilleran volcanic province(20 Ma?200 years B.P.) has been attributed tocrustal extension [Edwards and Russell, 2000].The aforementioned Wells Gray-Clearwater andChilcotin lavas may alternatively be a product ofback-arc extension [Bevier, 1983; Hickson, 1987].More recently, Thorkelson et al. [2011] proposed asingle model in which all of these volcanic provin-ces are related to upwelling of enriched mantlewithin and along the eroding margins of the1500 km long Northern Cordilleran slab window[Thorkelson and Taylor, 1989] that extends nearlyas far south as the BSM volcanic centers.[8] In this study, we investigate the roles of themantle and subducting slab in generating thealkalic compositions of the BSM basalts with highprecision whole-rock Sr-Nd-Pb-Hf isotope ratiosand trace element data. Radiogenic isotope signa-tures of primitive basalts are sensitive indicators ofmantle source heterogeneity [e.g., Hofmann, 2003]and the presence of components derived from sub-ducted oceanic crust and sediment [e.g., Kay et al.,1978]. Green and Sinha [2005] showed that theBSM alkalic basalts record less slab input thancalc-alkaline basalts of the southern GVB, butminimized the possible role of mantle heterogene-ity. However, recent improvements in the precisionof isotopic measurements have revealed mantleheterogeneities that were previously difficult to dis-cern [e.g., Abouchami et al., 2005]. Alkalic basaltsin British Columbia and the Cascade arc are typi-cally ascribed to mantle sources that are moreenriched in incompatible elements than the mantlewedge sampled by calc-alkaline basalts [e.g., Thor-kelson et al., 2011; Sluggett, 2008; Edwards andRussell, 2000; Leeman et al., 1990, 2005; Baconet al., 1997; Borg et al., 1997; Conrey et al.,1997; Schmidt et al., 2008; Jicha, et al. 2008].However, Reiners et al. [2000] proposed that bothbasalt types can be derived from a homogeneousmantle variably fluxed by slab-derived fluids.[9] We compare the BSM basalts to calc-alkalinebasalts from Mt. Baker, a stratovolcano located inthe ??cooler?? southern GVB and representative ofmore typical Cascade arc basalts, and to pub-lished data for other intraplate alkalic basaltsfrom British Columbia. Our new isotope and traceelement data show minimal subduction influenceon the source of the most primitive basalts atSalal Glacier and Bridge River. The BSM basaltsalso have a mantle source that is isotopically dis-tinct from, and more incompatible elementenriched, than the mantle underlying much of theCascade arc. These results have important impli-cations for the physical configuration of the sub-ducting slab and mantle flow patterns in northernCascadia.2. Geology of the Bridge River, SalalGlacier, and Mt. Meager VolcanicCenters[10] The Bridge River Cones, Salal Glacier, andMt. Meager are located 150 km north of Van-couver, British Columbia (Figure 1b). At the twonorthernmost centers (Salal Glacier and BridgeRiver), lavas are almost exclusively mafic. TheMt. Meager volcanic field includes basalt throughrhyolite but is dominated by intermediate compo-sitions, and Mt. Meager proper is a compositeandesitic stratovolcano [Ke, 1992].[11] The Salal Glacier volcanic field includes pil-low lavas, tuffs, and variably palagonitized andbrecciated flow remnants that survived continentalice sheet advances as high-altitude nunataks. Atlower altitudes, severe glacial erosion has revealedrhyolite and andesite dikes. Age dates for an alkalibasalt and overlying hawaiite are 0.97 and 0.59Ma (K-Ar), respectively [Lawrence, 1979].[12] Lavas at the Bridge River Cones are exclu-sively alkalic [Roddick and Souther, 1987]. Theterm ??cones?? is a misnomer because none of thedeposits is a true volcanic cone; rather, glacialerosion has produced cone-like forms. Columnarlavas of the Sham Hill plug and Tuber Hill expo-sure are dated at 1 Ma and 600 ka (K-Ar), respec-tively [Roddick and Souther, 1987].[13] At Mt. Meager, intermediate to silicic lavasspan the alkalic-subalkalic boundary [Stasiuk et al.,1994]. Mafic lavas are exclusively alkalic,however, and occur as four flow remnantscollectively known as the Mosaic Assemblage[Stasiuk and Russell, 1989; Stasiuk et al., 1994].Two of the basalts are dated at 90 and 140 ka (K-Ar) [Anderson, 1975; Woodsworth, 1977].Evidence for recent involvement of mafic magmain the form of mafic enclaves and banded pumicesis preserved in the 2360 years B.P. thatexplosively released 10 km3 of dacite [Clague etal., 1995; Michol et al., 2008]. Banded pumicesand mafic enclaves indicate that the intrusion of abasaltic magma may have triggered the eruption[Stasiuk et al., 1994].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131293. Major Element Compositions andPetrography[14] Samples analyzed for this study are from theBridge River, Salal Glacier, Mt. Meager, and Mt.Baker sample suites of Green and Sinha [2005]and from the Mt. Baker sample suite of E. K.Mullen and I. S. McCallum (Origin of basalts in ahot subduction setting: Petrologic and geochemi-cal insights from Mt. Baker, northern Cascade arc,submitted to Journal of Petrology, 2013, herein-after referred to as Mullen and McCallum, submit-ted manuscript, 2013). Major element data anddetailed petrographic descriptions for all samplesare provided in those references.[15] The BSM basalts are alkalic [Macdonald,1968] and nepheline normative with Na2O>K2O(Figure 2a). The basalts have distinctly lower SiO2(Figure 2) and Al2O3 than the calc-alkaline, hyper-sthene-normative Mt. Baker basalts.[16] Bridge River alkali olivine basalts and hawai-ites encompass the largest range of compositionaldiversity (Figure 2). Molar Mg/(Mg?Fe2?) valuesrange from 0.44 to 0.62; we consider two sampleswith Mg/(Mg?Fe2?)> 0.6 as primitive. Phenocrystand microphenocryst minerals are limited to olivine(1?2%) and rare plagioclase. Except for one sam-ple with a brown glass matrix, the basalts have hol-ocrystalline groundmasses containing olivine,plagioclase, titanaugite, magnetite, and ilmenite.The most primitive basalt (BRC10) contains biotiteand amphibole in the groundmass. Two of the moreevolved samples contain quartz xenocrysts andgranodiorite xenoliths (BRC03?4, BRC01?3).[17] Mt. Meager alkali basalts and hawaiites con-tain <1% microphenocrysts of olivine, clinopyr-oxene, and plagioclase. The groundmass containsglass and magnetite and, in the least primitivesample (MM01-1), biotite and amphibole. Mg/(Mg?Fe2?) values are 0.59?0.63.[18] Salal Glacier samples are the most primitiveamong the BSM basalts with Mg/(Mg?Fe2?)? 0.58to 0.66 and have the highest normative nephelinecontents. The most primitive samples are glassy andvesicular with phenocryst assemblages including<15% olivine, <1% plagioclase, <1% clinopyrox-ene, and rare orthopyroxene xenocrysts. Lessrimitive samples contain orthopyroxene phenocrystsand more abundant plagioclase, and olivine is eitherrimmed by clinopyroxene or absent.[19] At Mt. Baker, the most mafic lavas includemedium-K calc-alkaline basalt, high-Mg basalticandesite, and low-K olivine tholeiite, with Mg/(Mg? Fe2?)? 0.56?0.70. All samples contain oli-vine and plagioclase phenocrysts and some alsohave clinopyroxene phenocrysts (Mullen andMcCallum, submitted manuscript, 2013).4. Analytical Methods[20] Trace element abundances and Sr-Nd-Hf-Pbisotope ratios were measured on 19 BSM basalts,using splits of sample powders analyzed by Greenand Sinha [2005] for major and trace elements andSr isotope ratios. Larger symbols in Figure 2 des-ignate samples analyzed for the present study. HfFigure 2. Major element variation diagrams for the basaltsof Bridge River (red), Salal Glacier (orange), Mt. Meager(yellow), and Mt. Baker (lavender). (a) wt % Na2O?K2Oversus SiO2 with discriminant line of Macdonald [1968] andfields of Le Bas et al. [1986]. (b) Miyashiro diagram (FeO/MgO versus SiO2) with discriminant line of Miyashiro[1974]. Bridge River, Salal Glacier, and Mt. Meager data arefrom Green and Sinha [2005]. Mt. Baker data are fromMullen and McCallum (submitted manuscript, 2013) exceptLib21 from Green and Sinha [2005]. Large symbols (circlesand diamonds) indicate samples analyzed in this study fortrace elements and isotope ratios; diamonds are accompaniedby sample numbers and are the Suite 2 samples discussed inthe text. Small circles indicate samples of Green and Sinha[2005] not analyzed for the present study.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913130Figure 3. (a) 208Pb/204Pb versus 206Pb/204Pb, (b) 207Pb/204Pb versus 206Pb/204Pb. Insets show the same datafor the BSM and Mt. Baker basalts but on an expanded scale. Symbols for the BSM and Mt. Baker samplesgiven in legend; circles are used for primitive basalts. The more evolved basalts are subdivided into Suite 1(circles; isotopically similar to the primitive basalts) and Suite 2 (diamonds accompanied by sample numbers;isotopically distinct from the primitive basalts). 2 error bars (external reproducibilities) are smaller thansymbols in all plots. NHRL is the Northern Hemisphere Reference Line of Hart [1984]. Cascade arc basaltdata (pink crosses; only those with >8 wt % MgO are included) are from Conrey et al. [1997], Jicha et al.[2008], Bacon et al. [1994, 1997], Leeman et al. [1990, 2005], Baker et al. [1991],Magna et al. [2006], Groveet al. [2002], and Borg et al. [1997, 2000]. Northern Juan de Fuca MORB data (dark blue ?) are from Cou-sens et al. [1995]. Explorer MORB data (dark gray filled squares) are from B. Cousens (unpublished data2007). N. Gorda MORB (black ?) are from Allan et al. [1993]. Northern Cascadia sediment data (orangecircles) are from ODP sites 1027 and 888 [Carpentier et al., 2010, 2013]. Note that all isotope data are nor-malized to the same isotope standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913131isotope ratios were also measured on splits of pow-ders of three Mt. Baker basalts previously ana-lyzed for major and trace elements and Sr-Nd-Pbisotopes [Mullen and McCallum, submitted manu-script, 2013; Green and Sinha, 2005].[21] All chemical separations and mass spectro-metric analyses were carried out in Class 100 and10,000 clean laboratories, respectively, at the Pa-cific Centre for Isotopic and GeochemicalResearch at the University of British Columbia.Rock powders (100 mg) were digested in sub-boiled concentrated HF?HNO3 in 15 mL screw-top Savillex beakers on a hotplate for 48 h at130C. Samples were dried down on a hotplateand brought up in subboiled 6 N HCl and fluxedon a hotplate for at least 24 h. Sample aliquots of5?10% were diluted 5000X with an HNO3?HFsolution for analysis on a Thermo Finnigan Ele-ment2 HR-ICP-MS or an Agilent 7700 quadrupoleICP-MS. Sr isotope ratios were measured on aThermo Finnigan Triton TIMS and Pb, Nd, and Hfisotope ratios on a Nu Instruments MC-ICP-MS(Nu 021) following the procedures of Weis et al.[2006]. Pb, Sr, Hf, and Nd were separated fromsingle powder dissolutions by sequential ionexchange column chemistry as described in Weiset al. [2006, 2007]. All solutions were passedtwice through Pb exchange columns to ensure Pbpurification. Although thin sections of the ana-lyzed samples indicate little or no alteration in allsamples (minor iddingsite in olivine) and LOIvalues are low (<1%), even minimally alteredsamples can yield isotopic compositions that arenot representative of magmatic isotopic signatures,particularly in the case of Sr and Pb isotopes[Hanano et al., 2009; Nobre Silva et al., 2009].Therefore, we measured isotope ratios on bothunleached and leached powders of some samples.Leaching was conducted prior to powder dissolu-tion following the procedures of Nobre Silva et al.[2009, 2010]. Leached samples gave isotope ratioswithin analytical error of respective unleachedsamples for Sr and Nd (Figure S1).1 Hf isotoperatios are also within analytical error except forone sample (BRC10) that gave a higher value inthe leached sample. 207Pb/204Pb in leached sam-ples is systematically lower than in unleachedsamples while 208Pb/204Pb and 206Pb/204Pb arewithin error of unleached samples, with the excep-tion of one Mt. Baker sample (Lib21) (FigureS1).1 All isotope plots in the main text show dataobtained on leached samples except for cases inwhich only unleached samples were analyzed.Blank contributions to isotope ratios were negligi-ble with total procedural blanks of 50, 400, 90,and 15 pg for Pb, Sr, Nd, and Hf, respectively.5. Results5.1 Isotopes[22] Isotope ratios are reported in Table 1 and plot-ted in Figures 3?5. 87Sr/86Sr values measured in theBSM basalts are systematically lower than reportedby Green and Sinha [2005] for the same samplesand lie outside their reported uncertainties (Figure4a inset). For direct comparison among datasets, allliterature data are normalized to standard values of87Sr/86Sr? 0.710248 for SRM987 and 0.708028 forEimer and Amend; 143Nd/144Nd? 0.511973for Rennes, 0.511858 for La Jolla, 0.512633 forBCR-1, and 0.512130 for Ames [Weis et al., 2006,2007]; 176Hf/177Hf? 0.282160 for JMC 475[Vervoort and Blichert-Toft, 1999]; and208Pb/204Pb? 36.7219, 207Pb/204Pb? 15.4963,206Pb/204Pb? 16.9405 for SRM981 [Galer andAbouchami, 1998].[23] Primitive BSM basalts (Mg/Mg?Fe2?> 0.60)form an isotopic cluster (Figures 3 and 4) with anarrow range of 87Sr/86Sr? 0.70299?0.70314,ENd??7.1 to ?7.7, EHf??8.3 to ?10.0,208Pb/204Pb? 38.075?38.172, 207Pb/204Pb? 15.541?15.557, 206Pb/204Pb? 18.690?18.774. The primitiveBSM basalts overlap in 208Pb/204Pb and 206Pb/204Pbwith N. Juan de Fuca MORB [Cousens et al., 1995]and Explorer MORB (B. Cousens, unpublished data2007) (Figure 3a) but have slightly higher207Pb/204Pb and 87Sr/86Sr and lower ENd (Figures 3band 4a). Primitive BSM basalts plot near thedepleted end of the Sr-Nd-Pb isotopic arrays definedby other Cascade arc basalts (Figures 3 and 4).Along with Mt. Baker, primitive BSM basalts haveamong the highest ENd values reported for the Cas-cade arc. Mt. Baker basalts have slightly higher208Pb/204Pb and 206Pb/204Pb than primitive BSMsamples, but overlap in 207Pb/204Pb. Mt. Baker hassignificantly higher EHf (?11.1 to? 12.1) andslightly higher 87Sr/86Sr. In EHf-ENd isotopic space(Figure 4b), primitive BSM and Mt. Baker basaltsoverlap with only one outlier among data previouslypublished for the Cascade arc (Lassen Peak [Borg etal., 2002] and Mt. Adams [Jicha et al., 2008]). Mt.Adams EHf values cluster between primitive BSMbasalts and Mt. Baker. Together, the BSM, Mt.Baker, and Mt. Adams basalts define an EHf range1Additional supporting information may be found in the onlineversion of this article.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913132Table 1. Sr, Nd, Hf, and Pb Isotope Ratios87Sr/86Srb 143Nd/144NdcSample#Lat(N)Long(W)SiO2(wt%)MgNumbera Leached 2SEh Unleached 2SE Leached 2SE"NdLeachedf Unleached 2SE"NdUnleachedfBridge River ConesBRC01-3 50.93 123.45 50.51 56.5 0.703212 9BRC02 50.91 123.45 49.60 52.8BRC03-4 50.93 123.45 50.07 54.4 0.703219 6 0.512966 7 6.4BRC04 50.93 123.45 49.48 52.4 0.703184 9BRC05-1 50.92 123.45 46.66 48.3 0.703119 9BRC06 50.90 123.45 49.35 53.5 0.703175 8BRC07-2 50.92 123.45 47.24 47.7 0.703098 9 0.703124 7 0.513012 5 7.3 0.513012 6 7.3BRC09-3 50.92 123.41 48.02 59.6 0.702985 7 0.703012 9 0.513024 7 7.5BRC10 50.92 123.38 45.10 61.2 0.703052 7 0.703054 7 0.513031 7 7.7dupi 0.703055 7 0.703056 8Salal GlacierSG01-2 50.81 123.45 46.64 65.1 0.703140 7 0.513021 6 7.5SG01-3 50.81 123.45 46.62 64.5 0.703143 8 0.703149 9 0.513001 7 7.1SG10 50.78 123.39 46.04 65.7 0.703122 9dup 0.703117 8SG12 50.77 123.40 46.71 66.5 0.703065 7 0.703067 8 0.513013 6 7.3SG16 50.77 123.39 46.59 65.9 0.703101 10Mt. MeagerMM01-1 50.65 123.59 48.84 58.8 0.703764 8 0.703758 9 0.512930 5 5.7 0.512926 7 5.7dup 0.703762 6 0.703763 7 0.512936 5 5.8 0.512941 6 5.6MM02 50.69 123.57 48.64 63.0 0.703132MM04 50.69 123.57 48.94 60.8 0.703144 6 0.703146 0.513030 9 7.6MM08 50.55 123.53 49.68 60.6 0.703164Mount BakerLIB-21 48.67 121.74 51.04 63.5 0.703964 9 0.703970 7 0.512834 6 3.8 0.512834 6 3.802-MB-5 48.72 121.85 53.69 69.7 0.703109 7 0.513001 6 7.107-MB-112 48.66 121.70 52.56 56.6 0.703240 7 0.513034 7 7.702-MB-1 48.72 121.85 53.30 56.0 0.703513 8 0.512899 5.106-MB-82 48.72 121.85 50.57 65.1 0.703156 7 0.512986 6.807-MB-114 48.64 121.73 52.06 61.3 0.703213 7 0.513037 7.806-MB-97 48.78 121.88 54.45 49.0 0.703173 10 0.512993 6.9176Hf/177HfdSample # Leached 2SE "Hf Leachedg Unleached 2SE "Hf UnleachedgBridge River ConesBRC01?3 0.283067 8 10.4BRC02BRC03?4 0.283052 5 9.9BRC04 0.283026 7 9.0BRC05-1 0.283040 7 9.5BRC06 0.283018 6 8.7BRC07-2 0.283027 4 9.0 0.283030 5 9.1BRC09-3 0.283007 6 8.3 0.283030 5 9.1BRC10 0.283025 6 8.9 0.282985 4 7.5Salal GlacierSG01?2 0.283017 5 8.7SG01?3 0.283021 5 8.8 0.283025 24 8.9SG10 0.283012 9 8.5SG12SG16 0.283023 7 8.9Mt. MeagerMM01-1 0.283063 9 10.3 0.283064 6 10.3dup 0.283050 8 9.9 0.283056 7 10.3MM02 0.283050 6 9.8MM04 0.283056 5 10.1MM08 0.283022 5 8.9Mount BakerLIB-21 0.283084 5 11.0 0.283094 5 11.402-MB-5 0.283100 5 11.607-MB-112 0.283114 4 12.1MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913133that is similar to Explorer MORB, but with slightlylower ENd.[24] More evolved BSM basalts define twogroups: Suite 1 has isotopic ratios that overlapwith primitive basalts from the same volcanic cen-ter, whereas Suite 2 has substantially lower ENdand higher Sr and Pb isotope ratios than otherbasalts from their respective volcanic centers (Fig-ures 3 and 4a). Suite 2 includes three BSM basalts(BRC03?4, BRC01?3 at Bridge River; MM01-1at Mt. Meager), all of which have Pb isotope ratiosTable 1. (continued)208Pb/204Pbe 207Pb/204Pbe 206Pb/204PbeSample # Leached 2SE Unleached 2SE Leached 2SE Unleached 2SE Leached 2SE Unleached 2SEBridge River ConesBRC01-3 38.2441 22 15.5615 8 18.7878 8dup 38.2433 24 15.5604 9 18.7863 10BRC02 38.1901 21 15.5554 8 18.7481 10BRC03-4 38.2671 18 15.5584 7 18.8334 6dup 38.2674 18 15.5585 6 18.8337 8BRC04 38.1961 27 15.5622 9 18.7379 13BRC05-1 38.1274 17 15.5481 7 18.7591 8BRC06 38.1828 28 15.5530 12 18.7463 14BRC07-2 38.1215 19 38.1260 26 15.5450 7 15.5484 9 18.7626 8 18.7539 10BRC09-3 38.1245 18 38.1432 40 15.5462 6 15.5561 15 18.7432 8 18.7340 13BRC10 38.1496 22 38.1580 20 15.5566 10 15.5610 7 18.7738 10 18.7754 8dup 38.1601 36 15.5620 14 18.7750 17Salal GlacierSG01-2 38.1087 20 15.5419 8 18.7465 9SG01-3 38.1088 24 38.1250 31 15.5407 8 15.5508 7 18.7536 10 18.7536 7SG10 38.1285 24 15.5492 9 18.7363 12SG12 38.1721 22 38.1492 38 15.5521 7 15.5621 14 18.7739 9 18.6997 14SG16 38.1585 33 15.5527 12 18.7591 12Mt. MeagerMM01-1 38.2664 21 38.2734 19 15.5692 8 15.5731 7 18.8033 9 18.8090 7dup 38.2697 18 38.2772 19 15.5703 7 15.5738 7 18.8102 8 18.8115 7MM02 38.1005 35 15.5470 13 18.7034 15MM04 38.1046 14 38.1370 36 15.5470 4 15.5580 12 18.7082 6 18.7271 13MM08 38.0747 25 15.5550 9 18.6896 15Mount BakerLIB-21 38.4708 24 38.5285 20 15.5851 9 15.5931 7 18.9235 11 18.9726 802-MB-5 38.2495 22 15.5505 8 18.7975 907-MB-112 38.3077 21 15.5575 7 18.8385 802-MB-1 38.3598 15.5645 18.851506-MB-82 38.2661 15.5518 18.828607-MB-114 38.2753 15.5560 18.846606-MB-97 38.2597 15.5529 18.8356aCalculated as 100Mg/Mg?Fe2? (molar), using major element data from Green and Sinha [2005] and Mullen and McCallum (submittedmanuscript, 2013) and assuming Fe3?/Fe? 0.15.bReported Sr isotope ratios are corrected for mass fractionation using 86Sr/88Sr? 0.1194. Repeat analysis of the Sr SRM987 standard yielded amean (6 2) of 87Sr/86Sr? 0.7102486 2 (n? 7), identical to the accepted value [Weis et al., 2006].cReported Nd isotope ratios are corrected for mass fractionation using 146Nd/144Nd? 0.7219 and are normalized to 143Nd/144Nd? 0.511973 forthe Rennes reference material [Chauvel and Blichert-Toft, 2001] using the daily average method. The Rennes standard was analyzed every twosamples and over the course of analysis gave a mean (6 2) value of 143Nd/144Nd? 0.5119806 65 (n? 16). On a per session basis, reproducibil-ity was significantly better with a maximum daily 2 value of627 (53 ppm).dReported Hf isotope ratios are corrected for mass bias using 179Hf/177Hf? 0.7325 [Patchett and Tatsumoto, 1981] and normalized to176Hf/177Hf? 0.282160 for the ULB-JMC 475 reference material [Vervoort and Blichert-Toft, 1999] using the daily average of standard analyses.JMC 475 was analyzed every two samples and over the course of analysis gave a mean (6 2) of 176Hf/177Hf? 0.28217326 24 (86 ppm)(n? 24). On a per session basis, reproducibility was significantly better with daily 2 values ranging from 34 to 61 ppm.eReported Pb isotope ratios were corrected for mass bias by Tl doping [White et al., 2000] and are normalized to 208Pb/204Pb? 36.7219,207Pb/204Pb ?15.4963, 206Pb/204Pb? 16.9405 for the SRM981 standard [Galer and Abouchami, 1998] by sample-standard bracketing. Replicateanalysis of SRM981 over the course of analysis yielded in-run mean6 2 values of 208Pb/204Pb? 36.71986 91 (247 ppm),207Pb/204Pb? 15.49986 34 (220 ppm), and 206Pb/204Pb? 16.94346 29 (168 ppm) (n? 43). On a per session basis, reproducibility was signifi-cantly better with daily 2 values ranging from 74 to 186 ppm, 62 to 176 ppm, and 45 to 139 ppm for 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb,respectively.fENd calculated using a CHUR value of143Nd/144Nd? 0.512638 [Jacobsen and Wasserburg, 1980].gEHf calculated using CHUR value of176Hf/177Hf? 0.282772 [Blichert-Toft and Albarede, 1997].h2SE values (twice the standard errors) apply to the last decimal place(s) and are the internal absolute errors values for individual sample analyses.idup designates full procedural duplicates starting with a new sample powder aliquot; reproducibilities are similar to, or better than, the reprodu-cibilities determined through repeat standard analysis (values listed above).Data in italics are from Mullen and McCallum (submitted manuscript, 2013).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913134Figure 4. (a) ENd versus87Sr/86Sr; (b) EHf versus ENd. Symbols and data references as in Figure 3, plus Las-sen and Adams data in Figure 4b from Borg et al. [2002] and Jicha et al. [2008]. Note that all isotope data arenormalized to the same isotope standard values as described in main text. For the BSM and Mt. Baker data,2 error bars (external reproducibilities) are smaller than symbols. Inset in Figure 4a compares 87Sr/86Srmeasured in the present study (2 error bars? 20 ppm; smaller than symbol size) to 87Sr/86Sr measured onthe same samples by Green and Sinha [2005] (2 error bars? 100 ppm). The mantle array in EHf versus ENdspace is from Chauvel et al. [2008]. Orange and blue curves show the effect of adding 2% bulk sediment(blue curve with long dashes), 2% sediment fluid (blue curves with short dashes), 2% sediment melt (solidblue curves), 10% metabasalt fluid (orange dashed curve), and 2% metabasalt melt (orange solid curve) to themantle prior to 5% equilibrium partial melting of a primitive mantle composition [Sun and McDonough,1989]. Each curve has two tick marks indicating 1% and 2% addition, except for the metabasalt fluid curve(ticks at 5% and 10% addition). Slab fluid and melt compositions calculated using equilibrium melting/dehy-dration equations with FL?0.05. Trace element compositions for sediment and metabasalt are from Carpent-ier et al. [2013] (average of bulk ODP sites 888 and 1027) and Becker et al. [2000] (900C eclogite),respectively. Sediment isotope composition is the average of ODP sites 888 and 1027 from Carpentier et al.[2010]. Metabasalt Sr and Nd isotope ratios are from Staudigel et al. [1995] and the Hf isotope ratio is the av-erage of Explorer MORB shown here. Partition coefficients from Kessel et al. [2005] at 700C, 4 GPa (all flu-ids); 1000C, 4 GPa (metabasalt melt and sediment melt 1), and Hermann and Rubatto [2009] at 1050C, 4.5GPa (sediment melt 2).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913135similar to Mt. Baker basalts, plotting within theCascade arc array and closer to the field definedby subducting sediment (Figure 3). Suite 2 alsoincludes two Mt. Baker basalts that are isotopi-cally anomalous (in Sr and Nd) relative to moreprimitive Mt. Baker basalts ; Cathedral Crag(MB1) and one Sulphur Creek sample (Lib21) plotcloser to the field defined by subducting sediment(Figure 4a). As a group, Suite 2 has lower MgOand generally higher SiO2 than other basalts fromtheir respective volcanic centers (Figure 5).5.2 Trace Elements[25] Trace element abundances are reported inTable 2 and plotted in Figures 6?9. With theexception of Suite 2 (discussed later), the BSMbasalts have LILE and Pb abundances similar toMt. Baker basalts, but much higher HFSE (Figures6 and 8a). BSM basalts have substantially lowerZr/Nb and Ba/Nb than Mt. Baker basalts (Figure8b) and higher (La/Yb)N and (Dy/Yb)N (Figure 7).Among the BSM basalts, Salal Glacier has thehighest (La/Yb)N and lowest Yb and displays theleast variability among samples (Figure 7a).Bridge River and Mt. Meager have variable REEabundances, and the lowest (La/Yb)N occurs atMt. Meager (Figure 7b). However, Mt. Baker sam-ples extend to lower (La/Yb)N and higher Yb val-ues than the BSM basalts (Figure 7). Salal Glacierand Bridge River have no Nb anomalies whilesmall negative anomalies occur at Mt. Meager andprominent ones at Mt. Baker (Figure 9a). Ba/Lavalues are lowest at Salal Glacier and BridgeRiver, intermediate at Mt. Meager, and highest atMt. Baker (Figure 9b). Abundances of all traceelements in Bridge River and Salal Glacier primi-tive basalts are similar to samples from nonsub-duction settings, including Hawaiian postshieldalkalic basalts [Hanano et al., 2010], and overlapwith alkalic basalts from the Anahim volcanic belt[Charland et al., 1995], Cascade-Columbia tran-sect [Leeman et al., 2005; Jones, 2002], andDalles Lakes north of Mt. Rainier [Reiners et al.,2000] (Figures 8, 9a, and 9c).[26] The five basalts comprising Suite 2(BRC03?4, BRC01?3, MM01-1, MB1, andLib21) have trace element abundances that con-trast with other basalts at their respective vol-canic centers, including significantly higherLILE (La/Yb)N and Ba/Nb, and lower HFSE(Figures 6b?6e). These samples are excludedfrom the following discussion of mantle sourcecharacteristics but are revisited later in the contextof crustal assimilation.6. Discussion6.1. Mantle Source Characteristics6.1.1. Temperatures and Pressures[27] The BSM basalts segregated from their man-tle source at significantly higher pressures andtemperatures than the Mt. Baker basalts (Figure10). Liquidus pressures and temperatures (i.e.,mantle potential temperatures) were calculated forthe two most primitive basalts at each BSM vol-canic center (Table 3) using the olivine-liquid geo-thermometer and silica activity geobarometer ofPutirka [2008]. Whole-rock data [Green andSinha, 2005] were first adjusted into equilibriumwith Fo90 mantle by incremental olivine additionFigure 5. 208Pb/204Pb versus (a) wt % SiO2, and (b) wt % MgO. Note reversed scale for MgO.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913136Table 2. Trace Element AbundancesSample Number BRC01?3 BRC02 BRC03?4 BRC04 BRC05-1 BRC06 BRC07-2 BRC09-3 BRC10 SG01-1Methoda 1 1 2 1 1 1 2 1 1 1Concentrationb (ppm)Li 7.2 8.8 10 8 7.7 8.0 8.2 6.1 6.4 4.4Cs 0.17 0.22 0.46 0.17 0.14 0.18 0.13 0.07 0.09 0.19Rb 11 15 16 13 11 14 12 4.0 7.7 12Ba 413 310 450 303 269 288 286 195 176 269Th 2.0 1.6 2.2 1.5 1.5 1.5 1.8 0.94 0.87 1.7U 0.66 0.64 0.65 0.41 0.57 0.60 0.63 0.34 0.35 0.63Nb 14 27 16 25 26 26 28 17 15 29Ta 0.87 1.9 0.92 1.4 1.7 1.8 1.6 1.2 1.1 1.8La 24 20 24 18 20 17 22 14 12 20Ce 54 45 53 43 48 42 50 30 26 43Pb 4.2 2.8 3.9 3.0 2.2 2.6 2.3 1.7 1.3 2.1Pr 6.7 5.7 6.8 5.6 6.3 5.6 6.7 4.2 3.6 5.5Sr 1251 626 1213 577 587 650 628 399 388 545Nd 28 26 28 25 28 24 29 18 16 24Sm 5.2 6.0 5.5 5.9 6.7 5.9 6.8 4.7 4.3 5.5Zr 127 174 143 168 200 168 213 127 108 154Hf 3.4 4.1 3.3 4.1 4.6 3.9 4.8 3.2 2.9 3.7Eu 1.6 2.0 1.8 1.9 2.2 1.8 2.3 1.7 1.5 1.7Gd 4.2 6.2 4.8 6.0 6.5 5.8 6.6 5.1 4.7 5.2Tb 0.58 0.89 0.68 0.86 0.98 0.85 0.96 0.75 0.74 0.73Dy 3.4 4.7 3.9 4.9 5.6 4.9 5.7 4.3 4.0 4.0Y 19 25 21 27 31 26 30 25 23 22Ho 0.66 0.91 0.74 0.90 1.0 0.84 1.1 0.82 0.75 0.69Er 1.7 2.3 2.0 2.3 2.8 2.4 2.9 2.2 1.9 1.9Tm 0.24 0.32 0.31 0.39 0.31 0.31 0.28 0.27Yb 1.4 1.9 1.7 1.8 2.3 1.9 2.4 1.8 1.5 1.5Lu 0.20 0.27 0.23 0.26 0.33 0.24 0.34 0.26 0.22 0.22Sc 18 20 19 20 24 20 23 23 22 20Zn 85 101 98 111 122 106 126 98 107 100Cr 48 98 47 100 44 115 34 325 310 321Ni 44 50 41 51 36 56 32 134 228 281V 202 211 216 207 284 207 282 196 200 199Ga 23 24 24 27 24 21 22 21Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Concentration (ppm)Li 7.1 6.5 6.6 5.6 6.4 6.5 7.5 7.7 5.4 8.3Cs 0.20 0.17 0.15 0.14 0.18 0.10 0.08 0.13 0.08 0.44Rb 18 17 16 12 17 8.8 7.6 12 8.2 15Ba 339 330 330 247 264 676 249 265 154 659Th 2.2 2.1 1.9 1.7 1.7 1.7 1.1 1.2 0.65 2.4U 0.73 0.76 0.71 0.54 0.66 0.63 0.71 0.54 0.34 0.79Nb 31 31 32 26 26 12 17 19 9.1 6.3Ta 1.7 1.5 1.6 1.9 1.4 0.7 1.1 1.3 0.63 0.31La 23 24 22 18 18 27 13 15 8.4 24Ce 49 49 47 40 43 62 30 31 21 53Pb 2.2 2.1 2.2 2.3 2.0 5.2 1.9 2.0 1.4 3.8Pr 6.2 6.1 6.0 5.2 5.6 8.4 4.0 4.2 3.0 7.1Sr 658 589 585 508 624 1588 479 545 467 1455Nd 26 25 26 23 24 35 17 18 14 30Sm 5.5 5.4 5.7 5.1 5.3 6.7 4.2 4.4 4.0 5.8Zr 171 161 164 141 150 147 112 122 92 93Hf 3.6 3.8 3.7 3.3 3.5 3.4 2.9 3.0 2.7 2.4Eu 1.8 1.9 1.9 1.6 1.8 2.1 1.5 1.6 1.4 1.8Gd 5.1 5.3 5.1 5.0 5.2 5.5 4.2 4.8 4.3 4.7Tb 0.72 0.76 0.78 0.68 0.76 0.71 0.65 0.68 0.64 0.61Dy 4.1 4.0 4.4 3.7 4.3 4.0 3.8 3.8 3.9 3.4Y 22 23 25 21 21 21 20 22 21 18Ho 0.77 0.77 0.81 0.69 0.73 0.77 0.69 0.74 0.69 0.65Er 2.0 2.0 2.2 1.7 2.0 2.0 1.8 1.9 1.9 1.8Tm 0.29 0.30 0.24 0.26 0.24 0.27 0.25MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913137assuming Fe3?/PFe? 0.15. Amounts of olivineadded range from 11.5% (SG16) to 17.7%(BRC09). Primary biotite and amphibole in thegroundmass of some primitive BSM samples attestto the presence of water, so pressures and tempera-tures were calculated for several possible H2Ocontents (listed in Table 3). The plagioclase-liquidhygrometer of Lange et al. [2009] applied to themost primitive Salal Glacier basalt gives 1 wt %H2O at the liquidus assuming plagioclase satura-tion at P? 100 MPa and maximum An60 in plagio-clase cores [Lawrence, 1979]. At this watercontent, pressures calculated for primitive BSMbasalts correspond to depths ranging from 70 km(Mt. Meager) to 105 km (Bridge River) (Figure10). Decreasing melt SiO2 with increasing pres-sure [Longhi, 2002] is consistent with the P-Tdata. Calculated mantle potential temperatures are100?200C higher than predicted for the Cas-cade subarc mantle wedge [Syracuse et al., 2010],and similar to those of average MORB (1454C)[Putirka, 2008]. Intraplate basalts of the westernBasin and Range province give a broadly similarP-T range (60?90 km, 1350C1450C) as theBSM basalts [Lee et al., 2009]. Intraplate basaltsin the Cascade-Columbia transect have lower max-imum segregation depths (75?80 km) but similarmaximum temperatures (1460C), although an-hydrous conditions were assumed [Leeman et al.,2005]. Simcoe intraplate basalts record P-T condi-tions similar to the BSM basalts (max 100 km,1500C) [Leeman et al., 2005].[28] For Mt. Baker basalts, liquidus water contentsare 1.5 to 3.7 wt % (Mullen and McCallum, sub-mitted manuscript, 2013) and mantle potentialtemperatures are 1273C to 134C (Figure 10,Table 3), within the range for the subarc mantlewedge [Syracuse et al., 2010]. Mantle segregationdepths are 35 to 52 km, i.e., ranging from theMoho to just above the hot core of the mantlewedge. The shallower depths recorded by the Mt.Baker basalts are consistent with trace elementmodeling (below) that indicates residual garnet forthe BSM basalts but not Mt. Baker.6.1.2. Mantle Isotopic Characteristics[29] Primitive BSM basalts (Mg/[Mg?Fe2?]> 0.6)have isotope ratios that define a narrow range, con-sistent with a common mantle source and differen-tiation dominated by fractional crystallization. Pbisotope ratios overlap with Explorer and northernJuan de Fuca MORB [Cousens et al., 1995; B.Cousens, unpublished data 2007], Chilcotin plateaubasalts [Bevier, 1983], and the least radiogenic sam-ples from the Anahim volcanic belt [Bevier, 1989](Figure 11). The isotopic similarity among thesevolcanic provinces confirms that the northwesternmargin of North America is underlain by uppermantle that is relatively depleted and generally sim-ilar to northeastern Pacific mantle [Cousens andBevier, 1995; Bevier, 1989].[30] Although the BSM basalts have 208Pb/204Pbsimilar to local MORBs at a given 206Pb/204Pb,207Pb/204Pb is slightly higher (Figure 3).Relatively high 207Pb/204Pb could be interpreted asreflecting subducting sediment input, but thisshould increase 208Pb/204Pb along with207Pb/204Pb, and the BSM basalts overlap withMORB in 208Pb/204Pb.[31] High 207Pb/204Pb relative to 208Pb/204Pb mayinstead indicate a higher time-integrated U/Th inthe BSM source than in the MORB sources.Table 2. (continued)Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Yb 1.7 1.7 1.7 1.4 1.5 1.7 1.5 1.5 1.6 1.5Lu 0.24 0.23 0.26 0.21 0.22 0.25 0.21 0.22 0.22 0.22Sc 20 20 22 19 20 23 20 22 21 25Zn 102 93 103 95 106 100 106 102 106 91Cr 248 255 239 332 330 112 274 271 255 55Ni 227 216 130 301 285 49 188 146 156 24V 198 188 227 157 212 208 176 185 164 199Ga 20 22 17 21 20 21 22aMethod 1: Thermo Finnigan Element2 HR-ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS.bData were corrected for blank contributions and by sample-standard bracketing to published values for the USGS BCR2 reference material[Raczek et al., 2001] analyzed every eight samples (method 1), or the USGS AGV1 reference material [Chauvel et al., 2011] analyzed every sixsamples (method 2). Repeat analysis of the USGS BHVO2 standard gave RSD values of <5% and concentrations within 5% (relative) of pub-lished values (as compiled by Carpentier et al. [2013] from GeoRem) for most elements. The average BHVO2 values obtained during analyticalsessions are reported in Table S1. Duplicate analyses gave reproducibilities better than 5% (Table S1). Total procedural blanks (Table S1) werenegligible relative to analyzed sample concentrations.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913138Enrichment of the BSM mantle source in U relativeto Th at some time in the past could be accom-plished through addition of fluid or melt compo-nents derived from subducting sediment and/oroceanic crust, since U is slightly more incompatiblethan Th during dehydration and melting [Brenan etal., 1995; Kessel et al., 2005; Hermann andRubatto, 2009]. However, this situation wouldresult in the presence of a subduction signature inthe BSM mantle source, which is not observed.[32] A more plausible explanation may be meltingof the BSM mantle source in the presence of resid-ual garnet at some time in the past. Unlike othertypical mantle minerals, which do not fractionateU and Th appreciably, U is more compatible ingarnet than Th [Beattie, 1993; LaTourrette et al.,1993]. The BSM basalts also plot at the lower mar-gin of the Hf-Nd mantle array (Figure 4b), consist-ent with the isotopic evolution of mantle thatproduced melts within the garnet stability field[Carlson and Nowell, 2001].[33] Because BSM basalts have lower EHf (3 ep-silon units) than Mt. Baker but similar ENd (Figure4b), two distinct mantle sources are required. Par-titioning experiments show that Hf is preferen-tially retained in the subducting slab relative toNd, most effectively during slab dehydration butalso during slab melting [e.g., Kessel et al., 2005;Hermann and Rubatto, 2009]. High Nd/Hf in thesubduction component is further enhanced by theFigure 6. (a?d) Extended N-MORB normalized [Sun and McDonough, 1989] trace element diagrams, sub-divided by volcanic center. (d) Light gray field in each panel encompasses the range defined by Mt. Bakerbasalts. The darkest colors (with sample numbers) signify Suite 2 samples discussed in the text. (e) All of theSuite 2 samples are plotted together for comparison to Mt. Baker basalts.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913139preexisting negative HFSE anomalies that charac-terize Cascadia sediment [Carpentier et al., 2013;Prytulak et al., 2006]. As a consequence, additionof a sediment component to the BSM mantlesource generates mixing curves that extendtowards lower ENd values but with smaller changesin EHf. Most importantly, mixing trajectoriesextend away from the Mt. Baker data. Thus sub-duction input cannot account for the Hf isotopicdistinction between the BSM and Mt. Bakerbasalts; the difference is instead a primary featureof their respective mantle sources. Since mantleHf isotope ratios can be affected by both fluidsand melts derived from the slab (Figure 4b), Hfisotopes do not always directly record the isotopiccomposition of the mantle as is commonlyassumed.6.1.3. Mantle Source Fertility[34] Zr/Nb in basalts provides a useful indicator ofmantle source fertility because this ratio is mini-mally affected by subduction input or fractionalcrystallization (Figure 8b). Although Zr/Nb is con-trolled to some extent by melt fraction, the Zr/Nbrange defined by melts of average depleted mantledoes not overlap the range for melts of moreenriched mantle tapped by ocean island basalts.Mt. Baker basalts have Zr/Nb consistent with10% partial melting of average depleted mantlewith an additional slab component (Figure 8b). Zr/Nb is too low in the primitive BSM basalts to beproduced from the same mantle source as Mt.Baker, requiring a more incompatible elementenriched mantle source. The relatively high Nbcontents of the BSM basalts also indicate a sourcerelatively enriched in incompatible elements (Fig-ure 8a), as do high Na2O and TiO2 [Prytulak andElliott, 2007].6.1.4. Assessment of Subduction Input[35] For primitive samples at Salal Glacier andBridge River, Ba/Nb values lie within the range ofHawaiian basalts and coincide with melting curvesfor enriched mantle (at 2?5% partial melt) (Fig-ure 8), pointing to the likelihood that a slab-derived component was not present in the mantlesource. The absence of slab input is supported bythe absence of negative Ta-Nb anomalies (FigureFigure 7. (a?d) Chondrite-normalized [McDonough and Sun, 1995] rare-earth element diagrams subdividedby volcanic center.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913140Figure 8. (a) Ba (ppm) versus Nb (ppm); (b) Zr/Nb versus Ba/Nb. Except for Suite 2 samples (diamonds),plus MM08 from Mt. Meager, the BSM basalts are similar to basalts from MORB and OIB settings, i.e.,essentially no subduction component. MORB data (gray filled circles) and Hawaiian basalt data (shield andpostshield lavas shown as filled green and pink circles, respectively) were obtained from the PETDB (http://www.earthchem.org/petdb) and GEOROC databases (georoc.mpchmainz.gwdg.de/georoc), respectively,accessed in May 2012. Average OIB composition [Sun and McDonough, 1989] is shown as a black asterisk inFigure 8b. Black arrows in lower right corner of Figure 8a and upper right corner of Figure 8b show the effectof 15% fractionation of each mineral indicated, calculated using the Rayleigh equation, the starting composi-tion of BRC09-3, and partition coefficients listed in Table 4 plus ilmenite from McCallum and Charette[1978]. Only ilmenite and magnetite vectors are shown in Figure 8b because the other phases shown in Figure8a have a negligible effect. Orange and blue curves are for addition of subduction components to a depletedmantle source (average depleted MORB mantle of Salters and Stracke, 2004) prior to 10% partial melting,calculated as described in Fig. 4 caption. Most subduction components increase Ba at a given Nb, and Ba/Nbat a given Zr/Nb. The heavy black curves are the compositions of DM melts at 1 GPa and PM melts at 3 GPa,respectively (tick marks indicate % partial melt). Melt compositions were calculated using the equilibriummelting equation with mineral/melt partition coefficients from Table 4 and residual mantle mineral assemb-lages determined by BATCH modeling [Longhi, 2002] of starting compositions of Wasylenki et al. [2003]and Kinzler [1997]. Filled black squares with white (cross) and (plus) symbol are for DM (depleted MORBmantle) of Salters and Stracke [2004] and PM (primitive mantle) of Sun and McDonough [1989], respec-tively. Inset diagrams include data for alkalic basalts (molar Mg/(Mg? Fe2?)> 0.60, assuming Fe3?/PFe? 0.15) from the Anahim volcanic belt (dark blue squares) [Charland et al., 1995], Cascade-Columbiatransect (light blue squares) [Leeman et al., 2005; Jones, 2002], and Dalles Lakes north of Mt. Rainier (purplesquares) [Reiners et al., 2000]. Abbreviations: ol (olivine), opx (orthopyroxene), cpx (clinopyroxene), plag(plagioclase), ilm (ilmenite), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131419a) that are observed at Mt. Baker and in othercalc-alkaline Cascade arc basalts [e.g., Schmidt etal., 2008]. The absence of slab input is further sup-ported by the similarity of Salal Glacier andBridge River primitive basalts to Mauna Kea post-shield alkalic basalts [Hanano et al., 2010], whichsample the same mantle source as shield lavas,that is, a composition comparable to the PREMA,or C, mantle component [Nobre Silva et al., 2013](Figure 9c). The only major difference is in Pb,which is deficient in Hawaii (a ubiquitous featureof oceanic basalts [Hofmann, 1997]) but showssmall positive spikes at Bridge River and SalalGlacier. The Pb spikes are successfully modeledwithout subduction input (see later). However, wecannot definitively rule out the presence of a verysmall subduction component in the mantle source.An ancient subduction component may have beenadded to the source in the past, or the primaryalkalic magmas may have acquired a small sub-duction component during migration through themantle.Figure 9. (a) EHf versus Nb/Nb (niobium anomaly) for BSM and Mt. Baker basalts, compared to the rangesdefined by Hawaiian shield and postshield basalts (green and pink lines, respectively) with >8 wt % MgO.Nb/Nb calculated as 2(Nbsample/NbPM)/(Basample/BaPM?Lasample/LaPM) [Verma, 2009] where PM refers toPrimitive Mantle. Hawaii data were obtained from the GEOROC database accessed in May 2012(georoc.mpchmainz.gwdg.de/georoc). Also shown are values for average depleted mantle (DM, black square)[Salters and Stracke, 2004] and average N-MORB (gray square) and Primitive Mantle (PM, black square withwhite ?) of Hofmann et al. [1988]. (b) Ba/La versus 208Pb/204Pb for BSM and Mt. Baker basalts. (c) N-MORB normalized [Sun and McDonough, 1989] extended trace element diagram comparing Salal Glacier(orange) and Bridge River (red) primitive basalts (molar Mg/[Mg?Fe2?]> 0.60) to alkalic postshield basaltsfrom Mauna Kea (light blue) [Hanano et al., 2010], Simcoe volcanic field (dark blue) [Battleground Lakesample of Jones, 2000], and Anahim volcanic belt (green) [sample 2278 of Charland et al., 1995].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913142[36] In contrast to Salal Glacier and Bridge River,even the most primitive Mt. Meager basalts haveslightly elevated Ba and Ba/Nb relative to mantlemelting curves and Hawaiian basalts (Figure 8), aswell as small negative Nb anomalies (Figure 9a),all of which point to subduction input (althoughsignificantly less than at Mt. Baker). Zr/Nb in thetwo of the three primitive Mt. Meager basalts isthe same as at Salal Glacier and Bridge River,indicating similar mantle sources. In the other Mt.Meager sample (MM08), higher Zr/Nb and lowerNb is consistent with a mantle source that is transi-tional between the Mt. Baker mantle source andthat of the other BSM basalts. An intermediate Hfisotopic composition for MM08 (Figure 4b) sup-ports this conclusion.[37] Mt. Meager basalts have the lowest206Pb/204Pb and 208Pb/204Pb of all the BSM vol-canic centers; sample MM08 has the lowest208Pb/204Pb, 206Pb/204Pb, and (La/Yb)N, coupledwith the highest Ba/La (Figure 9b). These charac-teristics are not consistent with addition of a sub-ducting sediment component to the mantle sourceand may instead reflect the influence of a fluidderived from altered oceanic crust (AOC). AOCfluid input can increase LILE in the mantle sourcewithout affecting LREE [Kessel et al., 2005], andsince recent AOC has MORB-like Pb isotoperatios, it is capable of ??pulling?? Pb isotope ratiosof the mantle source to lower values. Mt. Meageralso has similar ENd to Bridge River and Salal Gla-cier but slightly higher 87Sr/86Sr (Figure 4a), con-sistent with the involvement of AOC that acquireshigh 87Sr/86Sr with minimal change in ENd duringprogressive seafloor alteration [Staudigel et al.,1995].6.1.5. Trace Element Modeling[38] Mantle melt fractions and residual mantle min-eral assemblages were determined by modeling theabundances of 28 trace elements in the two mostprimitive basalts at each BSM center (BRC09 andBRC10 at Bridge River, MM04 and MM02 at Mt.Meager, SG10 and SG16 at Salal Glacier). We alsomodeled MM08 at Mt. Meager because it may havea slightly more depleted mantle source.[39] The model is based upon the mass balanceequation for equilibrium melting, CLi = C0i ?Figure 10. Pressure versus temperature plot illustrating theconditions at which the BSM and Mt. Baker magmas segre-gated from the mantle. P and T (from Table 3) were calcu-lated using the silica activity geobarometer and olivine-liquidgeothermometer calibrations of Putirka [2008] for 1% and2% dissolved water (BSM basalts) or for the specific H2Ocontent given (Mt. Baker basalts). Standard estimates of errorare 43C and 0.29 GPa [Putirka, 2008].Table 3. Liquidus pressures and temperaturesH2O (wt.%)0.0 1.0 2.0P(GPa) T(C) P(GPa) T(C) P(GPa) T(C)Bridge RiverBRC10 3.49 1576 3.19 1535 2.89 1496BRC09 2.68 1505 2.50 1470 2.31 1437Salal GlacierSG10 2.82 1514 2.61 1478 2.41 1443SG16 2.67 1502 2.48 1467 2.29 1433Mt. MeagerMM02 2.04 1441 1.91 1409 1.79 1379MM04 2.15 1460 2.02 1427 1.86 1395a Mt. Baker P (GPa) T (C) H2O (wt.%)MB5 0.90 1273 2.7MB82 1.4 1326 2.1MB1 1.1 1274 3.7MB97 1.2 1309 2.1MB114 1.4 1326 15MB112 1.5 1350 1.5aP-T data for Mt. Baker (from Mullen and McCallum, submitted manuscript, 2013) are calculated only at the specific water content listed foreach sample.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131431= FL 1 Di ? Di [Shaw, 1970], where CLi isthe concentration of trace element (i) in the liquid(L), C0i is the initial concentration of trace element(i), FL is melt fraction, and Di is the bulk partitioncoefficient, defined as crystalline assemblage/melt.The model does not require a priori knowledge ofinitial mantle mineral assemblages but does requireinitial trace element abundances. The primitivemantle composition of Sun and McDonough [1989]was used as the source for all BSM samples, and amixture of 50% primitive mantle and 50% depletedmantle [Salters and Stracke, 2004] was also testedfor MM08. Distribution coefficients used in the cal-culations are listed in Table 4. Least-squares mini-mization was used to generate best fit models forthe basalts by varying the mantle mineral modesand melt fractions (see the caption of Figure 12).Note that the substitution of fractional melting inour model results in negligible change to modeloutcomes. Melt fractions are within 0.5% and resid-ual mantle modal abundances change by less than afew percent, with overall residual mineral assemb-lages remaining identical.6.1.5.1. Modeling Results[40] Representative best fit trace element solutionsare shown in Figure 12. Melt fractions are 2?4%for Salal Glacier, 4?5% for Mt. Meager, and 7?8%for Bridge River, all with residual garnet lherzo-lite. Lower melt fractions for Salal Glacier basaltsare consistent with their higher alkali elementabundances. For Mt. Meager sample MM08, aprimitive mantle source indicates 8% partial meltand the mixed PM-DM source gives 4%. The latterresult is preferred because it is more consistentwith the results for other Mt. Meager samples. Re-sidual garnet in all samples is consistent with thepressures of melting (2?3 GPa) calculated for theBSM basalts, as garnet is stable at the solidus ofhydrous mantle at pressures above 1.6 GPa [Gae-tani and Grove, 1998].[41] Results of similar modeling for Mt. Bakerbasalts, using a depleted mantle source, indicate5?12% partial melting of depleted lherzolite orharzburgite. Best fit solutions require overprintingby a subduction component consisting of AOCfluid, AOC melt, and sediment melt (Mullen andMcCallum, submitted manuscript, 2013). No re-sidual garnet is present in the Mt. Baker source,consistent with calculated melt segregation pres-sures (1?1.5 GPa) and with the lower (Dy/Yb)N,and higher Yb and Sc contents of the Mt. Bakersamples (24?33 ppm Sc) (Mullen and McCallum,Figure 11. Plot of 208Pb/204Pb versus 206Pb/204Pb comparing Pb isotope ratios for the BSM and Mt. Bakerbasalts (symbols as in Figure 9) to other basalts from the northeastern Pacific and southwestern British Colum-bia: Anahim volcanic belt (purple diamonds; Bevier [1989]); Chilcotin Plateau (black diamonds; Bevier[1983]); Wells Gray-Clearwater volcanic field (blue diamonds; Hickson [1987]); and Tuzo Wilson volcanicfield [Allan et al., 1993]. Other data and references as in Figure 3. Note that all isotope data are normalized tothe same standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913144submitted manuscript, 2013) as compared with theBSM basalts (18?24 ppm Sc; Table 2).6.4. Nonprimitive BSM Basalts: CrustalContamination or Subduction Input?[42] Although we make the case above that theprimitive Salal Glacier and Bridge River basaltsessentially lack a subduction component, some ofthe nonprimitive BSM basalts have geochemicalcharacteristics that could be interpreted as an ??arcsignature.?? Do these reflect subduction input thatis not displayed by more primitive samples?[43] BSM samples with Mg/(Mg? Fe2?)< 0.60are subdivided into two suites based upon isotopicand trace element compositions: Suite 1 has iso-tope and trace element ratios similar to primitiveBSM basalts, indicating minimal crustal contami-nation and differentiation processes dominated byfractional crystallization. Relative to the mostprimitive basalts, Suite 2 has high Sr-Pb isotoperatios, (La/Yb)N, and incompatible element abun-dances, coupled with low Nd-Hf isotope ratios andHFSE abundances.[44] Pearce element ratio diagrams [Russell andNicholls, 1988] show that the Suite 1 basalts areconsistent with fractionation of olivine? plagio-clase (6 minor clinopyroxene) from parental mag-mas that were similar to the most primitive basaltsat each volcanic center. The crystallizing assemb-lages are consistent with the presence of the sameminerals as phenocryst phases.[45] For Suite 2 samples, trace element abundan-ces and Pb isotope ratios are nearly indistinguish-able from the most primitive Mt. Baker basalts(Figures 3, 6e, and 8), pointing to the possibilitythat they may record input from the subductingslab as does Mt. Baker. However, Suite 2 has sig-nificantly higher 87Sr/86Sr and lower ENd than themost primitive Mt. Baker and BSM basalts (Figure4a). The lower MgO contents of Suite 2 lavas rela-tive to the most primitive lavas are consistent withTable 4. Partition Coefficients (Mineral/Melt)acpx opx oliv sp gar amph phlog mt plagCs 0.000201 0.00091 0.00004525 0.000625 0.00011 0.02325 2.261 0.00121 0.00623Rb 0.000603 0.00381 0.0000453 0.000625 0.00022 0.0232 1.702 0.00121 0.01823Ba 0.000683 0.00361 0.0000433 0.00067 0.000072 0.012 1.502 0.00121 0.3223Th 0.0124 0.00051 0.000053 0.0107 0.00212 0.00102 0.000201 0.002420 0.1923U 0.0134 0.00071 0.000053 0.0147 0.00947 0.00122 0.000201 0.01220 0.3423Nb 0.0051 0.00071 0.000413 0.0217 0.00315 0.082 0.0551 0.8620 0.00823Ta 0.0211 0.00081 0.00021 0.0217 0.0199 0.0831 0.0621 0.9520 0.02723K 0.00725 0.00012 0.000022 0.00125 0.0132 0.222 1.502 0.00121 0.09721La 0.0546 0.00061 0.000053 0.0119 0.001615 0.05524 0.0002525 0.001220 0.1123Ce 0.08625 0.00171 0.000063 0.0119 0.00515 0.09625 0.000301 0.001920 0.08525Pb 0.0104 0.00011 0.0000712 0.00057 0.000315 0.0424 0.091 0.02221 0.108523Pr 0.1425 0.002625 0.0001325 0.0119 0.02925 0.1316 0.000425 0.002320 0.06525Sr 0.04811 0.0093 0.0002512 0.00477 0.002515 0.3015 0.161 0.003020 1.9423Nd 0.1925 0.0041 0.000203 0.0119 0.05215 0.18716 0.000551 0.00425 0.05223Sm 0.276 0.0111 0.000603 0.0119 0.2515 0.3216 0.000701 0.007020 0.04123Zr 0.061 0.0133 0.0006810 0.00817 0.6614 0.1816 0.0111 0.5620 0.003923Hf 0.121 0.0133 0.001110 0.00307 0.6814 0.6316 0.0161 0.6520 0.001523Eu 0.4525 0.01625 0.0008025 0.0119 0.4015 0.4316 0.000725 0.01025 1.4223Ti 0.308 0.06110 0.00221 0.04819 0.2915 0.952 0.791 2021 0.04723Gd 0.5025 0.02225 0.000993 0.0119 0.9025 0.5416 0.000725 0.01620 0.03521Tb 0.5625 0.0301 0.0023 0.0119 1.415 0.6025 0.00071 0.02325 0.03121Dy 0.6125 0.03825 0.0043 0.0119 2.215 0.6325 0.000825 0.03325 0.02621Y 0.6525 0.0461 0.0073 0.00207 3.115 0.5215 0.0031 0.0520 0.02621Ho 0.656 0.0481 0.0063 0.0119 2.815 0.6224 0.00091 0.0525 0.01821Er 0.6925 0.05825 0.00873 0.0119 3.6 15 0.5724 0.001025 0.0725 0.014521Tm 0.7225 0.0711 0.01325 0.0119 3.725 0.5325 0.00141 0.01125 0.01221Yb 0.7425 0.0771 0.0173 0.0119 3.94 0.4825 0.001625 0.1725 0.009721Lu 0.756 0.0901 0.0203 0.0119 3.84 0.4324 0.00171 0.2820 0.00821aAbbreviations: cpx (clinopyroxene), opx (orthopyroxene), oliv (olivine), sp (spinel), gar (garnet), amph (amphibole), phl (phlogopite), mt(magnetite), plag (plagioclase).Data sources: 1Adam and Green [2006]; 2Halliday et al. [1995] compilation; 3Donnelly et al. [2004] compilation; 4Hauri et al. [1994]; 5Hartand Dunn [1993]; 6Gaetani [2004]; 7Elkins et al. [2008]; 8McDade et al. [2003]; 9Green et al. [2000]; 10Kennedy et al. [1993]; 11Beattie[1993];12Beattie [1994]; 13Canil and Fedortchouk [2001]; 14Salters and Longhi [1999]; 15Abraham et al. [2005] compilation; 16Chazot et al.[1996]; 17Horn et al. [1994]; 18Nagasawa et al. [1980]; 19McKenzie and O?Nions [1991]; 20Klemme et al. [2006]; 21Claeson and Meurer [2004]compilation; 22Dunn and Sen [1994]; 23Tepley et al. [2010]; 24LaTourrette et al. [1995]; 25Interpolated from neighboring elements.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913145crustal contamination (Figure 5b). Furthermore,Suite 2 contains the only two BSM basalts withxenocrysts (BRC01?3, BRC03?4).[46] The GVB crustal basement is a collage of Pa-leozoic and Mesozoic accreted terranes [Mongeret al., 1982]. At depths greater than 10 km, theGVB is underlain by the composite Wrangellia?Harrison terranes [Mullen, 2011; Miller et al.,2009; Monger and Price, 2000]. With the excep-tion of the Mt. Baker region, the terranes areintruded extensively by Jurassic to Cretaceousgranitoids of the Coast Plutonic Complex, the larg-est composite batholith in North America [Barkerand Arth, 1984; Friedman et al., 1995; Cui andRussell, 1995a, 1995b]. Because the crust is rela-tively young and Cascadia subducting sediment ismainly terrigenous [Carpentier et al., 2013; Pry-tulak et al., 2006], the isotopic effects of crustalassimilation are similar in many respects to theeffects of subducting sediment input. However,sediment input cannot account for the 87Sr/86Srversus Sr systematics of the Suite 2 lavas (Figure13a). Assimilation-fractional crystallization (AFC)modeling [DePaolo, 1981] using a granodioriticassimilant from the Coast Plutonic Complex canreproduce the Suite 2 trace element and isotopicdata, but the volume of assimilant required(>20%) would increase the SiO2 content beyondthe range of the Suite 2 samples (Figure 5a).Assimilation that takes place in the deep crust,Figure 12. Best fit trace element solutions for four of the most primitive BSM basalts, shown on N-MORBnormalized extended element diagrams and accompanying inset chondrite-normalized REE diagrams. Actualdata are shown with colored lines and symbols; modeling solutions shown as heavy black lines with blacksquares. Each BSM sample has been adjusted into equilibrium with Fo90 mantle using olivine/melt partitioncoefficients from Table 4. The mantle source composition used in the model for all BSM basalts (PM of Sunand McDonough [1989]) is shown as a thin black line in each panel; DM (source used for Mt. Baker) is alsoshown for reference [Salters and Stracke, 2004]. The Generalized Reduced Gradient (GRG2) nonlinear opti-mization code in Microsoft Excel Solver was used to obtain the best fit for each basalt by minimizing the sumof squares of residuals for 28 trace elements, i.e.,Pi Cliqi calc? ?  Cliqi obs? ?h i=Cliqi ?obs?n o2. The denomina-tor in the equation normalizes the concentrations of the elements so that each trace element has an equivalentimpact on the solution regardless of its absolute concentration. Best fit melt fractions (FL) and residual mantlemineral modes are given in lower right corner of each N-MORB-normalized panel. Abbreviations: ol (oli-vine), opx (orthopyroxene), cpx (clinopyroxene), gar (garnet).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913146where the country rock is mafic, can minimizechanges to the major element abundances of theoriginal basaltic magma [Reiners et al., 1995,1996]. AFC models in which the assimilant is agabbro from the lower crustal section of the Bo-nanza arc (Westcoast crystalline complex) ofWrangellia [DeBari et al., 1999] provide good fitsto Suite 2 trace element and isotope data with15% gabbro assimilated at Bridge River and21% at Mt. Meager (Figure 13). Modeling pa-rameters and results are provided in Table 5. Frac-tionating mineral phases (olivine, clinopyroxene,orthopyroxene, and minor magnetite) are consist-ent with experimental results for partial melting ofmafic compositions under lower crustal conditions[Rapp, 1995; Rapp and Watson, 1995]. Becausethe assimilant has a low SiO2 content (45 wt %)and the fractionating mineral assemblages havebulk SiO2 contents similar to the basalts, the finalmagmas maintain an overall basaltic compositionin the magmas.6.5. Relationship Between Tectonics andVolcanism[47] An incompatible element-enriched, garnet-bearing mantle source essentially free of subduc-tion input, coupled with relatively high mantlemelting temperatures and pressures, is consistentwith decompression melting of an upwelling as-thenosphere source for the primitive BSM basalts.Upwelling mantle is potentially consistent with aslab edge effect as proposed by Lawrence et al.[1984] for Salal Glacier basalts. Seismic anisot-ropy measurements reveal toroidal mantle flowaround the descending edges of subducted platesthat are undergoing rollback, thereby drawingexternal mantle (subslab) into the mantle wedgeFigure 13. AFC modeling results for (a) 87Sr/86Sr versus Sr; (b) ENd versus87Sr/86Sr; (c) Zr/Nb versus Ba/Nb. Heavy green curves with triangles and squares (AFC1 and AFC2, respectively) are the best fitassimilation-fractional crystallization pathways for Suite 2 samples BRC03?4 and MM01-1, respectively,using a gabbroic assimilant (from Table 5). The large green triangle and square are the gabbro compositionsused as assimilants in AFC 1 and 2, respectively. Orange and blue curves in Figure 13a are slab fluid/meltaddition curves, calculated as described in Fig. 4 caption. Heavy black curves in Figure 13c are from Figure8b. Data for the Coast Plutonic Complex shown as small blue triangles and light blue field [Friedman et al.,1995; Cui and Russell, 1995a, 1995b]. Sources of other data shown are given in Figure 4a caption.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913147[Long and Silver, 2008]. In other arcs, influx ofexternal mantle has been implicated in the genesisof lavas that are atypical for an arc setting [e.g.,Leat et al., 2004; Smith et al., 2001; Ferrari et al.,2001]. Slab rollback is occurring in the Cascadearc [Schellart, 2007], but the limited mantle ani-sotropy measurements in the GVB are inconclu-sive as to mantle wedge flow patterns [Currie etal., 2004]. Toroidal mantle flow has been docu-mented at the southern Juan de Fuca plate edge[Zandt and Humphreys, 2008], yet alkalic basaltsare not present [Hildreth, 2007] indicating that thetwo phenomena are not necessarily interrelated.[48] A slab edge origin may be improbable for theBSM volcanic centers in light of recent seismic to-mography, which indicates the northernmost slabedge in the Cascades (placed at the northernmostlimit of the Explorer plate) is located farther norththan the BSM volcanic centers [Mercier et al.,2009; Audet et al., 2008]. Toroidal mantle flowhas been proposed for the northern Explorer plateedge [Audet et al., 2008] and could be responsiblefor the alkalic basalts of the 500 km long Anahimvolcanic belt, which defines an east-west trendnearly orthogonal to, and north of, the GVB. Thisinterpretation is consistent with that of Thorkelsonet al. [2011] who proposed that Anahim magma-tism is related to mantle upwelling along the ther-mally eroding plate margins of the NorthernCordilleran slab window. However, eruption agesin the Anahim volcanic belt define an easterlytime progression that has been attributed to a hot-spot [Bevier, 1989], and tomographic results areconsistent with either interpretation [Mercier etal., 2009].[49] The BSM alkalic basalts may be related tomantle upwelling at the boundary between theJuan de Fuca and Explorer plates, as illustratedschematically in Figure 14. The northern segmentof the Juan de Fuca plate has had a complex tec-tonic history; about 4 Myr ago, the northernmostportion of the Juan de Fuca plate separated alongthe Nootka fault zone to form the independentExplorer microplate [Riddihough, 1984] (Figure1). Although convergence has ceased at the north-ern edge of the microplate, the southernmost partof the microplate continues to subduct slowly[Braunmiller and Nabelek, 2002], and the entireTable 5. AFC Modeling Parameters and ResultsCompositionAFC 1: Bridge River AFC 2: Mt. MeagerSample Modeled: BRC03?4 Sample Modeled: MM01-1Initial Magma: Assimilant: Initial Magma: Assimilant:BRC09-3a Gabbrob MM08a GabbrobSiO2 (wt %) 45.6 44.6 48.6 44.6TiO2 1.8 1.01 1.4 1.01MgO 15.6 6.57 14.5 6.57Na2O 2.6 1.31 2.9 1.31K2O 0.7 0.40 0.6 0.40Sr (ppm) 400 401 467 401Nd 18.5 5.0 14 5.0Ba 195 153 154 153Zr 127 21 92 21Nb 16.5 1.0 9 1.087Sr/86Sr 0.702986 0.7034 0.703164 0.7040143Nd/144Nd 0.513026 0.51286 0.513030 0.512820ENd ?7.6 ?4.3 ?7.6 ?3.6AFC resultsr c 0.90 0.89FLd 0.83 0.77olive 0.05cpx 0.05 0.10opx 0.05 0.10mt 0.02 0.03aTrace element and isotope data for initial magmas are from Table 1; major element data [from Green and Sinha, 2005] are corrected into equi-librium with Fo90 mantle.bMajor and trace element data for gabbro assimilant are from DeBari et al. [1999] for sample 91-17 of the Westcoast Crystalline Complexexcept Nd (interpolated); isotope ratios selected from within the range defined by the Coast Plutonic Complex [Cui and Russell, 1995b].cmass assimilated/mass crystallized.dfraction liquid remaining.eFraction of each mineral phase removed from magma; sum is equal to (1FL).fAbbreviations: oliv (olivine), cpx (clinopyroxene), opx (orthopyroxene), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913148Explorer region is a zone of strong shear deforma-tion [Dziak, 2006]. The offshore segment of theNootka fault shows left-lateral motion along a rup-ture and the onshore extension of the fault ismarked by thinning and deformation of the sub-ducting plate [Audet et al., 2008]. Seismic dataindicate the Explorer plate currently has a shal-lower dip than the Juan de Fuca plate, which maymanifest itself in a near-vertical gap between theplates (Figure 14). The BSM volcanic centers lieon, or just south of, the Nootka fault zone as ex-trapolated to the northeast (Figure 1a). We suggestthat thinning, deformation, and possible rupture ofthe subducted Explorer plate fragment may pro-vide a pathway for asthenospheric upwellingaccompanied by decompression melting. Farthereast along the projected trace of the Nootka fault,the Wells Gray-Clearwater volcanic field and Chil-cotin basalts have been similarly attributed toenriched asthenosphere upwelling through a gapalong the fault [Madsen et al., 2006; Sluggett,2008; Thorkelson et al., 2011].[50] Seismic tomography is inconclusive as towhether the Nootka fault is ??leaky?? or whethercontinuity is maintained at depth between theExplorer and Juan de Fuca plates [Mercier et al.,2009]. However, as the Explorer plate is situatedat the southern edge of a slab window, it is subjectto progressive thermal and physical degradationthat would facilitate passage of mantle melts frombelow [Thorkelson et al., 2011; Thorkelson andBreitsprecher, 2005]. In an analogous situation inthe Mexican arc, seismic anisotropy measurementsare consistent with plate separation. Faults sepa-rate the subducting Cocos plate into several seg-ments, and each subducts at a different angle,resulting in a scissors-like effect in which gapsbetween the plates allow for mantle upwellingthrough toroidal flow [Stubailo et al., 2012].7. Summary and Conclusions[51] Alkalic basalts at the Bridge River, Salal Gla-cier, and Mt. Meager volcanic centers (BSM vol-canic centers) of the Canadian segment of theCascade arc, known as the Garibaldi volcanic belt,have intraplate characteristics that contrast withtypical calc-alkaline mafic Cascade arc lavas. Newhigh precision Sr-Nd-Hf-Pb isotope ratios andtrace element abundances reveal that the mostprimitive basalts at Salal Glacier and Bridge Riverare essentially free of components derived fromthe subducting slab. The apparent trace element??arc signature?? exhibited by several more evolvedBSM basalts is more likely a consequence ofassimilation of mafic deep crust rather than slabinput. At Mt. Meager, however, primitive basaltsmay include a small amount of fluid derived fromsubducted altered oceanic crust.[52] The mantle source of the BSM basalts isdeeper, hotter, and isotopically distinct from thesource of calc-alkaline basalts from Mt. Baker andthroughout the Cascade arc. The BSM mantlesource is also more enriched in incompatible ele-ments than the depleted mantle wedge tapped bycalc-alkaline Cascade arc basalts, and similar toocean island basalt sources. Similar trace elementabundances among the BSM and Anahim alkalicbasalts, and those in the Cascade-Columbia tran-sect and north of Mt. Rainier (Figures 8 and 9c),indicate mantle sources similarly enriched in in-compatible elements.[53] BSM and Cascade-Columbia intraplate lavashave been previously attributed to enriched mantledomains associated with the base of an accretedterrane [Schmidt et al., 2008]. We consider this hy-pothesis unlikely for the BSM volcanic centers fortwo reasons. First, the accreted terranes beneathFigure 14. Schematic representation of plate configurationat the northern end of the Cascade arc based on a model ofRiddihough [1984]. The Explorer plate detached from theJuan de Fuca Plate along the Nootka fault zone 3 to 4 Myrago as it became younger, hotter, and more buoyant at thetrench. The thick dashed black line indicates the surface traceof the Nootka fault. Convergence of the Explorer plate withNorth America has now nearly ceased. The vertical windowformed between the Explorer and Juan de Fuca plates maypromote upwelling of deep, hot mantle (large orange arrow)at the edge of the currently subducting plate. Decompressionmelting of this mantle accounts for the presence of hot alkalicbasalts essentially free of a subduction signature (red, orange,and yellow triangles along Nootka fault zone for each of theBSM volcanic centers).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913149the BSM centers and the Cascade-Columbia tran-sect are different (Wrangellia and Siletzia, respec-tively), and second, Mt. Baker and the BSM sharethe same accreted terrane at depth yet the state ofmantle source enrichment differs substantially.[54] Although major and trace element datarequire an enriched mantle source for the BSMbasalts, isotopic data provide evidence for long-term mantle depletion. Pb isotope ratios of theBSM basalts are broadly similar to oceanic andintraplate basalts of the northeastern Pacific (Fig-ure 11), indicating that isotopically depleted uppermantle of common origin is regionally wide-spread, albeit with small isotopic heterogeneities.[55] With isotopic data consistent with long-termdepletion, incompatible-element enrichment of theBSM mantle source must have occurred relativelyrecently. Recent mantle enrichment has been pro-posed for numerous other cases of isotopicallydepleted alkalic basalts [Roden and Murthy, 1985,and references therein], including those from theTuzo Wilson volcanic field [Allan et al., 1993](Figure 11) and the Bowie Seamount in the Gulfof Alaska [Cousens, 1988]. The BSM volcaniccenters are located along, and just south of, theprojected trace of the Nootka fault zone, whichseparates the subducting Juan de Fuca plate fromthe Explorer plate fragment. We attribute the BSMbasalts to upwelling asthenosphere through a gapalong the fault, which undergoes decompressionmelting to generate alkalic basalts that are free ofsubduction input yet located in an arc setting.Acknowledgments[56] We thank Bruno Kieffer for assistance with TIMS analy-ses, Vivian Lai for help with trace element analyses, JaneBarling, Kathy Gordon, and Liyan Xing for assistance withMC-ICP-MS analyses, and Ines Nobre Silva for instruction inthe clean laboratory. We are grateful to Marion Carpentier forprocessing and analyzing eight samples for trace elementsand five for isotopes. Derek Thorkelson, Martin Streck, andRichard Carlson provided constructive and thoughtfulreviews. Insightful discussions with Kelly Russell have beenmuch appreciated. We are particularly grateful to StewartMcCallum for detailed reviews, discussions, and advice thathave significantly improved the manuscript. This researchwas funded by an NSERC Discovery Grant to D. 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Mullen and Dominique WeisPacific Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences,University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, V6T 1Z4, Canada(emullen@eos.ubc.ca)[1] In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier,and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures arenegligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceaniccrust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display traceelement signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation ratherthan primary input from the subducted slab. Primary BSM basalts represent 3?8% melts that segregated fromenriched garnet lherzolite at significantly higher temperatures and pressures (70?105 km) than calc-alkalineCascade arc basalts. The BSM mantle source is significantly more incompatible element-enriched than thedepleted mantle tapped by calc-alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct fromcalc-alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NWNorthAmerica. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent withupwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, whichforms the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate. A gap orattenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low-degreedecompressionmelting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.Components: 17,992 words, 14 figures, 5 tables.Keywords: alkali basalt ; Garibaldi Belt ; cascade arc; Sr-Nd-Pb-Hf isotopes; trace elements.Index Terms: 1040 Radiogenic isotope geochemistry: Geochemistry; 1037 Magma genesis and partial melting: Geo-chemistry; 1031 Subduction zone processes: Geochemistry; 1033 Intra-plate processes: Geochemistry; 1065 Major andtrace element geochemistry: Geochemistry; 3613 Subduction zone processes: Mineralogy and Petrology; 3615 Intra-plateprocesses: Mineralogy and Petrology; 3619 Magma genesis and partial melting: Mineralogy and Petrology; 3060 Subduc-tion zone processes: Marine Geology and Geophysics; 8170 Subduction zone processes: Tectonophysics; 8413 Subductionzone processes: Volcanology; 8415 Intra-plate processes: Volcanology.Received 10 January 2013; Revised 29 May 2013; Accepted 30 May 2013; Published 28 August 2013.Mullen, E. K., and D. Weis (2013), Sr-Nd-Hf-Pb isotope and trace element evidence for the origin of alkalic basalts in theGaribaldi Belt, northern Cascade arc, Geochem. Geophys. Geosyst., 14, 3126?3155, doi:10.1002/ggge.20191.1. Introduction[2] The role of mantle heterogeneity in generatingthe compositional diversity observed in arc magmasis difficult to decipher because it can be masked bycontributions from the subducting slab. There is ageneral consensus that mantle melting in arcs occursin response to input of a hydrous component derived? 2013. American Geophysical Union. All Rights Reserved. 3126ArticleVolume 14, Number 828 August 2013doi: 10.1002/ggge.20191ISSN: 1525-2027from the subducting slab that generates hydrousminerals in the mantle, lowers the mantle solidustemperature, and produces melts by dehydrationmelting or ??flux?? melting [e.g., Kushiro, 1987;Grove et al., 2002]. As a consequence, basaltserupted in subduction zone settings are predomi-nantly aluminous, subalkaline, and bear a slab-derived trace element ??arc signature?? that includesenrichments in large ion lithophile elements (LILEs)and depletions in high field strength elements(HFSEs) [e.g., Gill, 1981; Pearce and Peate, 1995].The Cascade arc, which extends 1300 km fromnorthern California to southwestern British Colum-bia, is an example of an arc in which calc-alkalinelavas dominate [Schmidt et al., 2008; Hildreth,2007; Bacon et al., 1997; Conrey et al., 1997].However, the northernmost segment of the arc is un-usual in that mafic lavas are predominantly alkalic.Alkali olivine basalt and hawaiite occur at the vol-canic fields of Mt. Meager, Salal Glacier, andBridge River Cones (hereinafter referred to as theBSM volcanic centers) [Green and Sinha, 2005].[3] Alkalic lavas are uncommon in arc settings,particularly along the main arc axis, and are attrib-uted to a variety of phenomena that include tearsin the subducting plate, back-arc extension, slabwindows, entrainment of mantle hotspots, accretedenriched mantle, and intraarc rifting [e.g., Naka-mura et al., 1989; Abratis and W?rner, 2001; Fer-rari et al., 2001; Marquez et al., 1999; Turnerand Hawkesworth, 1998; Pearce and Stern, 2006;Thorkelson and Taylor, 1989; Skulski et al., 1991;Righter and Carmichael, 1992; Hughes, 1990;Pearce and Peate, 1995, and references therein].The BSM volcanic centers are located 110 kmabove the subducting plate, broadly similar toother Cascade arc volcanoes [McCrory et al.,2004], suggesting a potential connection to arcmagmatic processes. Elucidating the petrogenesisof the BSM basalts may provide valuable insightsinto mantle and slab processes under the dimin-ished subduction regime at the northern boundaryof the Cascade arc [Harry and Green, 1999].[4] The Cascade magmatic arc has been activesince 40 Ma and is a consequence of subductionof the Juan de Fuca oceanic plate beneath NorthAmerica [Hildreth, 2007]. In northwestern Wash-ington, an abrupt change in the orientation of thearc axis mirrors a bend in the continental margin,subdividing the arc into two major segments (Fig-ure 1a). The High Cascades segment (Mt. Rainierto Lassen Peak) is separated from the GaribaldiVolcanic Belt (GVB) (Glacier Peak to Silver-throne) by a 120 km gap in modern volcanism[Green and Harry, 1999]. Throughout much of theHigh Cascades, the subducting plate is 10 Myrold at the trench with ocean floor isochrons gener-ally parallel to the continental margin [Wilson,2002]. In the GVB, however, isochrons are obliqueto the plate margin and the slab age at the trenchdecreases northward to 6 Ma outboard of theBSM volcanic centers (Figure 1a), making it oneof the youngest and hottest subduction settings inthe world [Syracuse et al., 2010; Harry andGreen, 1999]. Subducted water is expected to belost at shallow depths from a hot slab, leading toreduced hydration of the subarc mantle wedge[Green and Harry, 1999]. A diminished subduc-tion regime may account for why the GVB, incomparison to the High Cascades, has a narrowerwidth, lower magma production rates, and magma-tism mainly restricted to the major volcanic cen-ters [Harry and Green, 1999].[5] In this paper, we assess three hypotheses forthe origin of the BSM alkalic basalts. In the firstmodel, the BSM alkalic basalts are essentiallylower melt fraction ??equivalents?? of more typicalCascade arc calc-alkaline basalts [Green andHarry, 1999; Green and Sinha, 2005]. Reducedhydration of the subarc mantle wedge may reduceits capacity for flux melting, resulting in lowermelt fractions that are enriched in alkali elementsbut display minimal arc signature. A similar modelhas been proposed to account for basalts elsewherein the Cascade arc that are geochemically indistin-guishable from intraplate basalts [Reiners et al.,2000]. Intraplate-type lavas dominate the back-arcSimcoe volcanic field east of Mt. Adams and areinterspersed with other basalt types in a swathextending west 150 km from Simcoe to Portlandthat has been referred to as the Cascades-Columbia transect [Leeman et al., 1990, 2005;Hildreth, 2007; Jicha et al., 2008; Conrey et al.,1997; Bacon et al., 1997]. A few other examplesoccur north of Mt. Rainier [Reiners et al., 2000]and in north-central Oregon [Conrey et al., 1997].[6] Second, a subducted plate boundary or fracturezone may trigger mantle upwelling, inducingdecompression melting that generates low-degree,alkali-rich melts. These processes have been pro-posed for other arcs, including the Mexican arc andLesser Antilles [e.g., Righter et al., 1995; DeLonget al., 1975; Pearce, 2005], and a version of thismodel is mentioned by Lawrence et al. [1984] in thecontext of the Salal Glacier alkalic basalts. The sub-ducted boundary between the Explorer and Juan deFuca plates intersects the BSM volcanic centers andhas been implicated in the origin of the WellsMULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913127Gray-Clearwater volcanic field (3.5 Ma?7.6 ka[Hickson and Souther, 1984]) and Chilcotin Groupbasalts (32?0.8 Ma [Mathews, 1989]) that arelocated in the GVB back-arc region [Madsen et al.,2006; Sluggett, 2008]. Near the triple junctionamong the Explorer, Pacific, and North Americaplates, the Quaternary alkalic seamounts of the TuzoWilson volcanic field are attributed to a ??leakytransform?? in an oceanic setting [Allan et al., 1993].[7] Third, the BSM basalts may reflect one of theother mechanisms proposed to account forMiocene-Holocene intraplate volcanic centers thatoccur in a broad swath extending from southernBritish Columbia to Alaska. The east-west trend-ing Anahim volcanic belt (14.5 Ma?7.2 ka [Bevier,1989]), located immediately north of the GVB,may be related to a hotspot [Bevier, 1989; Char-land et al., 1995], an edge effect of the Juan deFigure 1. (a) Map of the Cascade arc and its tectonic setting. The extents of the Garibaldi volcanic belt andHigh Cascades segments of the arc are indicated with pink arrows. Volcanic and plutonic rocks are shown inyellow and orange shading, respectively. Black triangles denote composite volcanoes. (b) The study area isenclosed by a small bold rectangle and is enlarged. Igneous rock distributions are compiled from Lawrence etal. [1984], Monger [1989], DuBray et al. [2006], Green et al. [1988], Wheeler and McFeely [1991]. Oceanicplate configurations are from Braunmiller and Nabelek [2002], Audet et al. [2008], and Wilson [2002]. Col-ored lines on the oceanic plates are isochrons; accompanying numbers indicate the age of oceanic crust in Ma(from Wilson [2002]). Pseudofaults are shown as thin gray lines. Four heavy gray arrows on the Juan de Fucaand Explorer plates are convergence vectors (mm/yr) obtained from McCrory et al. [2004], Riddihough andHyndman [1991], and Braunmiller and Nabelek [2002] for a reference frame fixed relative to North America.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913128Fuca plate [Stacey, 1974], or ridge subduction[Farrar and Dixon, 1992]. Farther north, theextensive northern Cordilleran volcanic province(20 Ma?200 years B.P.) has been attributed tocrustal extension [Edwards and Russell, 2000].The aforementioned Wells Gray-Clearwater andChilcotin lavas may alternatively be a product ofback-arc extension [Bevier, 1983; Hickson, 1987].More recently, Thorkelson et al. [2011] proposed asingle model in which all of these volcanic provin-ces are related to upwelling of enriched mantlewithin and along the eroding margins of the1500 km long Northern Cordilleran slab window[Thorkelson and Taylor, 1989] that extends nearlyas far south as the BSM volcanic centers.[8] In this study, we investigate the roles of themantle and subducting slab in generating thealkalic compositions of the BSM basalts with highprecision whole-rock Sr-Nd-Pb-Hf isotope ratiosand trace element data. Radiogenic isotope signa-tures of primitive basalts are sensitive indicators ofmantle source heterogeneity [e.g., Hofmann, 2003]and the presence of components derived from sub-ducted oceanic crust and sediment [e.g., Kay et al.,1978]. Green and Sinha [2005] showed that theBSM alkalic basalts record less slab input thancalc-alkaline basalts of the southern GVB, butminimized the possible role of mantle heterogene-ity. However, recent improvements in the precisionof isotopic measurements have revealed mantleheterogeneities that were previously difficult to dis-cern [e.g., Abouchami et al., 2005]. Alkalic basaltsin British Columbia and the Cascade arc are typi-cally ascribed to mantle sources that are moreenriched in incompatible elements than the mantlewedge sampled by calc-alkaline basalts [e.g., Thor-kelson et al., 2011; Sluggett, 2008; Edwards andRussell, 2000; Leeman et al., 1990, 2005; Baconet al., 1997; Borg et al., 1997; Conrey et al.,1997; Schmidt et al., 2008; Jicha, et al. 2008].However, Reiners et al. [2000] proposed that bothbasalt types can be derived from a homogeneousmantle variably fluxed by slab-derived fluids.[9] We compare the BSM basalts to calc-alkalinebasalts from Mt. Baker, a stratovolcano located inthe ??cooler?? southern GVB and representative ofmore typical Cascade arc basalts, and to pub-lished data for other intraplate alkalic basaltsfrom British Columbia. Our new isotope and traceelement data show minimal subduction influenceon the source of the most primitive basalts atSalal Glacier and Bridge River. The BSM basaltsalso have a mantle source that is isotopically dis-tinct from, and more incompatible elementenriched, than the mantle underlying much of theCascade arc. These results have important impli-cations for the physical configuration of the sub-ducting slab and mantle flow patterns in northernCascadia.2. Geology of the Bridge River, SalalGlacier, and Mt. Meager VolcanicCenters[10] The Bridge River Cones, Salal Glacier, andMt. Meager are located 150 km north of Van-couver, British Columbia (Figure 1b). At the twonorthernmost centers (Salal Glacier and BridgeRiver), lavas are almost exclusively mafic. TheMt. Meager volcanic field includes basalt throughrhyolite but is dominated by intermediate compo-sitions, and Mt. Meager proper is a compositeandesitic stratovolcano [Ke, 1992].[11] The Salal Glacier volcanic field includes pil-low lavas, tuffs, and variably palagonitized andbrecciated flow remnants that survived continentalice sheet advances as high-altitude nunataks. Atlower altitudes, severe glacial erosion has revealedrhyolite and andesite dikes. Age dates for an alkalibasalt and overlying hawaiite are 0.97 and 0.59Ma (K-Ar), respectively [Lawrence, 1979].[12] Lavas at the Bridge River Cones are exclu-sively alkalic [Roddick and Souther, 1987]. Theterm ??cones?? is a misnomer because none of thedeposits is a true volcanic cone; rather, glacialerosion has produced cone-like forms. Columnarlavas of the Sham Hill plug and Tuber Hill expo-sure are dated at 1 Ma and 600 ka (K-Ar), respec-tively [Roddick and Souther, 1987].[13] At Mt. Meager, intermediate to silicic lavasspan the alkalic-subalkalic boundary [Stasiuk et al.,1994]. Mafic lavas are exclusively alkalic,however, and occur as four flow remnantscollectively known as the Mosaic Assemblage[Stasiuk and Russell, 1989; Stasiuk et al., 1994].Two of the basalts are dated at 90 and 140 ka (K-Ar) [Anderson, 1975; Woodsworth, 1977].Evidence for recent involvement of mafic magmain the form of mafic enclaves and banded pumicesis preserved in the 2360 years B.P. thatexplosively released 10 km3 of dacite [Clague etal., 1995; Michol et al., 2008]. Banded pumicesand mafic enclaves indicate that the intrusion of abasaltic magma may have triggered the eruption[Stasiuk et al., 1994].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131293. Major Element Compositions andPetrography[14] Samples analyzed for this study are from theBridge River, Salal Glacier, Mt. Meager, and Mt.Baker sample suites of Green and Sinha [2005]and from the Mt. Baker sample suite of E. K.Mullen and I. S. McCallum (Origin of basalts in ahot subduction setting: Petrologic and geochemi-cal insights from Mt. Baker, northern Cascade arc,submitted to Journal of Petrology, 2013, herein-after referred to as Mullen and McCallum, submit-ted manuscript, 2013). Major element data anddetailed petrographic descriptions for all samplesare provided in those references.[15] The BSM basalts are alkalic [Macdonald,1968] and nepheline normative with Na2O>K2O(Figure 2a). The basalts have distinctly lower SiO2(Figure 2) and Al2O3 than the calc-alkaline, hyper-sthene-normative Mt. Baker basalts.[16] Bridge River alkali olivine basalts and hawai-ites encompass the largest range of compositionaldiversity (Figure 2). Molar Mg/(Mg?Fe2?) valuesrange from 0.44 to 0.62; we consider two sampleswith Mg/(Mg?Fe2?)> 0.6 as primitive. Phenocrystand microphenocryst minerals are limited to olivine(1?2%) and rare plagioclase. Except for one sam-ple with a brown glass matrix, the basalts have hol-ocrystalline groundmasses containing olivine,plagioclase, titanaugite, magnetite, and ilmenite.The most primitive basalt (BRC10) contains biotiteand amphibole in the groundmass. Two of the moreevolved samples contain quartz xenocrysts andgranodiorite xenoliths (BRC03?4, BRC01?3).[17] Mt. Meager alkali basalts and hawaiites con-tain <1% microphenocrysts of olivine, clinopyr-oxene, and plagioclase. The groundmass containsglass and magnetite and, in the least primitivesample (MM01-1), biotite and amphibole. Mg/(Mg?Fe2?) values are 0.59?0.63.[18] Salal Glacier samples are the most primitiveamong the BSM basalts with Mg/(Mg?Fe2?)? 0.58to 0.66 and have the highest normative nephelinecontents. The most primitive samples are glassy andvesicular with phenocryst assemblages including<15% olivine, <1% plagioclase, <1% clinopyrox-ene, and rare orthopyroxene xenocrysts. Lessrimitive samples contain orthopyroxene phenocrystsand more abundant plagioclase, and olivine is eitherrimmed by clinopyroxene or absent.[19] At Mt. Baker, the most mafic lavas includemedium-K calc-alkaline basalt, high-Mg basalticandesite, and low-K olivine tholeiite, with Mg/(Mg? Fe2?)? 0.56?0.70. All samples contain oli-vine and plagioclase phenocrysts and some alsohave clinopyroxene phenocrysts (Mullen andMcCallum, submitted manuscript, 2013).4. Analytical Methods[20] Trace element abundances and Sr-Nd-Hf-Pbisotope ratios were measured on 19 BSM basalts,using splits of sample powders analyzed by Greenand Sinha [2005] for major and trace elements andSr isotope ratios. Larger symbols in Figure 2 des-ignate samples analyzed for the present study. HfFigure 2. Major element variation diagrams for the basaltsof Bridge River (red), Salal Glacier (orange), Mt. Meager(yellow), and Mt. Baker (lavender). (a) wt % Na2O?K2Oversus SiO2 with discriminant line of Macdonald [1968] andfields of Le Bas et al. [1986]. (b) Miyashiro diagram (FeO/MgO versus SiO2) with discriminant line of Miyashiro[1974]. Bridge River, Salal Glacier, and Mt. Meager data arefrom Green and Sinha [2005]. Mt. Baker data are fromMullen and McCallum (submitted manuscript, 2013) exceptLib21 from Green and Sinha [2005]. Large symbols (circlesand diamonds) indicate samples analyzed in this study fortrace elements and isotope ratios; diamonds are accompaniedby sample numbers and are the Suite 2 samples discussed inthe text. Small circles indicate samples of Green and Sinha[2005] not analyzed for the present study.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913130Figure 3. (a) 208Pb/204Pb versus 206Pb/204Pb, (b) 207Pb/204Pb versus 206Pb/204Pb. Insets show the same datafor the BSM and Mt. Baker basalts but on an expanded scale. Symbols for the BSM and Mt. Baker samplesgiven in legend; circles are used for primitive basalts. The more evolved basalts are subdivided into Suite 1(circles; isotopically similar to the primitive basalts) and Suite 2 (diamonds accompanied by sample numbers;isotopically distinct from the primitive basalts). 2 error bars (external reproducibilities) are smaller thansymbols in all plots. NHRL is the Northern Hemisphere Reference Line of Hart [1984]. Cascade arc basaltdata (pink crosses; only those with >8 wt % MgO are included) are from Conrey et al. [1997], Jicha et al.[2008], Bacon et al. [1994, 1997], Leeman et al. [1990, 2005], Baker et al. [1991],Magna et al. [2006], Groveet al. [2002], and Borg et al. [1997, 2000]. Northern Juan de Fuca MORB data (dark blue ?) are from Cou-sens et al. [1995]. Explorer MORB data (dark gray filled squares) are from B. Cousens (unpublished data2007). N. Gorda MORB (black ?) are from Allan et al. [1993]. Northern Cascadia sediment data (orangecircles) are from ODP sites 1027 and 888 [Carpentier et al., 2010, 2013]. Note that all isotope data are nor-malized to the same isotope standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913131isotope ratios were also measured on splits of pow-ders of three Mt. Baker basalts previously ana-lyzed for major and trace elements and Sr-Nd-Pbisotopes [Mullen and McCallum, submitted manu-script, 2013; Green and Sinha, 2005].[21] All chemical separations and mass spectro-metric analyses were carried out in Class 100 and10,000 clean laboratories, respectively, at the Pa-cific Centre for Isotopic and GeochemicalResearch at the University of British Columbia.Rock powders (100 mg) were digested in sub-boiled concentrated HF?HNO3 in 15 mL screw-top Savillex beakers on a hotplate for 48 h at130C. Samples were dried down on a hotplateand brought up in subboiled 6 N HCl and fluxedon a hotplate for at least 24 h. Sample aliquots of5?10% were diluted 5000X with an HNO3?HFsolution for analysis on a Thermo Finnigan Ele-ment2 HR-ICP-MS or an Agilent 7700 quadrupoleICP-MS. Sr isotope ratios were measured on aThermo Finnigan Triton TIMS and Pb, Nd, and Hfisotope ratios on a Nu Instruments MC-ICP-MS(Nu 021) following the procedures of Weis et al.[2006]. Pb, Sr, Hf, and Nd were separated fromsingle powder dissolutions by sequential ionexchange column chemistry as described in Weiset al. [2006, 2007]. All solutions were passedtwice through Pb exchange columns to ensure Pbpurification. Although thin sections of the ana-lyzed samples indicate little or no alteration in allsamples (minor iddingsite in olivine) and LOIvalues are low (<1%), even minimally alteredsamples can yield isotopic compositions that arenot representative of magmatic isotopic signatures,particularly in the case of Sr and Pb isotopes[Hanano et al., 2009; Nobre Silva et al., 2009].Therefore, we measured isotope ratios on bothunleached and leached powders of some samples.Leaching was conducted prior to powder dissolu-tion following the procedures of Nobre Silva et al.[2009, 2010]. Leached samples gave isotope ratioswithin analytical error of respective unleachedsamples for Sr and Nd (Figure S1).1 Hf isotoperatios are also within analytical error except forone sample (BRC10) that gave a higher value inthe leached sample. 207Pb/204Pb in leached sam-ples is systematically lower than in unleachedsamples while 208Pb/204Pb and 206Pb/204Pb arewithin error of unleached samples, with the excep-tion of one Mt. Baker sample (Lib21) (FigureS1).1 All isotope plots in the main text show dataobtained on leached samples except for cases inwhich only unleached samples were analyzed.Blank contributions to isotope ratios were negligi-ble with total procedural blanks of 50, 400, 90,and 15 pg for Pb, Sr, Nd, and Hf, respectively.5. Results5.1 Isotopes[22] Isotope ratios are reported in Table 1 and plot-ted in Figures 3?5. 87Sr/86Sr values measured in theBSM basalts are systematically lower than reportedby Green and Sinha [2005] for the same samplesand lie outside their reported uncertainties (Figure4a inset). For direct comparison among datasets, allliterature data are normalized to standard values of87Sr/86Sr? 0.710248 for SRM987 and 0.708028 forEimer and Amend; 143Nd/144Nd? 0.511973for Rennes, 0.511858 for La Jolla, 0.512633 forBCR-1, and 0.512130 for Ames [Weis et al., 2006,2007]; 176Hf/177Hf? 0.282160 for JMC 475[Vervoort and Blichert-Toft, 1999]; and208Pb/204Pb? 36.7219, 207Pb/204Pb? 15.4963,206Pb/204Pb? 16.9405 for SRM981 [Galer andAbouchami, 1998].[23] Primitive BSM basalts (Mg/Mg?Fe2?> 0.60)form an isotopic cluster (Figures 3 and 4) with anarrow range of 87Sr/86Sr? 0.70299?0.70314,ENd??7.1 to ?7.7, EHf??8.3 to ?10.0,208Pb/204Pb? 38.075?38.172, 207Pb/204Pb? 15.541?15.557, 206Pb/204Pb? 18.690?18.774. The primitiveBSM basalts overlap in 208Pb/204Pb and 206Pb/204Pbwith N. Juan de Fuca MORB [Cousens et al., 1995]and Explorer MORB (B. Cousens, unpublished data2007) (Figure 3a) but have slightly higher207Pb/204Pb and 87Sr/86Sr and lower ENd (Figures 3band 4a). Primitive BSM basalts plot near thedepleted end of the Sr-Nd-Pb isotopic arrays definedby other Cascade arc basalts (Figures 3 and 4).Along with Mt. Baker, primitive BSM basalts haveamong the highest ENd values reported for the Cas-cade arc. Mt. Baker basalts have slightly higher208Pb/204Pb and 206Pb/204Pb than primitive BSMsamples, but overlap in 207Pb/204Pb. Mt. Baker hassignificantly higher EHf (?11.1 to? 12.1) andslightly higher 87Sr/86Sr. In EHf-ENd isotopic space(Figure 4b), primitive BSM and Mt. Baker basaltsoverlap with only one outlier among data previouslypublished for the Cascade arc (Lassen Peak [Borg etal., 2002] and Mt. Adams [Jicha et al., 2008]). Mt.Adams EHf values cluster between primitive BSMbasalts and Mt. Baker. Together, the BSM, Mt.Baker, and Mt. Adams basalts define an EHf range1Additional supporting information may be found in the onlineversion of this article.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913132Table 1. Sr, Nd, Hf, and Pb Isotope Ratios87Sr/86Srb 143Nd/144NdcSample#Lat(N)Long(W)SiO2(wt%)MgNumbera Leached 2SEh Unleached 2SE Leached 2SE"NdLeachedf Unleached 2SE"NdUnleachedfBridge River ConesBRC01-3 50.93 123.45 50.51 56.5 0.703212 9BRC02 50.91 123.45 49.60 52.8BRC03-4 50.93 123.45 50.07 54.4 0.703219 6 0.512966 7 6.4BRC04 50.93 123.45 49.48 52.4 0.703184 9BRC05-1 50.92 123.45 46.66 48.3 0.703119 9BRC06 50.90 123.45 49.35 53.5 0.703175 8BRC07-2 50.92 123.45 47.24 47.7 0.703098 9 0.703124 7 0.513012 5 7.3 0.513012 6 7.3BRC09-3 50.92 123.41 48.02 59.6 0.702985 7 0.703012 9 0.513024 7 7.5BRC10 50.92 123.38 45.10 61.2 0.703052 7 0.703054 7 0.513031 7 7.7dupi 0.703055 7 0.703056 8Salal GlacierSG01-2 50.81 123.45 46.64 65.1 0.703140 7 0.513021 6 7.5SG01-3 50.81 123.45 46.62 64.5 0.703143 8 0.703149 9 0.513001 7 7.1SG10 50.78 123.39 46.04 65.7 0.703122 9dup 0.703117 8SG12 50.77 123.40 46.71 66.5 0.703065 7 0.703067 8 0.513013 6 7.3SG16 50.77 123.39 46.59 65.9 0.703101 10Mt. MeagerMM01-1 50.65 123.59 48.84 58.8 0.703764 8 0.703758 9 0.512930 5 5.7 0.512926 7 5.7dup 0.703762 6 0.703763 7 0.512936 5 5.8 0.512941 6 5.6MM02 50.69 123.57 48.64 63.0 0.703132MM04 50.69 123.57 48.94 60.8 0.703144 6 0.703146 0.513030 9 7.6MM08 50.55 123.53 49.68 60.6 0.703164Mount BakerLIB-21 48.67 121.74 51.04 63.5 0.703964 9 0.703970 7 0.512834 6 3.8 0.512834 6 3.802-MB-5 48.72 121.85 53.69 69.7 0.703109 7 0.513001 6 7.107-MB-112 48.66 121.70 52.56 56.6 0.703240 7 0.513034 7 7.702-MB-1 48.72 121.85 53.30 56.0 0.703513 8 0.512899 5.106-MB-82 48.72 121.85 50.57 65.1 0.703156 7 0.512986 6.807-MB-114 48.64 121.73 52.06 61.3 0.703213 7 0.513037 7.806-MB-97 48.78 121.88 54.45 49.0 0.703173 10 0.512993 6.9176Hf/177HfdSample # Leached 2SE "Hf Leachedg Unleached 2SE "Hf UnleachedgBridge River ConesBRC01?3 0.283067 8 10.4BRC02BRC03?4 0.283052 5 9.9BRC04 0.283026 7 9.0BRC05-1 0.283040 7 9.5BRC06 0.283018 6 8.7BRC07-2 0.283027 4 9.0 0.283030 5 9.1BRC09-3 0.283007 6 8.3 0.283030 5 9.1BRC10 0.283025 6 8.9 0.282985 4 7.5Salal GlacierSG01?2 0.283017 5 8.7SG01?3 0.283021 5 8.8 0.283025 24 8.9SG10 0.283012 9 8.5SG12SG16 0.283023 7 8.9Mt. MeagerMM01-1 0.283063 9 10.3 0.283064 6 10.3dup 0.283050 8 9.9 0.283056 7 10.3MM02 0.283050 6 9.8MM04 0.283056 5 10.1MM08 0.283022 5 8.9Mount BakerLIB-21 0.283084 5 11.0 0.283094 5 11.402-MB-5 0.283100 5 11.607-MB-112 0.283114 4 12.1MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913133that is similar to Explorer MORB, but with slightlylower ENd.[24] More evolved BSM basalts define twogroups: Suite 1 has isotopic ratios that overlapwith primitive basalts from the same volcanic cen-ter, whereas Suite 2 has substantially lower ENdand higher Sr and Pb isotope ratios than otherbasalts from their respective volcanic centers (Fig-ures 3 and 4a). Suite 2 includes three BSM basalts(BRC03?4, BRC01?3 at Bridge River; MM01-1at Mt. Meager), all of which have Pb isotope ratiosTable 1. (continued)208Pb/204Pbe 207Pb/204Pbe 206Pb/204PbeSample # Leached 2SE Unleached 2SE Leached 2SE Unleached 2SE Leached 2SE Unleached 2SEBridge River ConesBRC01-3 38.2441 22 15.5615 8 18.7878 8dup 38.2433 24 15.5604 9 18.7863 10BRC02 38.1901 21 15.5554 8 18.7481 10BRC03-4 38.2671 18 15.5584 7 18.8334 6dup 38.2674 18 15.5585 6 18.8337 8BRC04 38.1961 27 15.5622 9 18.7379 13BRC05-1 38.1274 17 15.5481 7 18.7591 8BRC06 38.1828 28 15.5530 12 18.7463 14BRC07-2 38.1215 19 38.1260 26 15.5450 7 15.5484 9 18.7626 8 18.7539 10BRC09-3 38.1245 18 38.1432 40 15.5462 6 15.5561 15 18.7432 8 18.7340 13BRC10 38.1496 22 38.1580 20 15.5566 10 15.5610 7 18.7738 10 18.7754 8dup 38.1601 36 15.5620 14 18.7750 17Salal GlacierSG01-2 38.1087 20 15.5419 8 18.7465 9SG01-3 38.1088 24 38.1250 31 15.5407 8 15.5508 7 18.7536 10 18.7536 7SG10 38.1285 24 15.5492 9 18.7363 12SG12 38.1721 22 38.1492 38 15.5521 7 15.5621 14 18.7739 9 18.6997 14SG16 38.1585 33 15.5527 12 18.7591 12Mt. MeagerMM01-1 38.2664 21 38.2734 19 15.5692 8 15.5731 7 18.8033 9 18.8090 7dup 38.2697 18 38.2772 19 15.5703 7 15.5738 7 18.8102 8 18.8115 7MM02 38.1005 35 15.5470 13 18.7034 15MM04 38.1046 14 38.1370 36 15.5470 4 15.5580 12 18.7082 6 18.7271 13MM08 38.0747 25 15.5550 9 18.6896 15Mount BakerLIB-21 38.4708 24 38.5285 20 15.5851 9 15.5931 7 18.9235 11 18.9726 802-MB-5 38.2495 22 15.5505 8 18.7975 907-MB-112 38.3077 21 15.5575 7 18.8385 802-MB-1 38.3598 15.5645 18.851506-MB-82 38.2661 15.5518 18.828607-MB-114 38.2753 15.5560 18.846606-MB-97 38.2597 15.5529 18.8356aCalculated as 100Mg/Mg?Fe2? (molar), using major element data from Green and Sinha [2005] and Mullen and McCallum (submittedmanuscript, 2013) and assuming Fe3?/Fe? 0.15.bReported Sr isotope ratios are corrected for mass fractionation using 86Sr/88Sr? 0.1194. Repeat analysis of the Sr SRM987 standard yielded amean (6 2) of 87Sr/86Sr? 0.7102486 2 (n? 7), identical to the accepted value [Weis et al., 2006].cReported Nd isotope ratios are corrected for mass fractionation using 146Nd/144Nd? 0.7219 and are normalized to 143Nd/144Nd? 0.511973 forthe Rennes reference material [Chauvel and Blichert-Toft, 2001] using the daily average method. The Rennes standard was analyzed every twosamples and over the course of analysis gave a mean (6 2) value of 143Nd/144Nd? 0.5119806 65 (n? 16). On a per session basis, reproducibil-ity was significantly better with a maximum daily 2 value of627 (53 ppm).dReported Hf isotope ratios are corrected for mass bias using 179Hf/177Hf? 0.7325 [Patchett and Tatsumoto, 1981] and normalized to176Hf/177Hf? 0.282160 for the ULB-JMC 475 reference material [Vervoort and Blichert-Toft, 1999] using the daily average of standard analyses.JMC 475 was analyzed every two samples and over the course of analysis gave a mean (6 2) of 176Hf/177Hf? 0.28217326 24 (86 ppm)(n? 24). On a per session basis, reproducibility was significantly better with daily 2 values ranging from 34 to 61 ppm.eReported Pb isotope ratios were corrected for mass bias by Tl doping [White et al., 2000] and are normalized to 208Pb/204Pb? 36.7219,207Pb/204Pb ?15.4963, 206Pb/204Pb? 16.9405 for the SRM981 standard [Galer and Abouchami, 1998] by sample-standard bracketing. Replicateanalysis of SRM981 over the course of analysis yielded in-run mean6 2 values of 208Pb/204Pb? 36.71986 91 (247 ppm),207Pb/204Pb? 15.49986 34 (220 ppm), and 206Pb/204Pb? 16.94346 29 (168 ppm) (n? 43). On a per session basis, reproducibility was signifi-cantly better with daily 2 values ranging from 74 to 186 ppm, 62 to 176 ppm, and 45 to 139 ppm for 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb,respectively.fENd calculated using a CHUR value of143Nd/144Nd? 0.512638 [Jacobsen and Wasserburg, 1980].gEHf calculated using CHUR value of176Hf/177Hf? 0.282772 [Blichert-Toft and Albarede, 1997].h2SE values (twice the standard errors) apply to the last decimal place(s) and are the internal absolute errors values for individual sample analyses.idup designates full procedural duplicates starting with a new sample powder aliquot; reproducibilities are similar to, or better than, the reprodu-cibilities determined through repeat standard analysis (values listed above).Data in italics are from Mullen and McCallum (submitted manuscript, 2013).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913134Figure 4. (a) ENd versus87Sr/86Sr; (b) EHf versus ENd. Symbols and data references as in Figure 3, plus Las-sen and Adams data in Figure 4b from Borg et al. [2002] and Jicha et al. [2008]. Note that all isotope data arenormalized to the same isotope standard values as described in main text. For the BSM and Mt. Baker data,2 error bars (external reproducibilities) are smaller than symbols. Inset in Figure 4a compares 87Sr/86Srmeasured in the present study (2 error bars? 20 ppm; smaller than symbol size) to 87Sr/86Sr measured onthe same samples by Green and Sinha [2005] (2 error bars? 100 ppm). The mantle array in EHf versus ENdspace is from Chauvel et al. [2008]. Orange and blue curves show the effect of adding 2% bulk sediment(blue curve with long dashes), 2% sediment fluid (blue curves with short dashes), 2% sediment melt (solidblue curves), 10% metabasalt fluid (orange dashed curve), and 2% metabasalt melt (orange solid curve) to themantle prior to 5% equilibrium partial melting of a primitive mantle composition [Sun and McDonough,1989]. Each curve has two tick marks indicating 1% and 2% addition, except for the metabasalt fluid curve(ticks at 5% and 10% addition). Slab fluid and melt compositions calculated using equilibrium melting/dehy-dration equations with FL?0.05. Trace element compositions for sediment and metabasalt are from Carpent-ier et al. [2013] (average of bulk ODP sites 888 and 1027) and Becker et al. [2000] (900C eclogite),respectively. Sediment isotope composition is the average of ODP sites 888 and 1027 from Carpentier et al.[2010]. Metabasalt Sr and Nd isotope ratios are from Staudigel et al. [1995] and the Hf isotope ratio is the av-erage of Explorer MORB shown here. Partition coefficients from Kessel et al. [2005] at 700C, 4 GPa (all flu-ids); 1000C, 4 GPa (metabasalt melt and sediment melt 1), and Hermann and Rubatto [2009] at 1050C, 4.5GPa (sediment melt 2).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913135similar to Mt. Baker basalts, plotting within theCascade arc array and closer to the field definedby subducting sediment (Figure 3). Suite 2 alsoincludes two Mt. Baker basalts that are isotopi-cally anomalous (in Sr and Nd) relative to moreprimitive Mt. Baker basalts ; Cathedral Crag(MB1) and one Sulphur Creek sample (Lib21) plotcloser to the field defined by subducting sediment(Figure 4a). As a group, Suite 2 has lower MgOand generally higher SiO2 than other basalts fromtheir respective volcanic centers (Figure 5).5.2 Trace Elements[25] Trace element abundances are reported inTable 2 and plotted in Figures 6?9. With theexception of Suite 2 (discussed later), the BSMbasalts have LILE and Pb abundances similar toMt. Baker basalts, but much higher HFSE (Figures6 and 8a). BSM basalts have substantially lowerZr/Nb and Ba/Nb than Mt. Baker basalts (Figure8b) and higher (La/Yb)N and (Dy/Yb)N (Figure 7).Among the BSM basalts, Salal Glacier has thehighest (La/Yb)N and lowest Yb and displays theleast variability among samples (Figure 7a).Bridge River and Mt. Meager have variable REEabundances, and the lowest (La/Yb)N occurs atMt. Meager (Figure 7b). However, Mt. Baker sam-ples extend to lower (La/Yb)N and higher Yb val-ues than the BSM basalts (Figure 7). Salal Glacierand Bridge River have no Nb anomalies whilesmall negative anomalies occur at Mt. Meager andprominent ones at Mt. Baker (Figure 9a). Ba/Lavalues are lowest at Salal Glacier and BridgeRiver, intermediate at Mt. Meager, and highest atMt. Baker (Figure 9b). Abundances of all traceelements in Bridge River and Salal Glacier primi-tive basalts are similar to samples from nonsub-duction settings, including Hawaiian postshieldalkalic basalts [Hanano et al., 2010], and overlapwith alkalic basalts from the Anahim volcanic belt[Charland et al., 1995], Cascade-Columbia tran-sect [Leeman et al., 2005; Jones, 2002], andDalles Lakes north of Mt. Rainier [Reiners et al.,2000] (Figures 8, 9a, and 9c).[26] The five basalts comprising Suite 2(BRC03?4, BRC01?3, MM01-1, MB1, andLib21) have trace element abundances that con-trast with other basalts at their respective vol-canic centers, including significantly higherLILE (La/Yb)N and Ba/Nb, and lower HFSE(Figures 6b?6e). These samples are excludedfrom the following discussion of mantle sourcecharacteristics but are revisited later in the contextof crustal assimilation.6. Discussion6.1. Mantle Source Characteristics6.1.1. Temperatures and Pressures[27] The BSM basalts segregated from their man-tle source at significantly higher pressures andtemperatures than the Mt. Baker basalts (Figure10). Liquidus pressures and temperatures (i.e.,mantle potential temperatures) were calculated forthe two most primitive basalts at each BSM vol-canic center (Table 3) using the olivine-liquid geo-thermometer and silica activity geobarometer ofPutirka [2008]. Whole-rock data [Green andSinha, 2005] were first adjusted into equilibriumwith Fo90 mantle by incremental olivine additionFigure 5. 208Pb/204Pb versus (a) wt % SiO2, and (b) wt % MgO. Note reversed scale for MgO.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913136Table 2. Trace Element AbundancesSample Number BRC01?3 BRC02 BRC03?4 BRC04 BRC05-1 BRC06 BRC07-2 BRC09-3 BRC10 SG01-1Methoda 1 1 2 1 1 1 2 1 1 1Concentrationb (ppm)Li 7.2 8.8 10 8 7.7 8.0 8.2 6.1 6.4 4.4Cs 0.17 0.22 0.46 0.17 0.14 0.18 0.13 0.07 0.09 0.19Rb 11 15 16 13 11 14 12 4.0 7.7 12Ba 413 310 450 303 269 288 286 195 176 269Th 2.0 1.6 2.2 1.5 1.5 1.5 1.8 0.94 0.87 1.7U 0.66 0.64 0.65 0.41 0.57 0.60 0.63 0.34 0.35 0.63Nb 14 27 16 25 26 26 28 17 15 29Ta 0.87 1.9 0.92 1.4 1.7 1.8 1.6 1.2 1.1 1.8La 24 20 24 18 20 17 22 14 12 20Ce 54 45 53 43 48 42 50 30 26 43Pb 4.2 2.8 3.9 3.0 2.2 2.6 2.3 1.7 1.3 2.1Pr 6.7 5.7 6.8 5.6 6.3 5.6 6.7 4.2 3.6 5.5Sr 1251 626 1213 577 587 650 628 399 388 545Nd 28 26 28 25 28 24 29 18 16 24Sm 5.2 6.0 5.5 5.9 6.7 5.9 6.8 4.7 4.3 5.5Zr 127 174 143 168 200 168 213 127 108 154Hf 3.4 4.1 3.3 4.1 4.6 3.9 4.8 3.2 2.9 3.7Eu 1.6 2.0 1.8 1.9 2.2 1.8 2.3 1.7 1.5 1.7Gd 4.2 6.2 4.8 6.0 6.5 5.8 6.6 5.1 4.7 5.2Tb 0.58 0.89 0.68 0.86 0.98 0.85 0.96 0.75 0.74 0.73Dy 3.4 4.7 3.9 4.9 5.6 4.9 5.7 4.3 4.0 4.0Y 19 25 21 27 31 26 30 25 23 22Ho 0.66 0.91 0.74 0.90 1.0 0.84 1.1 0.82 0.75 0.69Er 1.7 2.3 2.0 2.3 2.8 2.4 2.9 2.2 1.9 1.9Tm 0.24 0.32 0.31 0.39 0.31 0.31 0.28 0.27Yb 1.4 1.9 1.7 1.8 2.3 1.9 2.4 1.8 1.5 1.5Lu 0.20 0.27 0.23 0.26 0.33 0.24 0.34 0.26 0.22 0.22Sc 18 20 19 20 24 20 23 23 22 20Zn 85 101 98 111 122 106 126 98 107 100Cr 48 98 47 100 44 115 34 325 310 321Ni 44 50 41 51 36 56 32 134 228 281V 202 211 216 207 284 207 282 196 200 199Ga 23 24 24 27 24 21 22 21Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Concentration (ppm)Li 7.1 6.5 6.6 5.6 6.4 6.5 7.5 7.7 5.4 8.3Cs 0.20 0.17 0.15 0.14 0.18 0.10 0.08 0.13 0.08 0.44Rb 18 17 16 12 17 8.8 7.6 12 8.2 15Ba 339 330 330 247 264 676 249 265 154 659Th 2.2 2.1 1.9 1.7 1.7 1.7 1.1 1.2 0.65 2.4U 0.73 0.76 0.71 0.54 0.66 0.63 0.71 0.54 0.34 0.79Nb 31 31 32 26 26 12 17 19 9.1 6.3Ta 1.7 1.5 1.6 1.9 1.4 0.7 1.1 1.3 0.63 0.31La 23 24 22 18 18 27 13 15 8.4 24Ce 49 49 47 40 43 62 30 31 21 53Pb 2.2 2.1 2.2 2.3 2.0 5.2 1.9 2.0 1.4 3.8Pr 6.2 6.1 6.0 5.2 5.6 8.4 4.0 4.2 3.0 7.1Sr 658 589 585 508 624 1588 479 545 467 1455Nd 26 25 26 23 24 35 17 18 14 30Sm 5.5 5.4 5.7 5.1 5.3 6.7 4.2 4.4 4.0 5.8Zr 171 161 164 141 150 147 112 122 92 93Hf 3.6 3.8 3.7 3.3 3.5 3.4 2.9 3.0 2.7 2.4Eu 1.8 1.9 1.9 1.6 1.8 2.1 1.5 1.6 1.4 1.8Gd 5.1 5.3 5.1 5.0 5.2 5.5 4.2 4.8 4.3 4.7Tb 0.72 0.76 0.78 0.68 0.76 0.71 0.65 0.68 0.64 0.61Dy 4.1 4.0 4.4 3.7 4.3 4.0 3.8 3.8 3.9 3.4Y 22 23 25 21 21 21 20 22 21 18Ho 0.77 0.77 0.81 0.69 0.73 0.77 0.69 0.74 0.69 0.65Er 2.0 2.0 2.2 1.7 2.0 2.0 1.8 1.9 1.9 1.8Tm 0.29 0.30 0.24 0.26 0.24 0.27 0.25MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913137assuming Fe3?/PFe? 0.15. Amounts of olivineadded range from 11.5% (SG16) to 17.7%(BRC09). Primary biotite and amphibole in thegroundmass of some primitive BSM samples attestto the presence of water, so pressures and tempera-tures were calculated for several possible H2Ocontents (listed in Table 3). The plagioclase-liquidhygrometer of Lange et al. [2009] applied to themost primitive Salal Glacier basalt gives 1 wt %H2O at the liquidus assuming plagioclase satura-tion at P? 100 MPa and maximum An60 in plagio-clase cores [Lawrence, 1979]. At this watercontent, pressures calculated for primitive BSMbasalts correspond to depths ranging from 70 km(Mt. Meager) to 105 km (Bridge River) (Figure10). Decreasing melt SiO2 with increasing pres-sure [Longhi, 2002] is consistent with the P-Tdata. Calculated mantle potential temperatures are100?200C higher than predicted for the Cas-cade subarc mantle wedge [Syracuse et al., 2010],and similar to those of average MORB (1454C)[Putirka, 2008]. Intraplate basalts of the westernBasin and Range province give a broadly similarP-T range (60?90 km, 1350C1450C) as theBSM basalts [Lee et al., 2009]. Intraplate basaltsin the Cascade-Columbia transect have lower max-imum segregation depths (75?80 km) but similarmaximum temperatures (1460C), although an-hydrous conditions were assumed [Leeman et al.,2005]. Simcoe intraplate basalts record P-T condi-tions similar to the BSM basalts (max 100 km,1500C) [Leeman et al., 2005].[28] For Mt. Baker basalts, liquidus water contentsare 1.5 to 3.7 wt % (Mullen and McCallum, sub-mitted manuscript, 2013) and mantle potentialtemperatures are 1273C to 134C (Figure 10,Table 3), within the range for the subarc mantlewedge [Syracuse et al., 2010]. Mantle segregationdepths are 35 to 52 km, i.e., ranging from theMoho to just above the hot core of the mantlewedge. The shallower depths recorded by the Mt.Baker basalts are consistent with trace elementmodeling (below) that indicates residual garnet forthe BSM basalts but not Mt. Baker.6.1.2. Mantle Isotopic Characteristics[29] Primitive BSM basalts (Mg/[Mg?Fe2?]> 0.6)have isotope ratios that define a narrow range, con-sistent with a common mantle source and differen-tiation dominated by fractional crystallization. Pbisotope ratios overlap with Explorer and northernJuan de Fuca MORB [Cousens et al., 1995; B.Cousens, unpublished data 2007], Chilcotin plateaubasalts [Bevier, 1983], and the least radiogenic sam-ples from the Anahim volcanic belt [Bevier, 1989](Figure 11). The isotopic similarity among thesevolcanic provinces confirms that the northwesternmargin of North America is underlain by uppermantle that is relatively depleted and generally sim-ilar to northeastern Pacific mantle [Cousens andBevier, 1995; Bevier, 1989].[30] Although the BSM basalts have 208Pb/204Pbsimilar to local MORBs at a given 206Pb/204Pb,207Pb/204Pb is slightly higher (Figure 3).Relatively high 207Pb/204Pb could be interpreted asreflecting subducting sediment input, but thisshould increase 208Pb/204Pb along with207Pb/204Pb, and the BSM basalts overlap withMORB in 208Pb/204Pb.[31] High 207Pb/204Pb relative to 208Pb/204Pb mayinstead indicate a higher time-integrated U/Th inthe BSM source than in the MORB sources.Table 2. (continued)Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Yb 1.7 1.7 1.7 1.4 1.5 1.7 1.5 1.5 1.6 1.5Lu 0.24 0.23 0.26 0.21 0.22 0.25 0.21 0.22 0.22 0.22Sc 20 20 22 19 20 23 20 22 21 25Zn 102 93 103 95 106 100 106 102 106 91Cr 248 255 239 332 330 112 274 271 255 55Ni 227 216 130 301 285 49 188 146 156 24V 198 188 227 157 212 208 176 185 164 199Ga 20 22 17 21 20 21 22aMethod 1: Thermo Finnigan Element2 HR-ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS.bData were corrected for blank contributions and by sample-standard bracketing to published values for the USGS BCR2 reference material[Raczek et al., 2001] analyzed every eight samples (method 1), or the USGS AGV1 reference material [Chauvel et al., 2011] analyzed every sixsamples (method 2). Repeat analysis of the USGS BHVO2 standard gave RSD values of <5% and concentrations within 5% (relative) of pub-lished values (as compiled by Carpentier et al. [2013] from GeoRem) for most elements. The average BHVO2 values obtained during analyticalsessions are reported in Table S1. Duplicate analyses gave reproducibilities better than 5% (Table S1). Total procedural blanks (Table S1) werenegligible relative to analyzed sample concentrations.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913138Enrichment of the BSM mantle source in U relativeto Th at some time in the past could be accom-plished through addition of fluid or melt compo-nents derived from subducting sediment and/oroceanic crust, since U is slightly more incompatiblethan Th during dehydration and melting [Brenan etal., 1995; Kessel et al., 2005; Hermann andRubatto, 2009]. However, this situation wouldresult in the presence of a subduction signature inthe BSM mantle source, which is not observed.[32] A more plausible explanation may be meltingof the BSM mantle source in the presence of resid-ual garnet at some time in the past. Unlike othertypical mantle minerals, which do not fractionateU and Th appreciably, U is more compatible ingarnet than Th [Beattie, 1993; LaTourrette et al.,1993]. The BSM basalts also plot at the lower mar-gin of the Hf-Nd mantle array (Figure 4b), consist-ent with the isotopic evolution of mantle thatproduced melts within the garnet stability field[Carlson and Nowell, 2001].[33] Because BSM basalts have lower EHf (3 ep-silon units) than Mt. Baker but similar ENd (Figure4b), two distinct mantle sources are required. Par-titioning experiments show that Hf is preferen-tially retained in the subducting slab relative toNd, most effectively during slab dehydration butalso during slab melting [e.g., Kessel et al., 2005;Hermann and Rubatto, 2009]. High Nd/Hf in thesubduction component is further enhanced by theFigure 6. (a?d) Extended N-MORB normalized [Sun and McDonough, 1989] trace element diagrams, sub-divided by volcanic center. (d) Light gray field in each panel encompasses the range defined by Mt. Bakerbasalts. The darkest colors (with sample numbers) signify Suite 2 samples discussed in the text. (e) All of theSuite 2 samples are plotted together for comparison to Mt. Baker basalts.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913139preexisting negative HFSE anomalies that charac-terize Cascadia sediment [Carpentier et al., 2013;Prytulak et al., 2006]. As a consequence, additionof a sediment component to the BSM mantlesource generates mixing curves that extendtowards lower ENd values but with smaller changesin EHf. Most importantly, mixing trajectoriesextend away from the Mt. Baker data. Thus sub-duction input cannot account for the Hf isotopicdistinction between the BSM and Mt. Bakerbasalts; the difference is instead a primary featureof their respective mantle sources. Since mantleHf isotope ratios can be affected by both fluidsand melts derived from the slab (Figure 4b), Hfisotopes do not always directly record the isotopiccomposition of the mantle as is commonlyassumed.6.1.3. Mantle Source Fertility[34] Zr/Nb in basalts provides a useful indicator ofmantle source fertility because this ratio is mini-mally affected by subduction input or fractionalcrystallization (Figure 8b). Although Zr/Nb is con-trolled to some extent by melt fraction, the Zr/Nbrange defined by melts of average depleted mantledoes not overlap the range for melts of moreenriched mantle tapped by ocean island basalts.Mt. Baker basalts have Zr/Nb consistent with10% partial melting of average depleted mantlewith an additional slab component (Figure 8b). Zr/Nb is too low in the primitive BSM basalts to beproduced from the same mantle source as Mt.Baker, requiring a more incompatible elementenriched mantle source. The relatively high Nbcontents of the BSM basalts also indicate a sourcerelatively enriched in incompatible elements (Fig-ure 8a), as do high Na2O and TiO2 [Prytulak andElliott, 2007].6.1.4. Assessment of Subduction Input[35] For primitive samples at Salal Glacier andBridge River, Ba/Nb values lie within the range ofHawaiian basalts and coincide with melting curvesfor enriched mantle (at 2?5% partial melt) (Fig-ure 8), pointing to the likelihood that a slab-derived component was not present in the mantlesource. The absence of slab input is supported bythe absence of negative Ta-Nb anomalies (FigureFigure 7. (a?d) Chondrite-normalized [McDonough and Sun, 1995] rare-earth element diagrams subdividedby volcanic center.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913140Figure 8. (a) Ba (ppm) versus Nb (ppm); (b) Zr/Nb versus Ba/Nb. Except for Suite 2 samples (diamonds),plus MM08 from Mt. Meager, the BSM basalts are similar to basalts from MORB and OIB settings, i.e.,essentially no subduction component. MORB data (gray filled circles) and Hawaiian basalt data (shield andpostshield lavas shown as filled green and pink circles, respectively) were obtained from the PETDB (http://www.earthchem.org/petdb) and GEOROC databases (georoc.mpchmainz.gwdg.de/georoc), respectively,accessed in May 2012. Average OIB composition [Sun and McDonough, 1989] is shown as a black asterisk inFigure 8b. Black arrows in lower right corner of Figure 8a and upper right corner of Figure 8b show the effectof 15% fractionation of each mineral indicated, calculated using the Rayleigh equation, the starting composi-tion of BRC09-3, and partition coefficients listed in Table 4 plus ilmenite from McCallum and Charette[1978]. Only ilmenite and magnetite vectors are shown in Figure 8b because the other phases shown in Figure8a have a negligible effect. Orange and blue curves are for addition of subduction components to a depletedmantle source (average depleted MORB mantle of Salters and Stracke, 2004) prior to 10% partial melting,calculated as described in Fig. 4 caption. Most subduction components increase Ba at a given Nb, and Ba/Nbat a given Zr/Nb. The heavy black curves are the compositions of DM melts at 1 GPa and PM melts at 3 GPa,respectively (tick marks indicate % partial melt). Melt compositions were calculated using the equilibriummelting equation with mineral/melt partition coefficients from Table 4 and residual mantle mineral assemb-lages determined by BATCH modeling [Longhi, 2002] of starting compositions of Wasylenki et al. [2003]and Kinzler [1997]. Filled black squares with white (cross) and (plus) symbol are for DM (depleted MORBmantle) of Salters and Stracke [2004] and PM (primitive mantle) of Sun and McDonough [1989], respec-tively. Inset diagrams include data for alkalic basalts (molar Mg/(Mg? Fe2?)> 0.60, assuming Fe3?/PFe? 0.15) from the Anahim volcanic belt (dark blue squares) [Charland et al., 1995], Cascade-Columbiatransect (light blue squares) [Leeman et al., 2005; Jones, 2002], and Dalles Lakes north of Mt. Rainier (purplesquares) [Reiners et al., 2000]. Abbreviations: ol (olivine), opx (orthopyroxene), cpx (clinopyroxene), plag(plagioclase), ilm (ilmenite), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131419a) that are observed at Mt. Baker and in othercalc-alkaline Cascade arc basalts [e.g., Schmidt etal., 2008]. The absence of slab input is further sup-ported by the similarity of Salal Glacier andBridge River primitive basalts to Mauna Kea post-shield alkalic basalts [Hanano et al., 2010], whichsample the same mantle source as shield lavas,that is, a composition comparable to the PREMA,or C, mantle component [Nobre Silva et al., 2013](Figure 9c). The only major difference is in Pb,which is deficient in Hawaii (a ubiquitous featureof oceanic basalts [Hofmann, 1997]) but showssmall positive spikes at Bridge River and SalalGlacier. The Pb spikes are successfully modeledwithout subduction input (see later). However, wecannot definitively rule out the presence of a verysmall subduction component in the mantle source.An ancient subduction component may have beenadded to the source in the past, or the primaryalkalic magmas may have acquired a small sub-duction component during migration through themantle.Figure 9. (a) EHf versus Nb/Nb (niobium anomaly) for BSM and Mt. Baker basalts, compared to the rangesdefined by Hawaiian shield and postshield basalts (green and pink lines, respectively) with >8 wt % MgO.Nb/Nb calculated as 2(Nbsample/NbPM)/(Basample/BaPM?Lasample/LaPM) [Verma, 2009] where PM refers toPrimitive Mantle. Hawaii data were obtained from the GEOROC database accessed in May 2012(georoc.mpchmainz.gwdg.de/georoc). Also shown are values for average depleted mantle (DM, black square)[Salters and Stracke, 2004] and average N-MORB (gray square) and Primitive Mantle (PM, black square withwhite ?) of Hofmann et al. [1988]. (b) Ba/La versus 208Pb/204Pb for BSM and Mt. Baker basalts. (c) N-MORB normalized [Sun and McDonough, 1989] extended trace element diagram comparing Salal Glacier(orange) and Bridge River (red) primitive basalts (molar Mg/[Mg?Fe2?]> 0.60) to alkalic postshield basaltsfrom Mauna Kea (light blue) [Hanano et al., 2010], Simcoe volcanic field (dark blue) [Battleground Lakesample of Jones, 2000], and Anahim volcanic belt (green) [sample 2278 of Charland et al., 1995].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913142[36] In contrast to Salal Glacier and Bridge River,even the most primitive Mt. Meager basalts haveslightly elevated Ba and Ba/Nb relative to mantlemelting curves and Hawaiian basalts (Figure 8), aswell as small negative Nb anomalies (Figure 9a),all of which point to subduction input (althoughsignificantly less than at Mt. Baker). Zr/Nb in thetwo of the three primitive Mt. Meager basalts isthe same as at Salal Glacier and Bridge River,indicating similar mantle sources. In the other Mt.Meager sample (MM08), higher Zr/Nb and lowerNb is consistent with a mantle source that is transi-tional between the Mt. Baker mantle source andthat of the other BSM basalts. An intermediate Hfisotopic composition for MM08 (Figure 4b) sup-ports this conclusion.[37] Mt. Meager basalts have the lowest206Pb/204Pb and 208Pb/204Pb of all the BSM vol-canic centers; sample MM08 has the lowest208Pb/204Pb, 206Pb/204Pb, and (La/Yb)N, coupledwith the highest Ba/La (Figure 9b). These charac-teristics are not consistent with addition of a sub-ducting sediment component to the mantle sourceand may instead reflect the influence of a fluidderived from altered oceanic crust (AOC). AOCfluid input can increase LILE in the mantle sourcewithout affecting LREE [Kessel et al., 2005], andsince recent AOC has MORB-like Pb isotoperatios, it is capable of ??pulling?? Pb isotope ratiosof the mantle source to lower values. Mt. Meageralso has similar ENd to Bridge River and Salal Gla-cier but slightly higher 87Sr/86Sr (Figure 4a), con-sistent with the involvement of AOC that acquireshigh 87Sr/86Sr with minimal change in ENd duringprogressive seafloor alteration [Staudigel et al.,1995].6.1.5. Trace Element Modeling[38] Mantle melt fractions and residual mantle min-eral assemblages were determined by modeling theabundances of 28 trace elements in the two mostprimitive basalts at each BSM center (BRC09 andBRC10 at Bridge River, MM04 and MM02 at Mt.Meager, SG10 and SG16 at Salal Glacier). We alsomodeled MM08 at Mt. Meager because it may havea slightly more depleted mantle source.[39] The model is based upon the mass balanceequation for equilibrium melting, CLi = C0i ?Figure 10. Pressure versus temperature plot illustrating theconditions at which the BSM and Mt. Baker magmas segre-gated from the mantle. P and T (from Table 3) were calcu-lated using the silica activity geobarometer and olivine-liquidgeothermometer calibrations of Putirka [2008] for 1% and2% dissolved water (BSM basalts) or for the specific H2Ocontent given (Mt. Baker basalts). Standard estimates of errorare 43C and 0.29 GPa [Putirka, 2008].Table 3. Liquidus pressures and temperaturesH2O (wt.%)0.0 1.0 2.0P(GPa) T(C) P(GPa) T(C) P(GPa) T(C)Bridge RiverBRC10 3.49 1576 3.19 1535 2.89 1496BRC09 2.68 1505 2.50 1470 2.31 1437Salal GlacierSG10 2.82 1514 2.61 1478 2.41 1443SG16 2.67 1502 2.48 1467 2.29 1433Mt. MeagerMM02 2.04 1441 1.91 1409 1.79 1379MM04 2.15 1460 2.02 1427 1.86 1395a Mt. Baker P (GPa) T (C) H2O (wt.%)MB5 0.90 1273 2.7MB82 1.4 1326 2.1MB1 1.1 1274 3.7MB97 1.2 1309 2.1MB114 1.4 1326 15MB112 1.5 1350 1.5aP-T data for Mt. Baker (from Mullen and McCallum, submitted manuscript, 2013) are calculated only at the specific water content listed foreach sample.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131431= FL 1 Di ? Di [Shaw, 1970], where CLi isthe concentration of trace element (i) in the liquid(L), C0i is the initial concentration of trace element(i), FL is melt fraction, and Di is the bulk partitioncoefficient, defined as crystalline assemblage/melt.The model does not require a priori knowledge ofinitial mantle mineral assemblages but does requireinitial trace element abundances. The primitivemantle composition of Sun and McDonough [1989]was used as the source for all BSM samples, and amixture of 50% primitive mantle and 50% depletedmantle [Salters and Stracke, 2004] was also testedfor MM08. Distribution coefficients used in the cal-culations are listed in Table 4. Least-squares mini-mization was used to generate best fit models forthe basalts by varying the mantle mineral modesand melt fractions (see the caption of Figure 12).Note that the substitution of fractional melting inour model results in negligible change to modeloutcomes. Melt fractions are within 0.5% and resid-ual mantle modal abundances change by less than afew percent, with overall residual mineral assemb-lages remaining identical.6.1.5.1. Modeling Results[40] Representative best fit trace element solutionsare shown in Figure 12. Melt fractions are 2?4%for Salal Glacier, 4?5% for Mt. Meager, and 7?8%for Bridge River, all with residual garnet lherzo-lite. Lower melt fractions for Salal Glacier basaltsare consistent with their higher alkali elementabundances. For Mt. Meager sample MM08, aprimitive mantle source indicates 8% partial meltand the mixed PM-DM source gives 4%. The latterresult is preferred because it is more consistentwith the results for other Mt. Meager samples. Re-sidual garnet in all samples is consistent with thepressures of melting (2?3 GPa) calculated for theBSM basalts, as garnet is stable at the solidus ofhydrous mantle at pressures above 1.6 GPa [Gae-tani and Grove, 1998].[41] Results of similar modeling for Mt. Bakerbasalts, using a depleted mantle source, indicate5?12% partial melting of depleted lherzolite orharzburgite. Best fit solutions require overprintingby a subduction component consisting of AOCfluid, AOC melt, and sediment melt (Mullen andMcCallum, submitted manuscript, 2013). No re-sidual garnet is present in the Mt. Baker source,consistent with calculated melt segregation pres-sures (1?1.5 GPa) and with the lower (Dy/Yb)N,and higher Yb and Sc contents of the Mt. Bakersamples (24?33 ppm Sc) (Mullen and McCallum,Figure 11. Plot of 208Pb/204Pb versus 206Pb/204Pb comparing Pb isotope ratios for the BSM and Mt. Bakerbasalts (symbols as in Figure 9) to other basalts from the northeastern Pacific and southwestern British Colum-bia: Anahim volcanic belt (purple diamonds; Bevier [1989]); Chilcotin Plateau (black diamonds; Bevier[1983]); Wells Gray-Clearwater volcanic field (blue diamonds; Hickson [1987]); and Tuzo Wilson volcanicfield [Allan et al., 1993]. Other data and references as in Figure 3. Note that all isotope data are normalized tothe same standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913144submitted manuscript, 2013) as compared with theBSM basalts (18?24 ppm Sc; Table 2).6.4. Nonprimitive BSM Basalts: CrustalContamination or Subduction Input?[42] Although we make the case above that theprimitive Salal Glacier and Bridge River basaltsessentially lack a subduction component, some ofthe nonprimitive BSM basalts have geochemicalcharacteristics that could be interpreted as an ??arcsignature.?? Do these reflect subduction input thatis not displayed by more primitive samples?[43] BSM samples with Mg/(Mg? Fe2?)< 0.60are subdivided into two suites based upon isotopicand trace element compositions: Suite 1 has iso-tope and trace element ratios similar to primitiveBSM basalts, indicating minimal crustal contami-nation and differentiation processes dominated byfractional crystallization. Relative to the mostprimitive basalts, Suite 2 has high Sr-Pb isotoperatios, (La/Yb)N, and incompatible element abun-dances, coupled with low Nd-Hf isotope ratios andHFSE abundances.[44] Pearce element ratio diagrams [Russell andNicholls, 1988] show that the Suite 1 basalts areconsistent with fractionation of olivine? plagio-clase (6 minor clinopyroxene) from parental mag-mas that were similar to the most primitive basaltsat each volcanic center. The crystallizing assemb-lages are consistent with the presence of the sameminerals as phenocryst phases.[45] For Suite 2 samples, trace element abundan-ces and Pb isotope ratios are nearly indistinguish-able from the most primitive Mt. Baker basalts(Figures 3, 6e, and 8), pointing to the possibilitythat they may record input from the subductingslab as does Mt. Baker. However, Suite 2 has sig-nificantly higher 87Sr/86Sr and lower ENd than themost primitive Mt. Baker and BSM basalts (Figure4a). The lower MgO contents of Suite 2 lavas rela-tive to the most primitive lavas are consistent withTable 4. Partition Coefficients (Mineral/Melt)acpx opx oliv sp gar amph phlog mt plagCs 0.000201 0.00091 0.00004525 0.000625 0.00011 0.02325 2.261 0.00121 0.00623Rb 0.000603 0.00381 0.0000453 0.000625 0.00022 0.0232 1.702 0.00121 0.01823Ba 0.000683 0.00361 0.0000433 0.00067 0.000072 0.012 1.502 0.00121 0.3223Th 0.0124 0.00051 0.000053 0.0107 0.00212 0.00102 0.000201 0.002420 0.1923U 0.0134 0.00071 0.000053 0.0147 0.00947 0.00122 0.000201 0.01220 0.3423Nb 0.0051 0.00071 0.000413 0.0217 0.00315 0.082 0.0551 0.8620 0.00823Ta 0.0211 0.00081 0.00021 0.0217 0.0199 0.0831 0.0621 0.9520 0.02723K 0.00725 0.00012 0.000022 0.00125 0.0132 0.222 1.502 0.00121 0.09721La 0.0546 0.00061 0.000053 0.0119 0.001615 0.05524 0.0002525 0.001220 0.1123Ce 0.08625 0.00171 0.000063 0.0119 0.00515 0.09625 0.000301 0.001920 0.08525Pb 0.0104 0.00011 0.0000712 0.00057 0.000315 0.0424 0.091 0.02221 0.108523Pr 0.1425 0.002625 0.0001325 0.0119 0.02925 0.1316 0.000425 0.002320 0.06525Sr 0.04811 0.0093 0.0002512 0.00477 0.002515 0.3015 0.161 0.003020 1.9423Nd 0.1925 0.0041 0.000203 0.0119 0.05215 0.18716 0.000551 0.00425 0.05223Sm 0.276 0.0111 0.000603 0.0119 0.2515 0.3216 0.000701 0.007020 0.04123Zr 0.061 0.0133 0.0006810 0.00817 0.6614 0.1816 0.0111 0.5620 0.003923Hf 0.121 0.0133 0.001110 0.00307 0.6814 0.6316 0.0161 0.6520 0.001523Eu 0.4525 0.01625 0.0008025 0.0119 0.4015 0.4316 0.000725 0.01025 1.4223Ti 0.308 0.06110 0.00221 0.04819 0.2915 0.952 0.791 2021 0.04723Gd 0.5025 0.02225 0.000993 0.0119 0.9025 0.5416 0.000725 0.01620 0.03521Tb 0.5625 0.0301 0.0023 0.0119 1.415 0.6025 0.00071 0.02325 0.03121Dy 0.6125 0.03825 0.0043 0.0119 2.215 0.6325 0.000825 0.03325 0.02621Y 0.6525 0.0461 0.0073 0.00207 3.115 0.5215 0.0031 0.0520 0.02621Ho 0.656 0.0481 0.0063 0.0119 2.815 0.6224 0.00091 0.0525 0.01821Er 0.6925 0.05825 0.00873 0.0119 3.6 15 0.5724 0.001025 0.0725 0.014521Tm 0.7225 0.0711 0.01325 0.0119 3.725 0.5325 0.00141 0.01125 0.01221Yb 0.7425 0.0771 0.0173 0.0119 3.94 0.4825 0.001625 0.1725 0.009721Lu 0.756 0.0901 0.0203 0.0119 3.84 0.4324 0.00171 0.2820 0.00821aAbbreviations: cpx (clinopyroxene), opx (orthopyroxene), oliv (olivine), sp (spinel), gar (garnet), amph (amphibole), phl (phlogopite), mt(magnetite), plag (plagioclase).Data sources: 1Adam and Green [2006]; 2Halliday et al. [1995] compilation; 3Donnelly et al. [2004] compilation; 4Hauri et al. [1994]; 5Hartand Dunn [1993]; 6Gaetani [2004]; 7Elkins et al. [2008]; 8McDade et al. [2003]; 9Green et al. [2000]; 10Kennedy et al. [1993]; 11Beattie[1993];12Beattie [1994]; 13Canil and Fedortchouk [2001]; 14Salters and Longhi [1999]; 15Abraham et al. [2005] compilation; 16Chazot et al.[1996]; 17Horn et al. [1994]; 18Nagasawa et al. [1980]; 19McKenzie and O?Nions [1991]; 20Klemme et al. [2006]; 21Claeson and Meurer [2004]compilation; 22Dunn and Sen [1994]; 23Tepley et al. [2010]; 24LaTourrette et al. [1995]; 25Interpolated from neighboring elements.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913145crustal contamination (Figure 5b). Furthermore,Suite 2 contains the only two BSM basalts withxenocrysts (BRC01?3, BRC03?4).[46] The GVB crustal basement is a collage of Pa-leozoic and Mesozoic accreted terranes [Mongeret al., 1982]. At depths greater than 10 km, theGVB is underlain by the composite Wrangellia?Harrison terranes [Mullen, 2011; Miller et al.,2009; Monger and Price, 2000]. With the excep-tion of the Mt. Baker region, the terranes areintruded extensively by Jurassic to Cretaceousgranitoids of the Coast Plutonic Complex, the larg-est composite batholith in North America [Barkerand Arth, 1984; Friedman et al., 1995; Cui andRussell, 1995a, 1995b]. Because the crust is rela-tively young and Cascadia subducting sediment ismainly terrigenous [Carpentier et al., 2013; Pry-tulak et al., 2006], the isotopic effects of crustalassimilation are similar in many respects to theeffects of subducting sediment input. However,sediment input cannot account for the 87Sr/86Srversus Sr systematics of the Suite 2 lavas (Figure13a). Assimilation-fractional crystallization (AFC)modeling [DePaolo, 1981] using a granodioriticassimilant from the Coast Plutonic Complex canreproduce the Suite 2 trace element and isotopicdata, but the volume of assimilant required(>20%) would increase the SiO2 content beyondthe range of the Suite 2 samples (Figure 5a).Assimilation that takes place in the deep crust,Figure 12. Best fit trace element solutions for four of the most primitive BSM basalts, shown on N-MORBnormalized extended element diagrams and accompanying inset chondrite-normalized REE diagrams. Actualdata are shown with colored lines and symbols; modeling solutions shown as heavy black lines with blacksquares. Each BSM sample has been adjusted into equilibrium with Fo90 mantle using olivine/melt partitioncoefficients from Table 4. The mantle source composition used in the model for all BSM basalts (PM of Sunand McDonough [1989]) is shown as a thin black line in each panel; DM (source used for Mt. Baker) is alsoshown for reference [Salters and Stracke, 2004]. The Generalized Reduced Gradient (GRG2) nonlinear opti-mization code in Microsoft Excel Solver was used to obtain the best fit for each basalt by minimizing the sumof squares of residuals for 28 trace elements, i.e.,Pi Cliqi calc? ?  Cliqi obs? ?h i=Cliqi ?obs?n o2. The denomina-tor in the equation normalizes the concentrations of the elements so that each trace element has an equivalentimpact on the solution regardless of its absolute concentration. Best fit melt fractions (FL) and residual mantlemineral modes are given in lower right corner of each N-MORB-normalized panel. Abbreviations: ol (oli-vine), opx (orthopyroxene), cpx (clinopyroxene), gar (garnet).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913146where the country rock is mafic, can minimizechanges to the major element abundances of theoriginal basaltic magma [Reiners et al., 1995,1996]. AFC models in which the assimilant is agabbro from the lower crustal section of the Bo-nanza arc (Westcoast crystalline complex) ofWrangellia [DeBari et al., 1999] provide good fitsto Suite 2 trace element and isotope data with15% gabbro assimilated at Bridge River and21% at Mt. Meager (Figure 13). Modeling pa-rameters and results are provided in Table 5. Frac-tionating mineral phases (olivine, clinopyroxene,orthopyroxene, and minor magnetite) are consist-ent with experimental results for partial melting ofmafic compositions under lower crustal conditions[Rapp, 1995; Rapp and Watson, 1995]. Becausethe assimilant has a low SiO2 content (45 wt %)and the fractionating mineral assemblages havebulk SiO2 contents similar to the basalts, the finalmagmas maintain an overall basaltic compositionin the magmas.6.5. Relationship Between Tectonics andVolcanism[47] An incompatible element-enriched, garnet-bearing mantle source essentially free of subduc-tion input, coupled with relatively high mantlemelting temperatures and pressures, is consistentwith decompression melting of an upwelling as-thenosphere source for the primitive BSM basalts.Upwelling mantle is potentially consistent with aslab edge effect as proposed by Lawrence et al.[1984] for Salal Glacier basalts. Seismic anisot-ropy measurements reveal toroidal mantle flowaround the descending edges of subducted platesthat are undergoing rollback, thereby drawingexternal mantle (subslab) into the mantle wedgeFigure 13. AFC modeling results for (a) 87Sr/86Sr versus Sr; (b) ENd versus87Sr/86Sr; (c) Zr/Nb versus Ba/Nb. Heavy green curves with triangles and squares (AFC1 and AFC2, respectively) are the best fitassimilation-fractional crystallization pathways for Suite 2 samples BRC03?4 and MM01-1, respectively,using a gabbroic assimilant (from Table 5). The large green triangle and square are the gabbro compositionsused as assimilants in AFC 1 and 2, respectively. Orange and blue curves in Figure 13a are slab fluid/meltaddition curves, calculated as described in Fig. 4 caption. Heavy black curves in Figure 13c are from Figure8b. Data for the Coast Plutonic Complex shown as small blue triangles and light blue field [Friedman et al.,1995; Cui and Russell, 1995a, 1995b]. Sources of other data shown are given in Figure 4a caption.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913147[Long and Silver, 2008]. In other arcs, influx ofexternal mantle has been implicated in the genesisof lavas that are atypical for an arc setting [e.g.,Leat et al., 2004; Smith et al., 2001; Ferrari et al.,2001]. Slab rollback is occurring in the Cascadearc [Schellart, 2007], but the limited mantle ani-sotropy measurements in the GVB are inconclu-sive as to mantle wedge flow patterns [Currie etal., 2004]. Toroidal mantle flow has been docu-mented at the southern Juan de Fuca plate edge[Zandt and Humphreys, 2008], yet alkalic basaltsare not present [Hildreth, 2007] indicating that thetwo phenomena are not necessarily interrelated.[48] A slab edge origin may be improbable for theBSM volcanic centers in light of recent seismic to-mography, which indicates the northernmost slabedge in the Cascades (placed at the northernmostlimit of the Explorer plate) is located farther norththan the BSM volcanic centers [Mercier et al.,2009; Audet et al., 2008]. Toroidal mantle flowhas been proposed for the northern Explorer plateedge [Audet et al., 2008] and could be responsiblefor the alkalic basalts of the 500 km long Anahimvolcanic belt, which defines an east-west trendnearly orthogonal to, and north of, the GVB. Thisinterpretation is consistent with that of Thorkelsonet al. [2011] who proposed that Anahim magma-tism is related to mantle upwelling along the ther-mally eroding plate margins of the NorthernCordilleran slab window. However, eruption agesin the Anahim volcanic belt define an easterlytime progression that has been attributed to a hot-spot [Bevier, 1989], and tomographic results areconsistent with either interpretation [Mercier etal., 2009].[49] The BSM alkalic basalts may be related tomantle upwelling at the boundary between theJuan de Fuca and Explorer plates, as illustratedschematically in Figure 14. The northern segmentof the Juan de Fuca plate has had a complex tec-tonic history; about 4 Myr ago, the northernmostportion of the Juan de Fuca plate separated alongthe Nootka fault zone to form the independentExplorer microplate [Riddihough, 1984] (Figure1). Although convergence has ceased at the north-ern edge of the microplate, the southernmost partof the microplate continues to subduct slowly[Braunmiller and Nabelek, 2002], and the entireTable 5. AFC Modeling Parameters and ResultsCompositionAFC 1: Bridge River AFC 2: Mt. MeagerSample Modeled: BRC03?4 Sample Modeled: MM01-1Initial Magma: Assimilant: Initial Magma: Assimilant:BRC09-3a Gabbrob MM08a GabbrobSiO2 (wt %) 45.6 44.6 48.6 44.6TiO2 1.8 1.01 1.4 1.01MgO 15.6 6.57 14.5 6.57Na2O 2.6 1.31 2.9 1.31K2O 0.7 0.40 0.6 0.40Sr (ppm) 400 401 467 401Nd 18.5 5.0 14 5.0Ba 195 153 154 153Zr 127 21 92 21Nb 16.5 1.0 9 1.087Sr/86Sr 0.702986 0.7034 0.703164 0.7040143Nd/144Nd 0.513026 0.51286 0.513030 0.512820ENd ?7.6 ?4.3 ?7.6 ?3.6AFC resultsr c 0.90 0.89FLd 0.83 0.77olive 0.05cpx 0.05 0.10opx 0.05 0.10mt 0.02 0.03aTrace element and isotope data for initial magmas are from Table 1; major element data [from Green and Sinha, 2005] are corrected into equi-librium with Fo90 mantle.bMajor and trace element data for gabbro assimilant are from DeBari et al. [1999] for sample 91-17 of the Westcoast Crystalline Complexexcept Nd (interpolated); isotope ratios selected from within the range defined by the Coast Plutonic Complex [Cui and Russell, 1995b].cmass assimilated/mass crystallized.dfraction liquid remaining.eFraction of each mineral phase removed from magma; sum is equal to (1FL).fAbbreviations: oliv (olivine), cpx (clinopyroxene), opx (orthopyroxene), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913148Explorer region is a zone of strong shear deforma-tion [Dziak, 2006]. The offshore segment of theNootka fault shows left-lateral motion along a rup-ture and the onshore extension of the fault ismarked by thinning and deformation of the sub-ducting plate [Audet et al., 2008]. Seismic dataindicate the Explorer plate currently has a shal-lower dip than the Juan de Fuca plate, which maymanifest itself in a near-vertical gap between theplates (Figure 14). The BSM volcanic centers lieon, or just south of, the Nootka fault zone as ex-trapolated to the northeast (Figure 1a). We suggestthat thinning, deformation, and possible rupture ofthe subducted Explorer plate fragment may pro-vide a pathway for asthenospheric upwellingaccompanied by decompression melting. Farthereast along the projected trace of the Nootka fault,the Wells Gray-Clearwater volcanic field and Chil-cotin basalts have been similarly attributed toenriched asthenosphere upwelling through a gapalong the fault [Madsen et al., 2006; Sluggett,2008; Thorkelson et al., 2011].[50] Seismic tomography is inconclusive as towhether the Nootka fault is ??leaky?? or whethercontinuity is maintained at depth between theExplorer and Juan de Fuca plates [Mercier et al.,2009]. However, as the Explorer plate is situatedat the southern edge of a slab window, it is subjectto progressive thermal and physical degradationthat would facilitate passage of mantle melts frombelow [Thorkelson et al., 2011; Thorkelson andBreitsprecher, 2005]. In an analogous situation inthe Mexican arc, seismic anisotropy measurementsare consistent with plate separation. Faults sepa-rate the subducting Cocos plate into several seg-ments, and each subducts at a different angle,resulting in a scissors-like effect in which gapsbetween the plates allow for mantle upwellingthrough toroidal flow [Stubailo et al., 2012].7. Summary and Conclusions[51] Alkalic basalts at the Bridge River, Salal Gla-cier, and Mt. Meager volcanic centers (BSM vol-canic centers) of the Canadian segment of theCascade arc, known as the Garibaldi volcanic belt,have intraplate characteristics that contrast withtypical calc-alkaline mafic Cascade arc lavas. Newhigh precision Sr-Nd-Hf-Pb isotope ratios andtrace element abundances reveal that the mostprimitive basalts at Salal Glacier and Bridge Riverare essentially free of components derived fromthe subducting slab. The apparent trace element??arc signature?? exhibited by several more evolvedBSM basalts is more likely a consequence ofassimilation of mafic deep crust rather than slabinput. At Mt. Meager, however, primitive basaltsmay include a small amount of fluid derived fromsubducted altered oceanic crust.[52] The mantle source of the BSM basalts isdeeper, hotter, and isotopically distinct from thesource of calc-alkaline basalts from Mt. Baker andthroughout the Cascade arc. The BSM mantlesource is also more enriched in incompatible ele-ments than the depleted mantle wedge tapped bycalc-alkaline Cascade arc basalts, and similar toocean island basalt sources. Similar trace elementabundances among the BSM and Anahim alkalicbasalts, and those in the Cascade-Columbia tran-sect and north of Mt. Rainier (Figures 8 and 9c),indicate mantle sources similarly enriched in in-compatible elements.[53] BSM and Cascade-Columbia intraplate lavashave been previously attributed to enriched mantledomains associated with the base of an accretedterrane [Schmidt et al., 2008]. We consider this hy-pothesis unlikely for the BSM volcanic centers fortwo reasons. First, the accreted terranes beneathFigure 14. Schematic representation of plate configurationat the northern end of the Cascade arc based on a model ofRiddihough [1984]. The Explorer plate detached from theJuan de Fuca Plate along the Nootka fault zone 3 to 4 Myrago as it became younger, hotter, and more buoyant at thetrench. The thick dashed black line indicates the surface traceof the Nootka fault. Convergence of the Explorer plate withNorth America has now nearly ceased. The vertical windowformed between the Explorer and Juan de Fuca plates maypromote upwelling of deep, hot mantle (large orange arrow)at the edge of the currently subducting plate. Decompressionmelting of this mantle accounts for the presence of hot alkalicbasalts essentially free of a subduction signature (red, orange,and yellow triangles along Nootka fault zone for each of theBSM volcanic centers).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913149the BSM centers and the Cascade-Columbia tran-sect are different (Wrangellia and Siletzia, respec-tively), and second, Mt. Baker and the BSM sharethe same accreted terrane at depth yet the state ofmantle source enrichment differs substantially.[54] Although major and trace element datarequire an enriched mantle source for the BSMbasalts, isotopic data provide evidence for long-term mantle depletion. Pb isotope ratios of theBSM basalts are broadly similar to oceanic andintraplate basalts of the northeastern Pacific (Fig-ure 11), indicating that isotopically depleted uppermantle of common origin is regionally wide-spread, albeit with small isotopic heterogeneities.[55] With isotopic data consistent with long-termdepletion, incompatible-element enrichment of theBSM mantle source must have occurred relativelyrecently. Recent mantle enrichment has been pro-posed for numerous other cases of isotopicallydepleted alkalic basalts [Roden and Murthy, 1985,and references therein], including those from theTuzo Wilson volcanic field [Allan et al., 1993](Figure 11) and the Bowie Seamount in the Gulfof Alaska [Cousens, 1988]. The BSM volcaniccenters are located along, and just south of, theprojected trace of the Nootka fault zone, whichseparates the subducting Juan de Fuca plate fromthe Explorer plate fragment. We attribute the BSMbasalts to upwelling asthenosphere through a gapalong the fault, which undergoes decompressionmelting to generate alkalic basalts that are free ofsubduction input yet located in an arc setting.Acknowledgments[56] We thank Bruno Kieffer for assistance with TIMS analy-ses, Vivian Lai for help with trace element analyses, JaneBarling, Kathy Gordon, and Liyan Xing for assistance withMC-ICP-MS analyses, and Ines Nobre Silva for instruction inthe clean laboratory. We are grateful to Marion Carpentier forprocessing and analyzing eight samples for trace elementsand five for isotopes. Derek Thorkelson, Martin Streck, andRichard Carlson provided constructive and thoughtfulreviews. Insightful discussions with Kelly Russell have beenmuch appreciated. We are particularly grateful to StewartMcCallum for detailed reviews, discussions, and advice thathave significantly improved the manuscript. This researchwas funded by an NSERC Discovery Grant to D. 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Humphreys (2008), Toroidal mantle flowthrough the Western U.S. slab window, Geology, 36, 295?298.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913155 18.7018.7518.8018.8518.9018.9519.0018.70 18.80 18.90 19.00206 Pb/204 Pb (leached)206Pb/204Pb (unleached)0.70250.70300.70350.70400.70450.7025 0.7035 0.704587Sr/86Sr(leached)87Sr/86Sr (unleached)0.51270.51290.51310.5127 0.5129 0.5131143 Nd/144 Nd (leached)143Nd/144Nd (unleached)38.0538.1538.2538.3538.4538.5538.05 38.15 38.25 38.35 38.45 38.55208 Pb/204 Pb (leached)208Pb/204Pb (unleached)0.282950.283000.283050.283100.283150.28295 0.28300 0.28305 0.28310 0.28315176 Hf/177Hf (leached)176Hf/177Hf (unleached)15.5415.5615.5815.6015.54 15.56 15.58 15.60207 Pb/204 Pb (leached)207Pb/204Pb (unleached)Mullen and Weis (2013) Figure S1 Sr-Nd-Hf-Pb isotope and trace element evidencefor the origin of alkalic basalts in the GaribaldiBelt, northern Cascade arcEmily K. Mullen and Dominique WeisPacific Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences,University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, V6T 1Z4, Canada(emullen@eos.ubc.ca)[1] In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier,and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures arenegligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceaniccrust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display traceelement signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation ratherthan primary input from the subducted slab. Primary BSM basalts represent 3?8% melts that segregated fromenriched garnet lherzolite at significantly higher temperatures and pressures (70?105 km) than calc-alkalineCascade arc basalts. The BSM mantle source is significantly more incompatible element-enriched than thedepleted mantle tapped by calc-alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct fromcalc-alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NWNorthAmerica. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent withupwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, whichforms the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate. A gap orattenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low-degreedecompressionmelting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.Components: 17,992 words, 14 figures, 5 tables.Keywords: alkali basalt ; Garibaldi Belt ; cascade arc; Sr-Nd-Pb-Hf isotopes; trace elements.Index Terms: 1040 Radiogenic isotope geochemistry: Geochemistry; 1037 Magma genesis and partial melting: Geo-chemistry; 1031 Subduction zone processes: Geochemistry; 1033 Intra-plate processes: Geochemistry; 1065 Major andtrace element geochemistry: Geochemistry; 3613 Subduction zone processes: Mineralogy and Petrology; 3615 Intra-plateprocesses: Mineralogy and Petrology; 3619 Magma genesis and partial melting: Mineralogy and Petrology; 3060 Subduc-tion zone processes: Marine Geology and Geophysics; 8170 Subduction zone processes: Tectonophysics; 8413 Subductionzone processes: Volcanology; 8415 Intra-plate processes: Volcanology.Received 10 January 2013; Revised 29 May 2013; Accepted 30 May 2013; Published 28 August 2013.Mullen, E. K., and D. Weis (2013), Sr-Nd-Hf-Pb isotope and trace element evidence for the origin of alkalic basalts in theGaribaldi Belt, northern Cascade arc, Geochem. Geophys. Geosyst., 14, 3126?3155, doi:10.1002/ggge.20191.1. Introduction[2] The role of mantle heterogeneity in generatingthe compositional diversity observed in arc magmasis difficult to decipher because it can be masked bycontributions from the subducting slab. There is ageneral consensus that mantle melting in arcs occursin response to input of a hydrous component derived? 2013. American Geophysical Union. All Rights Reserved. 3126ArticleVolume 14, Number 828 August 2013doi: 10.1002/ggge.20191ISSN: 1525-2027from the subducting slab that generates hydrousminerals in the mantle, lowers the mantle solidustemperature, and produces melts by dehydrationmelting or ??flux?? melting [e.g., Kushiro, 1987;Grove et al., 2002]. As a consequence, basaltserupted in subduction zone settings are predomi-nantly aluminous, subalkaline, and bear a slab-derived trace element ??arc signature?? that includesenrichments in large ion lithophile elements (LILEs)and depletions in high field strength elements(HFSEs) [e.g., Gill, 1981; Pearce and Peate, 1995].The Cascade arc, which extends 1300 km fromnorthern California to southwestern British Colum-bia, is an example of an arc in which calc-alkalinelavas dominate [Schmidt et al., 2008; Hildreth,2007; Bacon et al., 1997; Conrey et al., 1997].However, the northernmost segment of the arc is un-usual in that mafic lavas are predominantly alkalic.Alkali olivine basalt and hawaiite occur at the vol-canic fields of Mt. Meager, Salal Glacier, andBridge River Cones (hereinafter referred to as theBSM volcanic centers) [Green and Sinha, 2005].[3] Alkalic lavas are uncommon in arc settings,particularly along the main arc axis, and are attrib-uted to a variety of phenomena that include tearsin the subducting plate, back-arc extension, slabwindows, entrainment of mantle hotspots, accretedenriched mantle, and intraarc rifting [e.g., Naka-mura et al., 1989; Abratis and W?rner, 2001; Fer-rari et al., 2001; Marquez et al., 1999; Turnerand Hawkesworth, 1998; Pearce and Stern, 2006;Thorkelson and Taylor, 1989; Skulski et al., 1991;Righter and Carmichael, 1992; Hughes, 1990;Pearce and Peate, 1995, and references therein].The BSM volcanic centers are located 110 kmabove the subducting plate, broadly similar toother Cascade arc volcanoes [McCrory et al.,2004], suggesting a potential connection to arcmagmatic processes. Elucidating the petrogenesisof the BSM basalts may provide valuable insightsinto mantle and slab processes under the dimin-ished subduction regime at the northern boundaryof the Cascade arc [Harry and Green, 1999].[4] The Cascade magmatic arc has been activesince 40 Ma and is a consequence of subductionof the Juan de Fuca oceanic plate beneath NorthAmerica [Hildreth, 2007]. In northwestern Wash-ington, an abrupt change in the orientation of thearc axis mirrors a bend in the continental margin,subdividing the arc into two major segments (Fig-ure 1a). The High Cascades segment (Mt. Rainierto Lassen Peak) is separated from the GaribaldiVolcanic Belt (GVB) (Glacier Peak to Silver-throne) by a 120 km gap in modern volcanism[Green and Harry, 1999]. Throughout much of theHigh Cascades, the subducting plate is 10 Myrold at the trench with ocean floor isochrons gener-ally parallel to the continental margin [Wilson,2002]. In the GVB, however, isochrons are obliqueto the plate margin and the slab age at the trenchdecreases northward to 6 Ma outboard of theBSM volcanic centers (Figure 1a), making it oneof the youngest and hottest subduction settings inthe world [Syracuse et al., 2010; Harry andGreen, 1999]. Subducted water is expected to belost at shallow depths from a hot slab, leading toreduced hydration of the subarc mantle wedge[Green and Harry, 1999]. A diminished subduc-tion regime may account for why the GVB, incomparison to the High Cascades, has a narrowerwidth, lower magma production rates, and magma-tism mainly restricted to the major volcanic cen-ters [Harry and Green, 1999].[5] In this paper, we assess three hypotheses forthe origin of the BSM alkalic basalts. In the firstmodel, the BSM alkalic basalts are essentiallylower melt fraction ??equivalents?? of more typicalCascade arc calc-alkaline basalts [Green andHarry, 1999; Green and Sinha, 2005]. Reducedhydration of the subarc mantle wedge may reduceits capacity for flux melting, resulting in lowermelt fractions that are enriched in alkali elementsbut display minimal arc signature. A similar modelhas been proposed to account for basalts elsewherein the Cascade arc that are geochemically indistin-guishable from intraplate basalts [Reiners et al.,2000]. Intraplate-type lavas dominate the back-arcSimcoe volcanic field east of Mt. Adams and areinterspersed with other basalt types in a swathextending west 150 km from Simcoe to Portlandthat has been referred to as the Cascades-Columbia transect [Leeman et al., 1990, 2005;Hildreth, 2007; Jicha et al., 2008; Conrey et al.,1997; Bacon et al., 1997]. A few other examplesoccur north of Mt. Rainier [Reiners et al., 2000]and in north-central Oregon [Conrey et al., 1997].[6] Second, a subducted plate boundary or fracturezone may trigger mantle upwelling, inducingdecompression melting that generates low-degree,alkali-rich melts. These processes have been pro-posed for other arcs, including the Mexican arc andLesser Antilles [e.g., Righter et al., 1995; DeLonget al., 1975; Pearce, 2005], and a version of thismodel is mentioned by Lawrence et al. [1984] in thecontext of the Salal Glacier alkalic basalts. The sub-ducted boundary between the Explorer and Juan deFuca plates intersects the BSM volcanic centers andhas been implicated in the origin of the WellsMULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913127Gray-Clearwater volcanic field (3.5 Ma?7.6 ka[Hickson and Souther, 1984]) and Chilcotin Groupbasalts (32?0.8 Ma [Mathews, 1989]) that arelocated in the GVB back-arc region [Madsen et al.,2006; Sluggett, 2008]. Near the triple junctionamong the Explorer, Pacific, and North Americaplates, the Quaternary alkalic seamounts of the TuzoWilson volcanic field are attributed to a ??leakytransform?? in an oceanic setting [Allan et al., 1993].[7] Third, the BSM basalts may reflect one of theother mechanisms proposed to account forMiocene-Holocene intraplate volcanic centers thatoccur in a broad swath extending from southernBritish Columbia to Alaska. The east-west trend-ing Anahim volcanic belt (14.5 Ma?7.2 ka [Bevier,1989]), located immediately north of the GVB,may be related to a hotspot [Bevier, 1989; Char-land et al., 1995], an edge effect of the Juan deFigure 1. (a) Map of the Cascade arc and its tectonic setting. The extents of the Garibaldi volcanic belt andHigh Cascades segments of the arc are indicated with pink arrows. Volcanic and plutonic rocks are shown inyellow and orange shading, respectively. Black triangles denote composite volcanoes. (b) The study area isenclosed by a small bold rectangle and is enlarged. Igneous rock distributions are compiled from Lawrence etal. [1984], Monger [1989], DuBray et al. [2006], Green et al. [1988], Wheeler and McFeely [1991]. Oceanicplate configurations are from Braunmiller and Nabelek [2002], Audet et al. [2008], and Wilson [2002]. Col-ored lines on the oceanic plates are isochrons; accompanying numbers indicate the age of oceanic crust in Ma(from Wilson [2002]). Pseudofaults are shown as thin gray lines. Four heavy gray arrows on the Juan de Fucaand Explorer plates are convergence vectors (mm/yr) obtained from McCrory et al. [2004], Riddihough andHyndman [1991], and Braunmiller and Nabelek [2002] for a reference frame fixed relative to North America.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913128Fuca plate [Stacey, 1974], or ridge subduction[Farrar and Dixon, 1992]. Farther north, theextensive northern Cordilleran volcanic province(20 Ma?200 years B.P.) has been attributed tocrustal extension [Edwards and Russell, 2000].The aforementioned Wells Gray-Clearwater andChilcotin lavas may alternatively be a product ofback-arc extension [Bevier, 1983; Hickson, 1987].More recently, Thorkelson et al. [2011] proposed asingle model in which all of these volcanic provin-ces are related to upwelling of enriched mantlewithin and along the eroding margins of the1500 km long Northern Cordilleran slab window[Thorkelson and Taylor, 1989] that extends nearlyas far south as the BSM volcanic centers.[8] In this study, we investigate the roles of themantle and subducting slab in generating thealkalic compositions of the BSM basalts with highprecision whole-rock Sr-Nd-Pb-Hf isotope ratiosand trace element data. Radiogenic isotope signa-tures of primitive basalts are sensitive indicators ofmantle source heterogeneity [e.g., Hofmann, 2003]and the presence of components derived from sub-ducted oceanic crust and sediment [e.g., Kay et al.,1978]. Green and Sinha [2005] showed that theBSM alkalic basalts record less slab input thancalc-alkaline basalts of the southern GVB, butminimized the possible role of mantle heterogene-ity. However, recent improvements in the precisionof isotopic measurements have revealed mantleheterogeneities that were previously difficult to dis-cern [e.g., Abouchami et al., 2005]. Alkalic basaltsin British Columbia and the Cascade arc are typi-cally ascribed to mantle sources that are moreenriched in incompatible elements than the mantlewedge sampled by calc-alkaline basalts [e.g., Thor-kelson et al., 2011; Sluggett, 2008; Edwards andRussell, 2000; Leeman et al., 1990, 2005; Baconet al., 1997; Borg et al., 1997; Conrey et al.,1997; Schmidt et al., 2008; Jicha, et al. 2008].However, Reiners et al. [2000] proposed that bothbasalt types can be derived from a homogeneousmantle variably fluxed by slab-derived fluids.[9] We compare the BSM basalts to calc-alkalinebasalts from Mt. Baker, a stratovolcano located inthe ??cooler?? southern GVB and representative ofmore typical Cascade arc basalts, and to pub-lished data for other intraplate alkalic basaltsfrom British Columbia. Our new isotope and traceelement data show minimal subduction influenceon the source of the most primitive basalts atSalal Glacier and Bridge River. The BSM basaltsalso have a mantle source that is isotopically dis-tinct from, and more incompatible elementenriched, than the mantle underlying much of theCascade arc. These results have important impli-cations for the physical configuration of the sub-ducting slab and mantle flow patterns in northernCascadia.2. Geology of the Bridge River, SalalGlacier, and Mt. Meager VolcanicCenters[10] The Bridge River Cones, Salal Glacier, andMt. Meager are located 150 km north of Van-couver, British Columbia (Figure 1b). At the twonorthernmost centers (Salal Glacier and BridgeRiver), lavas are almost exclusively mafic. TheMt. Meager volcanic field includes basalt throughrhyolite but is dominated by intermediate compo-sitions, and Mt. Meager proper is a compositeandesitic stratovolcano [Ke, 1992].[11] The Salal Glacier volcanic field includes pil-low lavas, tuffs, and variably palagonitized andbrecciated flow remnants that survived continentalice sheet advances as high-altitude nunataks. Atlower altitudes, severe glacial erosion has revealedrhyolite and andesite dikes. Age dates for an alkalibasalt and overlying hawaiite are 0.97 and 0.59Ma (K-Ar), respectively [Lawrence, 1979].[12] Lavas at the Bridge River Cones are exclu-sively alkalic [Roddick and Souther, 1987]. Theterm ??cones?? is a misnomer because none of thedeposits is a true volcanic cone; rather, glacialerosion has produced cone-like forms. Columnarlavas of the Sham Hill plug and Tuber Hill expo-sure are dated at 1 Ma and 600 ka (K-Ar), respec-tively [Roddick and Souther, 1987].[13] At Mt. Meager, intermediate to silicic lavasspan the alkalic-subalkalic boundary [Stasiuk et al.,1994]. Mafic lavas are exclusively alkalic,however, and occur as four flow remnantscollectively known as the Mosaic Assemblage[Stasiuk and Russell, 1989; Stasiuk et al., 1994].Two of the basalts are dated at 90 and 140 ka (K-Ar) [Anderson, 1975; Woodsworth, 1977].Evidence for recent involvement of mafic magmain the form of mafic enclaves and banded pumicesis preserved in the 2360 years B.P. thatexplosively released 10 km3 of dacite [Clague etal., 1995; Michol et al., 2008]. Banded pumicesand mafic enclaves indicate that the intrusion of abasaltic magma may have triggered the eruption[Stasiuk et al., 1994].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131293. Major Element Compositions andPetrography[14] Samples analyzed for this study are from theBridge River, Salal Glacier, Mt. Meager, and Mt.Baker sample suites of Green and Sinha [2005]and from the Mt. Baker sample suite of E. K.Mullen and I. S. McCallum (Origin of basalts in ahot subduction setting: Petrologic and geochemi-cal insights from Mt. Baker, northern Cascade arc,submitted to Journal of Petrology, 2013, herein-after referred to as Mullen and McCallum, submit-ted manuscript, 2013). Major element data anddetailed petrographic descriptions for all samplesare provided in those references.[15] The BSM basalts are alkalic [Macdonald,1968] and nepheline normative with Na2O>K2O(Figure 2a). The basalts have distinctly lower SiO2(Figure 2) and Al2O3 than the calc-alkaline, hyper-sthene-normative Mt. Baker basalts.[16] Bridge River alkali olivine basalts and hawai-ites encompass the largest range of compositionaldiversity (Figure 2). Molar Mg/(Mg?Fe2?) valuesrange from 0.44 to 0.62; we consider two sampleswith Mg/(Mg?Fe2?)> 0.6 as primitive. Phenocrystand microphenocryst minerals are limited to olivine(1?2%) and rare plagioclase. Except for one sam-ple with a brown glass matrix, the basalts have hol-ocrystalline groundmasses containing olivine,plagioclase, titanaugite, magnetite, and ilmenite.The most primitive basalt (BRC10) contains biotiteand amphibole in the groundmass. Two of the moreevolved samples contain quartz xenocrysts andgranodiorite xenoliths (BRC03?4, BRC01?3).[17] Mt. Meager alkali basalts and hawaiites con-tain <1% microphenocrysts of olivine, clinopyr-oxene, and plagioclase. The groundmass containsglass and magnetite and, in the least primitivesample (MM01-1), biotite and amphibole. Mg/(Mg?Fe2?) values are 0.59?0.63.[18] Salal Glacier samples are the most primitiveamong the BSM basalts with Mg/(Mg?Fe2?)? 0.58to 0.66 and have the highest normative nephelinecontents. The most primitive samples are glassy andvesicular with phenocryst assemblages including<15% olivine, <1% plagioclase, <1% clinopyrox-ene, and rare orthopyroxene xenocrysts. Lessrimitive samples contain orthopyroxene phenocrystsand more abundant plagioclase, and olivine is eitherrimmed by clinopyroxene or absent.[19] At Mt. Baker, the most mafic lavas includemedium-K calc-alkaline basalt, high-Mg basalticandesite, and low-K olivine tholeiite, with Mg/(Mg? Fe2?)? 0.56?0.70. All samples contain oli-vine and plagioclase phenocrysts and some alsohave clinopyroxene phenocrysts (Mullen andMcCallum, submitted manuscript, 2013).4. Analytical Methods[20] Trace element abundances and Sr-Nd-Hf-Pbisotope ratios were measured on 19 BSM basalts,using splits of sample powders analyzed by Greenand Sinha [2005] for major and trace elements andSr isotope ratios. Larger symbols in Figure 2 des-ignate samples analyzed for the present study. HfFigure 2. Major element variation diagrams for the basaltsof Bridge River (red), Salal Glacier (orange), Mt. Meager(yellow), and Mt. Baker (lavender). (a) wt % Na2O?K2Oversus SiO2 with discriminant line of Macdonald [1968] andfields of Le Bas et al. [1986]. (b) Miyashiro diagram (FeO/MgO versus SiO2) with discriminant line of Miyashiro[1974]. Bridge River, Salal Glacier, and Mt. Meager data arefrom Green and Sinha [2005]. Mt. Baker data are fromMullen and McCallum (submitted manuscript, 2013) exceptLib21 from Green and Sinha [2005]. Large symbols (circlesand diamonds) indicate samples analyzed in this study fortrace elements and isotope ratios; diamonds are accompaniedby sample numbers and are the Suite 2 samples discussed inthe text. Small circles indicate samples of Green and Sinha[2005] not analyzed for the present study.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913130Figure 3. (a) 208Pb/204Pb versus 206Pb/204Pb, (b) 207Pb/204Pb versus 206Pb/204Pb. Insets show the same datafor the BSM and Mt. Baker basalts but on an expanded scale. Symbols for the BSM and Mt. Baker samplesgiven in legend; circles are used for primitive basalts. The more evolved basalts are subdivided into Suite 1(circles; isotopically similar to the primitive basalts) and Suite 2 (diamonds accompanied by sample numbers;isotopically distinct from the primitive basalts). 2 error bars (external reproducibilities) are smaller thansymbols in all plots. NHRL is the Northern Hemisphere Reference Line of Hart [1984]. Cascade arc basaltdata (pink crosses; only those with >8 wt % MgO are included) are from Conrey et al. [1997], Jicha et al.[2008], Bacon et al. [1994, 1997], Leeman et al. [1990, 2005], Baker et al. [1991],Magna et al. [2006], Groveet al. [2002], and Borg et al. [1997, 2000]. Northern Juan de Fuca MORB data (dark blue ?) are from Cou-sens et al. [1995]. Explorer MORB data (dark gray filled squares) are from B. Cousens (unpublished data2007). N. Gorda MORB (black ?) are from Allan et al. [1993]. Northern Cascadia sediment data (orangecircles) are from ODP sites 1027 and 888 [Carpentier et al., 2010, 2013]. Note that all isotope data are nor-malized to the same isotope standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913131isotope ratios were also measured on splits of pow-ders of three Mt. Baker basalts previously ana-lyzed for major and trace elements and Sr-Nd-Pbisotopes [Mullen and McCallum, submitted manu-script, 2013; Green and Sinha, 2005].[21] All chemical separations and mass spectro-metric analyses were carried out in Class 100 and10,000 clean laboratories, respectively, at the Pa-cific Centre for Isotopic and GeochemicalResearch at the University of British Columbia.Rock powders (100 mg) were digested in sub-boiled concentrated HF?HNO3 in 15 mL screw-top Savillex beakers on a hotplate for 48 h at130C. Samples were dried down on a hotplateand brought up in subboiled 6 N HCl and fluxedon a hotplate for at least 24 h. Sample aliquots of5?10% were diluted 5000X with an HNO3?HFsolution for analysis on a Thermo Finnigan Ele-ment2 HR-ICP-MS or an Agilent 7700 quadrupoleICP-MS. Sr isotope ratios were measured on aThermo Finnigan Triton TIMS and Pb, Nd, and Hfisotope ratios on a Nu Instruments MC-ICP-MS(Nu 021) following the procedures of Weis et al.[2006]. Pb, Sr, Hf, and Nd were separated fromsingle powder dissolutions by sequential ionexchange column chemistry as described in Weiset al. [2006, 2007]. All solutions were passedtwice through Pb exchange columns to ensure Pbpurification. Although thin sections of the ana-lyzed samples indicate little or no alteration in allsamples (minor iddingsite in olivine) and LOIvalues are low (<1%), even minimally alteredsamples can yield isotopic compositions that arenot representative of magmatic isotopic signatures,particularly in the case of Sr and Pb isotopes[Hanano et al., 2009; Nobre Silva et al., 2009].Therefore, we measured isotope ratios on bothunleached and leached powders of some samples.Leaching was conducted prior to powder dissolu-tion following the procedures of Nobre Silva et al.[2009, 2010]. Leached samples gave isotope ratioswithin analytical error of respective unleachedsamples for Sr and Nd (Figure S1).1 Hf isotoperatios are also within analytical error except forone sample (BRC10) that gave a higher value inthe leached sample. 207Pb/204Pb in leached sam-ples is systematically lower than in unleachedsamples while 208Pb/204Pb and 206Pb/204Pb arewithin error of unleached samples, with the excep-tion of one Mt. Baker sample (Lib21) (FigureS1).1 All isotope plots in the main text show dataobtained on leached samples except for cases inwhich only unleached samples were analyzed.Blank contributions to isotope ratios were negligi-ble with total procedural blanks of 50, 400, 90,and 15 pg for Pb, Sr, Nd, and Hf, respectively.5. Results5.1 Isotopes[22] Isotope ratios are reported in Table 1 and plot-ted in Figures 3?5. 87Sr/86Sr values measured in theBSM basalts are systematically lower than reportedby Green and Sinha [2005] for the same samplesand lie outside their reported uncertainties (Figure4a inset). For direct comparison among datasets, allliterature data are normalized to standard values of87Sr/86Sr? 0.710248 for SRM987 and 0.708028 forEimer and Amend; 143Nd/144Nd? 0.511973for Rennes, 0.511858 for La Jolla, 0.512633 forBCR-1, and 0.512130 for Ames [Weis et al., 2006,2007]; 176Hf/177Hf? 0.282160 for JMC 475[Vervoort and Blichert-Toft, 1999]; and208Pb/204Pb? 36.7219, 207Pb/204Pb? 15.4963,206Pb/204Pb? 16.9405 for SRM981 [Galer andAbouchami, 1998].[23] Primitive BSM basalts (Mg/Mg?Fe2?> 0.60)form an isotopic cluster (Figures 3 and 4) with anarrow range of 87Sr/86Sr? 0.70299?0.70314,ENd??7.1 to ?7.7, EHf??8.3 to ?10.0,208Pb/204Pb? 38.075?38.172, 207Pb/204Pb? 15.541?15.557, 206Pb/204Pb? 18.690?18.774. The primitiveBSM basalts overlap in 208Pb/204Pb and 206Pb/204Pbwith N. Juan de Fuca MORB [Cousens et al., 1995]and Explorer MORB (B. Cousens, unpublished data2007) (Figure 3a) but have slightly higher207Pb/204Pb and 87Sr/86Sr and lower ENd (Figures 3band 4a). Primitive BSM basalts plot near thedepleted end of the Sr-Nd-Pb isotopic arrays definedby other Cascade arc basalts (Figures 3 and 4).Along with Mt. Baker, primitive BSM basalts haveamong the highest ENd values reported for the Cas-cade arc. Mt. Baker basalts have slightly higher208Pb/204Pb and 206Pb/204Pb than primitive BSMsamples, but overlap in 207Pb/204Pb. Mt. Baker hassignificantly higher EHf (?11.1 to? 12.1) andslightly higher 87Sr/86Sr. In EHf-ENd isotopic space(Figure 4b), primitive BSM and Mt. Baker basaltsoverlap with only one outlier among data previouslypublished for the Cascade arc (Lassen Peak [Borg etal., 2002] and Mt. Adams [Jicha et al., 2008]). Mt.Adams EHf values cluster between primitive BSMbasalts and Mt. Baker. Together, the BSM, Mt.Baker, and Mt. Adams basalts define an EHf range1Additional supporting information may be found in the onlineversion of this article.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913132Table 1. Sr, Nd, Hf, and Pb Isotope Ratios87Sr/86Srb 143Nd/144NdcSample#Lat(N)Long(W)SiO2(wt%)MgNumbera Leached 2SEh Unleached 2SE Leached 2SE"NdLeachedf Unleached 2SE"NdUnleachedfBridge River ConesBRC01-3 50.93 123.45 50.51 56.5 0.703212 9BRC02 50.91 123.45 49.60 52.8BRC03-4 50.93 123.45 50.07 54.4 0.703219 6 0.512966 7 6.4BRC04 50.93 123.45 49.48 52.4 0.703184 9BRC05-1 50.92 123.45 46.66 48.3 0.703119 9BRC06 50.90 123.45 49.35 53.5 0.703175 8BRC07-2 50.92 123.45 47.24 47.7 0.703098 9 0.703124 7 0.513012 5 7.3 0.513012 6 7.3BRC09-3 50.92 123.41 48.02 59.6 0.702985 7 0.703012 9 0.513024 7 7.5BRC10 50.92 123.38 45.10 61.2 0.703052 7 0.703054 7 0.513031 7 7.7dupi 0.703055 7 0.703056 8Salal GlacierSG01-2 50.81 123.45 46.64 65.1 0.703140 7 0.513021 6 7.5SG01-3 50.81 123.45 46.62 64.5 0.703143 8 0.703149 9 0.513001 7 7.1SG10 50.78 123.39 46.04 65.7 0.703122 9dup 0.703117 8SG12 50.77 123.40 46.71 66.5 0.703065 7 0.703067 8 0.513013 6 7.3SG16 50.77 123.39 46.59 65.9 0.703101 10Mt. MeagerMM01-1 50.65 123.59 48.84 58.8 0.703764 8 0.703758 9 0.512930 5 5.7 0.512926 7 5.7dup 0.703762 6 0.703763 7 0.512936 5 5.8 0.512941 6 5.6MM02 50.69 123.57 48.64 63.0 0.703132MM04 50.69 123.57 48.94 60.8 0.703144 6 0.703146 0.513030 9 7.6MM08 50.55 123.53 49.68 60.6 0.703164Mount BakerLIB-21 48.67 121.74 51.04 63.5 0.703964 9 0.703970 7 0.512834 6 3.8 0.512834 6 3.802-MB-5 48.72 121.85 53.69 69.7 0.703109 7 0.513001 6 7.107-MB-112 48.66 121.70 52.56 56.6 0.703240 7 0.513034 7 7.702-MB-1 48.72 121.85 53.30 56.0 0.703513 8 0.512899 5.106-MB-82 48.72 121.85 50.57 65.1 0.703156 7 0.512986 6.807-MB-114 48.64 121.73 52.06 61.3 0.703213 7 0.513037 7.806-MB-97 48.78 121.88 54.45 49.0 0.703173 10 0.512993 6.9176Hf/177HfdSample # Leached 2SE "Hf Leachedg Unleached 2SE "Hf UnleachedgBridge River ConesBRC01?3 0.283067 8 10.4BRC02BRC03?4 0.283052 5 9.9BRC04 0.283026 7 9.0BRC05-1 0.283040 7 9.5BRC06 0.283018 6 8.7BRC07-2 0.283027 4 9.0 0.283030 5 9.1BRC09-3 0.283007 6 8.3 0.283030 5 9.1BRC10 0.283025 6 8.9 0.282985 4 7.5Salal GlacierSG01?2 0.283017 5 8.7SG01?3 0.283021 5 8.8 0.283025 24 8.9SG10 0.283012 9 8.5SG12SG16 0.283023 7 8.9Mt. MeagerMM01-1 0.283063 9 10.3 0.283064 6 10.3dup 0.283050 8 9.9 0.283056 7 10.3MM02 0.283050 6 9.8MM04 0.283056 5 10.1MM08 0.283022 5 8.9Mount BakerLIB-21 0.283084 5 11.0 0.283094 5 11.402-MB-5 0.283100 5 11.607-MB-112 0.283114 4 12.1MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913133that is similar to Explorer MORB, but with slightlylower ENd.[24] More evolved BSM basalts define twogroups: Suite 1 has isotopic ratios that overlapwith primitive basalts from the same volcanic cen-ter, whereas Suite 2 has substantially lower ENdand higher Sr and Pb isotope ratios than otherbasalts from their respective volcanic centers (Fig-ures 3 and 4a). Suite 2 includes three BSM basalts(BRC03?4, BRC01?3 at Bridge River; MM01-1at Mt. Meager), all of which have Pb isotope ratiosTable 1. (continued)208Pb/204Pbe 207Pb/204Pbe 206Pb/204PbeSample # Leached 2SE Unleached 2SE Leached 2SE Unleached 2SE Leached 2SE Unleached 2SEBridge River ConesBRC01-3 38.2441 22 15.5615 8 18.7878 8dup 38.2433 24 15.5604 9 18.7863 10BRC02 38.1901 21 15.5554 8 18.7481 10BRC03-4 38.2671 18 15.5584 7 18.8334 6dup 38.2674 18 15.5585 6 18.8337 8BRC04 38.1961 27 15.5622 9 18.7379 13BRC05-1 38.1274 17 15.5481 7 18.7591 8BRC06 38.1828 28 15.5530 12 18.7463 14BRC07-2 38.1215 19 38.1260 26 15.5450 7 15.5484 9 18.7626 8 18.7539 10BRC09-3 38.1245 18 38.1432 40 15.5462 6 15.5561 15 18.7432 8 18.7340 13BRC10 38.1496 22 38.1580 20 15.5566 10 15.5610 7 18.7738 10 18.7754 8dup 38.1601 36 15.5620 14 18.7750 17Salal GlacierSG01-2 38.1087 20 15.5419 8 18.7465 9SG01-3 38.1088 24 38.1250 31 15.5407 8 15.5508 7 18.7536 10 18.7536 7SG10 38.1285 24 15.5492 9 18.7363 12SG12 38.1721 22 38.1492 38 15.5521 7 15.5621 14 18.7739 9 18.6997 14SG16 38.1585 33 15.5527 12 18.7591 12Mt. MeagerMM01-1 38.2664 21 38.2734 19 15.5692 8 15.5731 7 18.8033 9 18.8090 7dup 38.2697 18 38.2772 19 15.5703 7 15.5738 7 18.8102 8 18.8115 7MM02 38.1005 35 15.5470 13 18.7034 15MM04 38.1046 14 38.1370 36 15.5470 4 15.5580 12 18.7082 6 18.7271 13MM08 38.0747 25 15.5550 9 18.6896 15Mount BakerLIB-21 38.4708 24 38.5285 20 15.5851 9 15.5931 7 18.9235 11 18.9726 802-MB-5 38.2495 22 15.5505 8 18.7975 907-MB-112 38.3077 21 15.5575 7 18.8385 802-MB-1 38.3598 15.5645 18.851506-MB-82 38.2661 15.5518 18.828607-MB-114 38.2753 15.5560 18.846606-MB-97 38.2597 15.5529 18.8356aCalculated as 100Mg/Mg?Fe2? (molar), using major element data from Green and Sinha [2005] and Mullen and McCallum (submittedmanuscript, 2013) and assuming Fe3?/Fe? 0.15.bReported Sr isotope ratios are corrected for mass fractionation using 86Sr/88Sr? 0.1194. Repeat analysis of the Sr SRM987 standard yielded amean (6 2) of 87Sr/86Sr? 0.7102486 2 (n? 7), identical to the accepted value [Weis et al., 2006].cReported Nd isotope ratios are corrected for mass fractionation using 146Nd/144Nd? 0.7219 and are normalized to 143Nd/144Nd? 0.511973 forthe Rennes reference material [Chauvel and Blichert-Toft, 2001] using the daily average method. The Rennes standard was analyzed every twosamples and over the course of analysis gave a mean (6 2) value of 143Nd/144Nd? 0.5119806 65 (n? 16). On a per session basis, reproducibil-ity was significantly better with a maximum daily 2 value of627 (53 ppm).dReported Hf isotope ratios are corrected for mass bias using 179Hf/177Hf? 0.7325 [Patchett and Tatsumoto, 1981] and normalized to176Hf/177Hf? 0.282160 for the ULB-JMC 475 reference material [Vervoort and Blichert-Toft, 1999] using the daily average of standard analyses.JMC 475 was analyzed every two samples and over the course of analysis gave a mean (6 2) of 176Hf/177Hf? 0.28217326 24 (86 ppm)(n? 24). On a per session basis, reproducibility was significantly better with daily 2 values ranging from 34 to 61 ppm.eReported Pb isotope ratios were corrected for mass bias by Tl doping [White et al., 2000] and are normalized to 208Pb/204Pb? 36.7219,207Pb/204Pb ?15.4963, 206Pb/204Pb? 16.9405 for the SRM981 standard [Galer and Abouchami, 1998] by sample-standard bracketing. Replicateanalysis of SRM981 over the course of analysis yielded in-run mean6 2 values of 208Pb/204Pb? 36.71986 91 (247 ppm),207Pb/204Pb? 15.49986 34 (220 ppm), and 206Pb/204Pb? 16.94346 29 (168 ppm) (n? 43). On a per session basis, reproducibility was signifi-cantly better with daily 2 values ranging from 74 to 186 ppm, 62 to 176 ppm, and 45 to 139 ppm for 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb,respectively.fENd calculated using a CHUR value of143Nd/144Nd? 0.512638 [Jacobsen and Wasserburg, 1980].gEHf calculated using CHUR value of176Hf/177Hf? 0.282772 [Blichert-Toft and Albarede, 1997].h2SE values (twice the standard errors) apply to the last decimal place(s) and are the internal absolute errors values for individual sample analyses.idup designates full procedural duplicates starting with a new sample powder aliquot; reproducibilities are similar to, or better than, the reprodu-cibilities determined through repeat standard analysis (values listed above).Data in italics are from Mullen and McCallum (submitted manuscript, 2013).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913134Figure 4. (a) ENd versus87Sr/86Sr; (b) EHf versus ENd. Symbols and data references as in Figure 3, plus Las-sen and Adams data in Figure 4b from Borg et al. [2002] and Jicha et al. [2008]. Note that all isotope data arenormalized to the same isotope standard values as described in main text. For the BSM and Mt. Baker data,2 error bars (external reproducibilities) are smaller than symbols. Inset in Figure 4a compares 87Sr/86Srmeasured in the present study (2 error bars? 20 ppm; smaller than symbol size) to 87Sr/86Sr measured onthe same samples by Green and Sinha [2005] (2 error bars? 100 ppm). The mantle array in EHf versus ENdspace is from Chauvel et al. [2008]. Orange and blue curves show the effect of adding 2% bulk sediment(blue curve with long dashes), 2% sediment fluid (blue curves with short dashes), 2% sediment melt (solidblue curves), 10% metabasalt fluid (orange dashed curve), and 2% metabasalt melt (orange solid curve) to themantle prior to 5% equilibrium partial melting of a primitive mantle composition [Sun and McDonough,1989]. Each curve has two tick marks indicating 1% and 2% addition, except for the metabasalt fluid curve(ticks at 5% and 10% addition). Slab fluid and melt compositions calculated using equilibrium melting/dehy-dration equations with FL?0.05. Trace element compositions for sediment and metabasalt are from Carpent-ier et al. [2013] (average of bulk ODP sites 888 and 1027) and Becker et al. [2000] (900C eclogite),respectively. Sediment isotope composition is the average of ODP sites 888 and 1027 from Carpentier et al.[2010]. Metabasalt Sr and Nd isotope ratios are from Staudigel et al. [1995] and the Hf isotope ratio is the av-erage of Explorer MORB shown here. Partition coefficients from Kessel et al. [2005] at 700C, 4 GPa (all flu-ids); 1000C, 4 GPa (metabasalt melt and sediment melt 1), and Hermann and Rubatto [2009] at 1050C, 4.5GPa (sediment melt 2).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913135similar to Mt. Baker basalts, plotting within theCascade arc array and closer to the field definedby subducting sediment (Figure 3). Suite 2 alsoincludes two Mt. Baker basalts that are isotopi-cally anomalous (in Sr and Nd) relative to moreprimitive Mt. Baker basalts ; Cathedral Crag(MB1) and one Sulphur Creek sample (Lib21) plotcloser to the field defined by subducting sediment(Figure 4a). As a group, Suite 2 has lower MgOand generally higher SiO2 than other basalts fromtheir respective volcanic centers (Figure 5).5.2 Trace Elements[25] Trace element abundances are reported inTable 2 and plotted in Figures 6?9. With theexception of Suite 2 (discussed later), the BSMbasalts have LILE and Pb abundances similar toMt. Baker basalts, but much higher HFSE (Figures6 and 8a). BSM basalts have substantially lowerZr/Nb and Ba/Nb than Mt. Baker basalts (Figure8b) and higher (La/Yb)N and (Dy/Yb)N (Figure 7).Among the BSM basalts, Salal Glacier has thehighest (La/Yb)N and lowest Yb and displays theleast variability among samples (Figure 7a).Bridge River and Mt. Meager have variable REEabundances, and the lowest (La/Yb)N occurs atMt. Meager (Figure 7b). However, Mt. Baker sam-ples extend to lower (La/Yb)N and higher Yb val-ues than the BSM basalts (Figure 7). Salal Glacierand Bridge River have no Nb anomalies whilesmall negative anomalies occur at Mt. Meager andprominent ones at Mt. Baker (Figure 9a). Ba/Lavalues are lowest at Salal Glacier and BridgeRiver, intermediate at Mt. Meager, and highest atMt. Baker (Figure 9b). Abundances of all traceelements in Bridge River and Salal Glacier primi-tive basalts are similar to samples from nonsub-duction settings, including Hawaiian postshieldalkalic basalts [Hanano et al., 2010], and overlapwith alkalic basalts from the Anahim volcanic belt[Charland et al., 1995], Cascade-Columbia tran-sect [Leeman et al., 2005; Jones, 2002], andDalles Lakes north of Mt. Rainier [Reiners et al.,2000] (Figures 8, 9a, and 9c).[26] The five basalts comprising Suite 2(BRC03?4, BRC01?3, MM01-1, MB1, andLib21) have trace element abundances that con-trast with other basalts at their respective vol-canic centers, including significantly higherLILE (La/Yb)N and Ba/Nb, and lower HFSE(Figures 6b?6e). These samples are excludedfrom the following discussion of mantle sourcecharacteristics but are revisited later in the contextof crustal assimilation.6. Discussion6.1. Mantle Source Characteristics6.1.1. Temperatures and Pressures[27] The BSM basalts segregated from their man-tle source at significantly higher pressures andtemperatures than the Mt. Baker basalts (Figure10). Liquidus pressures and temperatures (i.e.,mantle potential temperatures) were calculated forthe two most primitive basalts at each BSM vol-canic center (Table 3) using the olivine-liquid geo-thermometer and silica activity geobarometer ofPutirka [2008]. Whole-rock data [Green andSinha, 2005] were first adjusted into equilibriumwith Fo90 mantle by incremental olivine additionFigure 5. 208Pb/204Pb versus (a) wt % SiO2, and (b) wt % MgO. Note reversed scale for MgO.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913136Table 2. Trace Element AbundancesSample Number BRC01?3 BRC02 BRC03?4 BRC04 BRC05-1 BRC06 BRC07-2 BRC09-3 BRC10 SG01-1Methoda 1 1 2 1 1 1 2 1 1 1Concentrationb (ppm)Li 7.2 8.8 10 8 7.7 8.0 8.2 6.1 6.4 4.4Cs 0.17 0.22 0.46 0.17 0.14 0.18 0.13 0.07 0.09 0.19Rb 11 15 16 13 11 14 12 4.0 7.7 12Ba 413 310 450 303 269 288 286 195 176 269Th 2.0 1.6 2.2 1.5 1.5 1.5 1.8 0.94 0.87 1.7U 0.66 0.64 0.65 0.41 0.57 0.60 0.63 0.34 0.35 0.63Nb 14 27 16 25 26 26 28 17 15 29Ta 0.87 1.9 0.92 1.4 1.7 1.8 1.6 1.2 1.1 1.8La 24 20 24 18 20 17 22 14 12 20Ce 54 45 53 43 48 42 50 30 26 43Pb 4.2 2.8 3.9 3.0 2.2 2.6 2.3 1.7 1.3 2.1Pr 6.7 5.7 6.8 5.6 6.3 5.6 6.7 4.2 3.6 5.5Sr 1251 626 1213 577 587 650 628 399 388 545Nd 28 26 28 25 28 24 29 18 16 24Sm 5.2 6.0 5.5 5.9 6.7 5.9 6.8 4.7 4.3 5.5Zr 127 174 143 168 200 168 213 127 108 154Hf 3.4 4.1 3.3 4.1 4.6 3.9 4.8 3.2 2.9 3.7Eu 1.6 2.0 1.8 1.9 2.2 1.8 2.3 1.7 1.5 1.7Gd 4.2 6.2 4.8 6.0 6.5 5.8 6.6 5.1 4.7 5.2Tb 0.58 0.89 0.68 0.86 0.98 0.85 0.96 0.75 0.74 0.73Dy 3.4 4.7 3.9 4.9 5.6 4.9 5.7 4.3 4.0 4.0Y 19 25 21 27 31 26 30 25 23 22Ho 0.66 0.91 0.74 0.90 1.0 0.84 1.1 0.82 0.75 0.69Er 1.7 2.3 2.0 2.3 2.8 2.4 2.9 2.2 1.9 1.9Tm 0.24 0.32 0.31 0.39 0.31 0.31 0.28 0.27Yb 1.4 1.9 1.7 1.8 2.3 1.9 2.4 1.8 1.5 1.5Lu 0.20 0.27 0.23 0.26 0.33 0.24 0.34 0.26 0.22 0.22Sc 18 20 19 20 24 20 23 23 22 20Zn 85 101 98 111 122 106 126 98 107 100Cr 48 98 47 100 44 115 34 325 310 321Ni 44 50 41 51 36 56 32 134 228 281V 202 211 216 207 284 207 282 196 200 199Ga 23 24 24 27 24 21 22 21Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Concentration (ppm)Li 7.1 6.5 6.6 5.6 6.4 6.5 7.5 7.7 5.4 8.3Cs 0.20 0.17 0.15 0.14 0.18 0.10 0.08 0.13 0.08 0.44Rb 18 17 16 12 17 8.8 7.6 12 8.2 15Ba 339 330 330 247 264 676 249 265 154 659Th 2.2 2.1 1.9 1.7 1.7 1.7 1.1 1.2 0.65 2.4U 0.73 0.76 0.71 0.54 0.66 0.63 0.71 0.54 0.34 0.79Nb 31 31 32 26 26 12 17 19 9.1 6.3Ta 1.7 1.5 1.6 1.9 1.4 0.7 1.1 1.3 0.63 0.31La 23 24 22 18 18 27 13 15 8.4 24Ce 49 49 47 40 43 62 30 31 21 53Pb 2.2 2.1 2.2 2.3 2.0 5.2 1.9 2.0 1.4 3.8Pr 6.2 6.1 6.0 5.2 5.6 8.4 4.0 4.2 3.0 7.1Sr 658 589 585 508 624 1588 479 545 467 1455Nd 26 25 26 23 24 35 17 18 14 30Sm 5.5 5.4 5.7 5.1 5.3 6.7 4.2 4.4 4.0 5.8Zr 171 161 164 141 150 147 112 122 92 93Hf 3.6 3.8 3.7 3.3 3.5 3.4 2.9 3.0 2.7 2.4Eu 1.8 1.9 1.9 1.6 1.8 2.1 1.5 1.6 1.4 1.8Gd 5.1 5.3 5.1 5.0 5.2 5.5 4.2 4.8 4.3 4.7Tb 0.72 0.76 0.78 0.68 0.76 0.71 0.65 0.68 0.64 0.61Dy 4.1 4.0 4.4 3.7 4.3 4.0 3.8 3.8 3.9 3.4Y 22 23 25 21 21 21 20 22 21 18Ho 0.77 0.77 0.81 0.69 0.73 0.77 0.69 0.74 0.69 0.65Er 2.0 2.0 2.2 1.7 2.0 2.0 1.8 1.9 1.9 1.8Tm 0.29 0.30 0.24 0.26 0.24 0.27 0.25MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913137assuming Fe3?/PFe? 0.15. Amounts of olivineadded range from 11.5% (SG16) to 17.7%(BRC09). Primary biotite and amphibole in thegroundmass of some primitive BSM samples attestto the presence of water, so pressures and tempera-tures were calculated for several possible H2Ocontents (listed in Table 3). The plagioclase-liquidhygrometer of Lange et al. [2009] applied to themost primitive Salal Glacier basalt gives 1 wt %H2O at the liquidus assuming plagioclase satura-tion at P? 100 MPa and maximum An60 in plagio-clase cores [Lawrence, 1979]. At this watercontent, pressures calculated for primitive BSMbasalts correspond to depths ranging from 70 km(Mt. Meager) to 105 km (Bridge River) (Figure10). Decreasing melt SiO2 with increasing pres-sure [Longhi, 2002] is consistent with the P-Tdata. Calculated mantle potential temperatures are100?200C higher than predicted for the Cas-cade subarc mantle wedge [Syracuse et al., 2010],and similar to those of average MORB (1454C)[Putirka, 2008]. Intraplate basalts of the westernBasin and Range province give a broadly similarP-T range (60?90 km, 1350C1450C) as theBSM basalts [Lee et al., 2009]. Intraplate basaltsin the Cascade-Columbia transect have lower max-imum segregation depths (75?80 km) but similarmaximum temperatures (1460C), although an-hydrous conditions were assumed [Leeman et al.,2005]. Simcoe intraplate basalts record P-T condi-tions similar to the BSM basalts (max 100 km,1500C) [Leeman et al., 2005].[28] For Mt. Baker basalts, liquidus water contentsare 1.5 to 3.7 wt % (Mullen and McCallum, sub-mitted manuscript, 2013) and mantle potentialtemperatures are 1273C to 134C (Figure 10,Table 3), within the range for the subarc mantlewedge [Syracuse et al., 2010]. Mantle segregationdepths are 35 to 52 km, i.e., ranging from theMoho to just above the hot core of the mantlewedge. The shallower depths recorded by the Mt.Baker basalts are consistent with trace elementmodeling (below) that indicates residual garnet forthe BSM basalts but not Mt. Baker.6.1.2. Mantle Isotopic Characteristics[29] Primitive BSM basalts (Mg/[Mg?Fe2?]> 0.6)have isotope ratios that define a narrow range, con-sistent with a common mantle source and differen-tiation dominated by fractional crystallization. Pbisotope ratios overlap with Explorer and northernJuan de Fuca MORB [Cousens et al., 1995; B.Cousens, unpublished data 2007], Chilcotin plateaubasalts [Bevier, 1983], and the least radiogenic sam-ples from the Anahim volcanic belt [Bevier, 1989](Figure 11). The isotopic similarity among thesevolcanic provinces confirms that the northwesternmargin of North America is underlain by uppermantle that is relatively depleted and generally sim-ilar to northeastern Pacific mantle [Cousens andBevier, 1995; Bevier, 1989].[30] Although the BSM basalts have 208Pb/204Pbsimilar to local MORBs at a given 206Pb/204Pb,207Pb/204Pb is slightly higher (Figure 3).Relatively high 207Pb/204Pb could be interpreted asreflecting subducting sediment input, but thisshould increase 208Pb/204Pb along with207Pb/204Pb, and the BSM basalts overlap withMORB in 208Pb/204Pb.[31] High 207Pb/204Pb relative to 208Pb/204Pb mayinstead indicate a higher time-integrated U/Th inthe BSM source than in the MORB sources.Table 2. (continued)Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Yb 1.7 1.7 1.7 1.4 1.5 1.7 1.5 1.5 1.6 1.5Lu 0.24 0.23 0.26 0.21 0.22 0.25 0.21 0.22 0.22 0.22Sc 20 20 22 19 20 23 20 22 21 25Zn 102 93 103 95 106 100 106 102 106 91Cr 248 255 239 332 330 112 274 271 255 55Ni 227 216 130 301 285 49 188 146 156 24V 198 188 227 157 212 208 176 185 164 199Ga 20 22 17 21 20 21 22aMethod 1: Thermo Finnigan Element2 HR-ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS.bData were corrected for blank contributions and by sample-standard bracketing to published values for the USGS BCR2 reference material[Raczek et al., 2001] analyzed every eight samples (method 1), or the USGS AGV1 reference material [Chauvel et al., 2011] analyzed every sixsamples (method 2). Repeat analysis of the USGS BHVO2 standard gave RSD values of <5% and concentrations within 5% (relative) of pub-lished values (as compiled by Carpentier et al. [2013] from GeoRem) for most elements. The average BHVO2 values obtained during analyticalsessions are reported in Table S1. Duplicate analyses gave reproducibilities better than 5% (Table S1). Total procedural blanks (Table S1) werenegligible relative to analyzed sample concentrations.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913138Enrichment of the BSM mantle source in U relativeto Th at some time in the past could be accom-plished through addition of fluid or melt compo-nents derived from subducting sediment and/oroceanic crust, since U is slightly more incompatiblethan Th during dehydration and melting [Brenan etal., 1995; Kessel et al., 2005; Hermann andRubatto, 2009]. However, this situation wouldresult in the presence of a subduction signature inthe BSM mantle source, which is not observed.[32] A more plausible explanation may be meltingof the BSM mantle source in the presence of resid-ual garnet at some time in the past. Unlike othertypical mantle minerals, which do not fractionateU and Th appreciably, U is more compatible ingarnet than Th [Beattie, 1993; LaTourrette et al.,1993]. The BSM basalts also plot at the lower mar-gin of the Hf-Nd mantle array (Figure 4b), consist-ent with the isotopic evolution of mantle thatproduced melts within the garnet stability field[Carlson and Nowell, 2001].[33] Because BSM basalts have lower EHf (3 ep-silon units) than Mt. Baker but similar ENd (Figure4b), two distinct mantle sources are required. Par-titioning experiments show that Hf is preferen-tially retained in the subducting slab relative toNd, most effectively during slab dehydration butalso during slab melting [e.g., Kessel et al., 2005;Hermann and Rubatto, 2009]. High Nd/Hf in thesubduction component is further enhanced by theFigure 6. (a?d) Extended N-MORB normalized [Sun and McDonough, 1989] trace element diagrams, sub-divided by volcanic center. (d) Light gray field in each panel encompasses the range defined by Mt. Bakerbasalts. The darkest colors (with sample numbers) signify Suite 2 samples discussed in the text. (e) All of theSuite 2 samples are plotted together for comparison to Mt. Baker basalts.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913139preexisting negative HFSE anomalies that charac-terize Cascadia sediment [Carpentier et al., 2013;Prytulak et al., 2006]. As a consequence, additionof a sediment component to the BSM mantlesource generates mixing curves that extendtowards lower ENd values but with smaller changesin EHf. Most importantly, mixing trajectoriesextend away from the Mt. Baker data. Thus sub-duction input cannot account for the Hf isotopicdistinction between the BSM and Mt. Bakerbasalts; the difference is instead a primary featureof their respective mantle sources. Since mantleHf isotope ratios can be affected by both fluidsand melts derived from the slab (Figure 4b), Hfisotopes do not always directly record the isotopiccomposition of the mantle as is commonlyassumed.6.1.3. Mantle Source Fertility[34] Zr/Nb in basalts provides a useful indicator ofmantle source fertility because this ratio is mini-mally affected by subduction input or fractionalcrystallization (Figure 8b). Although Zr/Nb is con-trolled to some extent by melt fraction, the Zr/Nbrange defined by melts of average depleted mantledoes not overlap the range for melts of moreenriched mantle tapped by ocean island basalts.Mt. Baker basalts have Zr/Nb consistent with10% partial melting of average depleted mantlewith an additional slab component (Figure 8b). Zr/Nb is too low in the primitive BSM basalts to beproduced from the same mantle source as Mt.Baker, requiring a more incompatible elementenriched mantle source. The relatively high Nbcontents of the BSM basalts also indicate a sourcerelatively enriched in incompatible elements (Fig-ure 8a), as do high Na2O and TiO2 [Prytulak andElliott, 2007].6.1.4. Assessment of Subduction Input[35] For primitive samples at Salal Glacier andBridge River, Ba/Nb values lie within the range ofHawaiian basalts and coincide with melting curvesfor enriched mantle (at 2?5% partial melt) (Fig-ure 8), pointing to the likelihood that a slab-derived component was not present in the mantlesource. The absence of slab input is supported bythe absence of negative Ta-Nb anomalies (FigureFigure 7. (a?d) Chondrite-normalized [McDonough and Sun, 1995] rare-earth element diagrams subdividedby volcanic center.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913140Figure 8. (a) Ba (ppm) versus Nb (ppm); (b) Zr/Nb versus Ba/Nb. Except for Suite 2 samples (diamonds),plus MM08 from Mt. Meager, the BSM basalts are similar to basalts from MORB and OIB settings, i.e.,essentially no subduction component. MORB data (gray filled circles) and Hawaiian basalt data (shield andpostshield lavas shown as filled green and pink circles, respectively) were obtained from the PETDB (http://www.earthchem.org/petdb) and GEOROC databases (georoc.mpchmainz.gwdg.de/georoc), respectively,accessed in May 2012. Average OIB composition [Sun and McDonough, 1989] is shown as a black asterisk inFigure 8b. Black arrows in lower right corner of Figure 8a and upper right corner of Figure 8b show the effectof 15% fractionation of each mineral indicated, calculated using the Rayleigh equation, the starting composi-tion of BRC09-3, and partition coefficients listed in Table 4 plus ilmenite from McCallum and Charette[1978]. Only ilmenite and magnetite vectors are shown in Figure 8b because the other phases shown in Figure8a have a negligible effect. Orange and blue curves are for addition of subduction components to a depletedmantle source (average depleted MORB mantle of Salters and Stracke, 2004) prior to 10% partial melting,calculated as described in Fig. 4 caption. Most subduction components increase Ba at a given Nb, and Ba/Nbat a given Zr/Nb. The heavy black curves are the compositions of DM melts at 1 GPa and PM melts at 3 GPa,respectively (tick marks indicate % partial melt). Melt compositions were calculated using the equilibriummelting equation with mineral/melt partition coefficients from Table 4 and residual mantle mineral assemb-lages determined by BATCH modeling [Longhi, 2002] of starting compositions of Wasylenki et al. [2003]and Kinzler [1997]. Filled black squares with white (cross) and (plus) symbol are for DM (depleted MORBmantle) of Salters and Stracke [2004] and PM (primitive mantle) of Sun and McDonough [1989], respec-tively. Inset diagrams include data for alkalic basalts (molar Mg/(Mg? Fe2?)> 0.60, assuming Fe3?/PFe? 0.15) from the Anahim volcanic belt (dark blue squares) [Charland et al., 1995], Cascade-Columbiatransect (light blue squares) [Leeman et al., 2005; Jones, 2002], and Dalles Lakes north of Mt. Rainier (purplesquares) [Reiners et al., 2000]. Abbreviations: ol (olivine), opx (orthopyroxene), cpx (clinopyroxene), plag(plagioclase), ilm (ilmenite), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131419a) that are observed at Mt. Baker and in othercalc-alkaline Cascade arc basalts [e.g., Schmidt etal., 2008]. The absence of slab input is further sup-ported by the similarity of Salal Glacier andBridge River primitive basalts to Mauna Kea post-shield alkalic basalts [Hanano et al., 2010], whichsample the same mantle source as shield lavas,that is, a composition comparable to the PREMA,or C, mantle component [Nobre Silva et al., 2013](Figure 9c). The only major difference is in Pb,which is deficient in Hawaii (a ubiquitous featureof oceanic basalts [Hofmann, 1997]) but showssmall positive spikes at Bridge River and SalalGlacier. The Pb spikes are successfully modeledwithout subduction input (see later). However, wecannot definitively rule out the presence of a verysmall subduction component in the mantle source.An ancient subduction component may have beenadded to the source in the past, or the primaryalkalic magmas may have acquired a small sub-duction component during migration through themantle.Figure 9. (a) EHf versus Nb/Nb (niobium anomaly) for BSM and Mt. Baker basalts, compared to the rangesdefined by Hawaiian shield and postshield basalts (green and pink lines, respectively) with >8 wt % MgO.Nb/Nb calculated as 2(Nbsample/NbPM)/(Basample/BaPM?Lasample/LaPM) [Verma, 2009] where PM refers toPrimitive Mantle. Hawaii data were obtained from the GEOROC database accessed in May 2012(georoc.mpchmainz.gwdg.de/georoc). Also shown are values for average depleted mantle (DM, black square)[Salters and Stracke, 2004] and average N-MORB (gray square) and Primitive Mantle (PM, black square withwhite ?) of Hofmann et al. [1988]. (b) Ba/La versus 208Pb/204Pb for BSM and Mt. Baker basalts. (c) N-MORB normalized [Sun and McDonough, 1989] extended trace element diagram comparing Salal Glacier(orange) and Bridge River (red) primitive basalts (molar Mg/[Mg?Fe2?]> 0.60) to alkalic postshield basaltsfrom Mauna Kea (light blue) [Hanano et al., 2010], Simcoe volcanic field (dark blue) [Battleground Lakesample of Jones, 2000], and Anahim volcanic belt (green) [sample 2278 of Charland et al., 1995].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913142[36] In contrast to Salal Glacier and Bridge River,even the most primitive Mt. Meager basalts haveslightly elevated Ba and Ba/Nb relative to mantlemelting curves and Hawaiian basalts (Figure 8), aswell as small negative Nb anomalies (Figure 9a),all of which point to subduction input (althoughsignificantly less than at Mt. Baker). Zr/Nb in thetwo of the three primitive Mt. Meager basalts isthe same as at Salal Glacier and Bridge River,indicating similar mantle sources. In the other Mt.Meager sample (MM08), higher Zr/Nb and lowerNb is consistent with a mantle source that is transi-tional between the Mt. Baker mantle source andthat of the other BSM basalts. An intermediate Hfisotopic composition for MM08 (Figure 4b) sup-ports this conclusion.[37] Mt. Meager basalts have the lowest206Pb/204Pb and 208Pb/204Pb of all the BSM vol-canic centers; sample MM08 has the lowest208Pb/204Pb, 206Pb/204Pb, and (La/Yb)N, coupledwith the highest Ba/La (Figure 9b). These charac-teristics are not consistent with addition of a sub-ducting sediment component to the mantle sourceand may instead reflect the influence of a fluidderived from altered oceanic crust (AOC). AOCfluid input can increase LILE in the mantle sourcewithout affecting LREE [Kessel et al., 2005], andsince recent AOC has MORB-like Pb isotoperatios, it is capable of ??pulling?? Pb isotope ratiosof the mantle source to lower values. Mt. Meageralso has similar ENd to Bridge River and Salal Gla-cier but slightly higher 87Sr/86Sr (Figure 4a), con-sistent with the involvement of AOC that acquireshigh 87Sr/86Sr with minimal change in ENd duringprogressive seafloor alteration [Staudigel et al.,1995].6.1.5. Trace Element Modeling[38] Mantle melt fractions and residual mantle min-eral assemblages were determined by modeling theabundances of 28 trace elements in the two mostprimitive basalts at each BSM center (BRC09 andBRC10 at Bridge River, MM04 and MM02 at Mt.Meager, SG10 and SG16 at Salal Glacier). We alsomodeled MM08 at Mt. Meager because it may havea slightly more depleted mantle source.[39] The model is based upon the mass balanceequation for equilibrium melting, CLi = C0i ?Figure 10. Pressure versus temperature plot illustrating theconditions at which the BSM and Mt. Baker magmas segre-gated from the mantle. P and T (from Table 3) were calcu-lated using the silica activity geobarometer and olivine-liquidgeothermometer calibrations of Putirka [2008] for 1% and2% dissolved water (BSM basalts) or for the specific H2Ocontent given (Mt. Baker basalts). Standard estimates of errorare 43C and 0.29 GPa [Putirka, 2008].Table 3. Liquidus pressures and temperaturesH2O (wt.%)0.0 1.0 2.0P(GPa) T(C) P(GPa) T(C) P(GPa) T(C)Bridge RiverBRC10 3.49 1576 3.19 1535 2.89 1496BRC09 2.68 1505 2.50 1470 2.31 1437Salal GlacierSG10 2.82 1514 2.61 1478 2.41 1443SG16 2.67 1502 2.48 1467 2.29 1433Mt. MeagerMM02 2.04 1441 1.91 1409 1.79 1379MM04 2.15 1460 2.02 1427 1.86 1395a Mt. Baker P (GPa) T (C) H2O (wt.%)MB5 0.90 1273 2.7MB82 1.4 1326 2.1MB1 1.1 1274 3.7MB97 1.2 1309 2.1MB114 1.4 1326 15MB112 1.5 1350 1.5aP-T data for Mt. Baker (from Mullen and McCallum, submitted manuscript, 2013) are calculated only at the specific water content listed foreach sample.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131431= FL 1 Di ? Di [Shaw, 1970], where CLi isthe concentration of trace element (i) in the liquid(L), C0i is the initial concentration of trace element(i), FL is melt fraction, and Di is the bulk partitioncoefficient, defined as crystalline assemblage/melt.The model does not require a priori knowledge ofinitial mantle mineral assemblages but does requireinitial trace element abundances. The primitivemantle composition of Sun and McDonough [1989]was used as the source for all BSM samples, and amixture of 50% primitive mantle and 50% depletedmantle [Salters and Stracke, 2004] was also testedfor MM08. Distribution coefficients used in the cal-culations are listed in Table 4. Least-squares mini-mization was used to generate best fit models forthe basalts by varying the mantle mineral modesand melt fractions (see the caption of Figure 12).Note that the substitution of fractional melting inour model results in negligible change to modeloutcomes. Melt fractions are within 0.5% and resid-ual mantle modal abundances change by less than afew percent, with overall residual mineral assemb-lages remaining identical.6.1.5.1. Modeling Results[40] Representative best fit trace element solutionsare shown in Figure 12. Melt fractions are 2?4%for Salal Glacier, 4?5% for Mt. Meager, and 7?8%for Bridge River, all with residual garnet lherzo-lite. Lower melt fractions for Salal Glacier basaltsare consistent with their higher alkali elementabundances. For Mt. Meager sample MM08, aprimitive mantle source indicates 8% partial meltand the mixed PM-DM source gives 4%. The latterresult is preferred because it is more consistentwith the results for other Mt. Meager samples. Re-sidual garnet in all samples is consistent with thepressures of melting (2?3 GPa) calculated for theBSM basalts, as garnet is stable at the solidus ofhydrous mantle at pressures above 1.6 GPa [Gae-tani and Grove, 1998].[41] Results of similar modeling for Mt. Bakerbasalts, using a depleted mantle source, indicate5?12% partial melting of depleted lherzolite orharzburgite. Best fit solutions require overprintingby a subduction component consisting of AOCfluid, AOC melt, and sediment melt (Mullen andMcCallum, submitted manuscript, 2013). No re-sidual garnet is present in the Mt. Baker source,consistent with calculated melt segregation pres-sures (1?1.5 GPa) and with the lower (Dy/Yb)N,and higher Yb and Sc contents of the Mt. Bakersamples (24?33 ppm Sc) (Mullen and McCallum,Figure 11. Plot of 208Pb/204Pb versus 206Pb/204Pb comparing Pb isotope ratios for the BSM and Mt. Bakerbasalts (symbols as in Figure 9) to other basalts from the northeastern Pacific and southwestern British Colum-bia: Anahim volcanic belt (purple diamonds; Bevier [1989]); Chilcotin Plateau (black diamonds; Bevier[1983]); Wells Gray-Clearwater volcanic field (blue diamonds; Hickson [1987]); and Tuzo Wilson volcanicfield [Allan et al., 1993]. Other data and references as in Figure 3. Note that all isotope data are normalized tothe same standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913144submitted manuscript, 2013) as compared with theBSM basalts (18?24 ppm Sc; Table 2).6.4. Nonprimitive BSM Basalts: CrustalContamination or Subduction Input?[42] Although we make the case above that theprimitive Salal Glacier and Bridge River basaltsessentially lack a subduction component, some ofthe nonprimitive BSM basalts have geochemicalcharacteristics that could be interpreted as an ??arcsignature.?? Do these reflect subduction input thatis not displayed by more primitive samples?[43] BSM samples with Mg/(Mg? Fe2?)< 0.60are subdivided into two suites based upon isotopicand trace element compositions: Suite 1 has iso-tope and trace element ratios similar to primitiveBSM basalts, indicating minimal crustal contami-nation and differentiation processes dominated byfractional crystallization. Relative to the mostprimitive basalts, Suite 2 has high Sr-Pb isotoperatios, (La/Yb)N, and incompatible element abun-dances, coupled with low Nd-Hf isotope ratios andHFSE abundances.[44] Pearce element ratio diagrams [Russell andNicholls, 1988] show that the Suite 1 basalts areconsistent with fractionation of olivine? plagio-clase (6 minor clinopyroxene) from parental mag-mas that were similar to the most primitive basaltsat each volcanic center. The crystallizing assemb-lages are consistent with the presence of the sameminerals as phenocryst phases.[45] For Suite 2 samples, trace element abundan-ces and Pb isotope ratios are nearly indistinguish-able from the most primitive Mt. Baker basalts(Figures 3, 6e, and 8), pointing to the possibilitythat they may record input from the subductingslab as does Mt. Baker. However, Suite 2 has sig-nificantly higher 87Sr/86Sr and lower ENd than themost primitive Mt. Baker and BSM basalts (Figure4a). The lower MgO contents of Suite 2 lavas rela-tive to the most primitive lavas are consistent withTable 4. Partition Coefficients (Mineral/Melt)acpx opx oliv sp gar amph phlog mt plagCs 0.000201 0.00091 0.00004525 0.000625 0.00011 0.02325 2.261 0.00121 0.00623Rb 0.000603 0.00381 0.0000453 0.000625 0.00022 0.0232 1.702 0.00121 0.01823Ba 0.000683 0.00361 0.0000433 0.00067 0.000072 0.012 1.502 0.00121 0.3223Th 0.0124 0.00051 0.000053 0.0107 0.00212 0.00102 0.000201 0.002420 0.1923U 0.0134 0.00071 0.000053 0.0147 0.00947 0.00122 0.000201 0.01220 0.3423Nb 0.0051 0.00071 0.000413 0.0217 0.00315 0.082 0.0551 0.8620 0.00823Ta 0.0211 0.00081 0.00021 0.0217 0.0199 0.0831 0.0621 0.9520 0.02723K 0.00725 0.00012 0.000022 0.00125 0.0132 0.222 1.502 0.00121 0.09721La 0.0546 0.00061 0.000053 0.0119 0.001615 0.05524 0.0002525 0.001220 0.1123Ce 0.08625 0.00171 0.000063 0.0119 0.00515 0.09625 0.000301 0.001920 0.08525Pb 0.0104 0.00011 0.0000712 0.00057 0.000315 0.0424 0.091 0.02221 0.108523Pr 0.1425 0.002625 0.0001325 0.0119 0.02925 0.1316 0.000425 0.002320 0.06525Sr 0.04811 0.0093 0.0002512 0.00477 0.002515 0.3015 0.161 0.003020 1.9423Nd 0.1925 0.0041 0.000203 0.0119 0.05215 0.18716 0.000551 0.00425 0.05223Sm 0.276 0.0111 0.000603 0.0119 0.2515 0.3216 0.000701 0.007020 0.04123Zr 0.061 0.0133 0.0006810 0.00817 0.6614 0.1816 0.0111 0.5620 0.003923Hf 0.121 0.0133 0.001110 0.00307 0.6814 0.6316 0.0161 0.6520 0.001523Eu 0.4525 0.01625 0.0008025 0.0119 0.4015 0.4316 0.000725 0.01025 1.4223Ti 0.308 0.06110 0.00221 0.04819 0.2915 0.952 0.791 2021 0.04723Gd 0.5025 0.02225 0.000993 0.0119 0.9025 0.5416 0.000725 0.01620 0.03521Tb 0.5625 0.0301 0.0023 0.0119 1.415 0.6025 0.00071 0.02325 0.03121Dy 0.6125 0.03825 0.0043 0.0119 2.215 0.6325 0.000825 0.03325 0.02621Y 0.6525 0.0461 0.0073 0.00207 3.115 0.5215 0.0031 0.0520 0.02621Ho 0.656 0.0481 0.0063 0.0119 2.815 0.6224 0.00091 0.0525 0.01821Er 0.6925 0.05825 0.00873 0.0119 3.6 15 0.5724 0.001025 0.0725 0.014521Tm 0.7225 0.0711 0.01325 0.0119 3.725 0.5325 0.00141 0.01125 0.01221Yb 0.7425 0.0771 0.0173 0.0119 3.94 0.4825 0.001625 0.1725 0.009721Lu 0.756 0.0901 0.0203 0.0119 3.84 0.4324 0.00171 0.2820 0.00821aAbbreviations: cpx (clinopyroxene), opx (orthopyroxene), oliv (olivine), sp (spinel), gar (garnet), amph (amphibole), phl (phlogopite), mt(magnetite), plag (plagioclase).Data sources: 1Adam and Green [2006]; 2Halliday et al. [1995] compilation; 3Donnelly et al. [2004] compilation; 4Hauri et al. [1994]; 5Hartand Dunn [1993]; 6Gaetani [2004]; 7Elkins et al. [2008]; 8McDade et al. [2003]; 9Green et al. [2000]; 10Kennedy et al. [1993]; 11Beattie[1993];12Beattie [1994]; 13Canil and Fedortchouk [2001]; 14Salters and Longhi [1999]; 15Abraham et al. [2005] compilation; 16Chazot et al.[1996]; 17Horn et al. [1994]; 18Nagasawa et al. [1980]; 19McKenzie and O?Nions [1991]; 20Klemme et al. [2006]; 21Claeson and Meurer [2004]compilation; 22Dunn and Sen [1994]; 23Tepley et al. [2010]; 24LaTourrette et al. [1995]; 25Interpolated from neighboring elements.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913145crustal contamination (Figure 5b). Furthermore,Suite 2 contains the only two BSM basalts withxenocrysts (BRC01?3, BRC03?4).[46] The GVB crustal basement is a collage of Pa-leozoic and Mesozoic accreted terranes [Mongeret al., 1982]. At depths greater than 10 km, theGVB is underlain by the composite Wrangellia?Harrison terranes [Mullen, 2011; Miller et al.,2009; Monger and Price, 2000]. With the excep-tion of the Mt. Baker region, the terranes areintruded extensively by Jurassic to Cretaceousgranitoids of the Coast Plutonic Complex, the larg-est composite batholith in North America [Barkerand Arth, 1984; Friedman et al., 1995; Cui andRussell, 1995a, 1995b]. Because the crust is rela-tively young and Cascadia subducting sediment ismainly terrigenous [Carpentier et al., 2013; Pry-tulak et al., 2006], the isotopic effects of crustalassimilation are similar in many respects to theeffects of subducting sediment input. However,sediment input cannot account for the 87Sr/86Srversus Sr systematics of the Suite 2 lavas (Figure13a). Assimilation-fractional crystallization (AFC)modeling [DePaolo, 1981] using a granodioriticassimilant from the Coast Plutonic Complex canreproduce the Suite 2 trace element and isotopicdata, but the volume of assimilant required(>20%) would increase the SiO2 content beyondthe range of the Suite 2 samples (Figure 5a).Assimilation that takes place in the deep crust,Figure 12. Best fit trace element solutions for four of the most primitive BSM basalts, shown on N-MORBnormalized extended element diagrams and accompanying inset chondrite-normalized REE diagrams. Actualdata are shown with colored lines and symbols; modeling solutions shown as heavy black lines with blacksquares. Each BSM sample has been adjusted into equilibrium with Fo90 mantle using olivine/melt partitioncoefficients from Table 4. The mantle source composition used in the model for all BSM basalts (PM of Sunand McDonough [1989]) is shown as a thin black line in each panel; DM (source used for Mt. Baker) is alsoshown for reference [Salters and Stracke, 2004]. The Generalized Reduced Gradient (GRG2) nonlinear opti-mization code in Microsoft Excel Solver was used to obtain the best fit for each basalt by minimizing the sumof squares of residuals for 28 trace elements, i.e.,Pi Cliqi calc? ?  Cliqi obs? ?h i=Cliqi ?obs?n o2. The denomina-tor in the equation normalizes the concentrations of the elements so that each trace element has an equivalentimpact on the solution regardless of its absolute concentration. Best fit melt fractions (FL) and residual mantlemineral modes are given in lower right corner of each N-MORB-normalized panel. Abbreviations: ol (oli-vine), opx (orthopyroxene), cpx (clinopyroxene), gar (garnet).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913146where the country rock is mafic, can minimizechanges to the major element abundances of theoriginal basaltic magma [Reiners et al., 1995,1996]. AFC models in which the assimilant is agabbro from the lower crustal section of the Bo-nanza arc (Westcoast crystalline complex) ofWrangellia [DeBari et al., 1999] provide good fitsto Suite 2 trace element and isotope data with15% gabbro assimilated at Bridge River and21% at Mt. Meager (Figure 13). Modeling pa-rameters and results are provided in Table 5. Frac-tionating mineral phases (olivine, clinopyroxene,orthopyroxene, and minor magnetite) are consist-ent with experimental results for partial melting ofmafic compositions under lower crustal conditions[Rapp, 1995; Rapp and Watson, 1995]. Becausethe assimilant has a low SiO2 content (45 wt %)and the fractionating mineral assemblages havebulk SiO2 contents similar to the basalts, the finalmagmas maintain an overall basaltic compositionin the magmas.6.5. Relationship Between Tectonics andVolcanism[47] An incompatible element-enriched, garnet-bearing mantle source essentially free of subduc-tion input, coupled with relatively high mantlemelting temperatures and pressures, is consistentwith decompression melting of an upwelling as-thenosphere source for the primitive BSM basalts.Upwelling mantle is potentially consistent with aslab edge effect as proposed by Lawrence et al.[1984] for Salal Glacier basalts. Seismic anisot-ropy measurements reveal toroidal mantle flowaround the descending edges of subducted platesthat are undergoing rollback, thereby drawingexternal mantle (subslab) into the mantle wedgeFigure 13. AFC modeling results for (a) 87Sr/86Sr versus Sr; (b) ENd versus87Sr/86Sr; (c) Zr/Nb versus Ba/Nb. Heavy green curves with triangles and squares (AFC1 and AFC2, respectively) are the best fitassimilation-fractional crystallization pathways for Suite 2 samples BRC03?4 and MM01-1, respectively,using a gabbroic assimilant (from Table 5). The large green triangle and square are the gabbro compositionsused as assimilants in AFC 1 and 2, respectively. Orange and blue curves in Figure 13a are slab fluid/meltaddition curves, calculated as described in Fig. 4 caption. Heavy black curves in Figure 13c are from Figure8b. Data for the Coast Plutonic Complex shown as small blue triangles and light blue field [Friedman et al.,1995; Cui and Russell, 1995a, 1995b]. Sources of other data shown are given in Figure 4a caption.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913147[Long and Silver, 2008]. In other arcs, influx ofexternal mantle has been implicated in the genesisof lavas that are atypical for an arc setting [e.g.,Leat et al., 2004; Smith et al., 2001; Ferrari et al.,2001]. Slab rollback is occurring in the Cascadearc [Schellart, 2007], but the limited mantle ani-sotropy measurements in the GVB are inconclu-sive as to mantle wedge flow patterns [Currie etal., 2004]. Toroidal mantle flow has been docu-mented at the southern Juan de Fuca plate edge[Zandt and Humphreys, 2008], yet alkalic basaltsare not present [Hildreth, 2007] indicating that thetwo phenomena are not necessarily interrelated.[48] A slab edge origin may be improbable for theBSM volcanic centers in light of recent seismic to-mography, which indicates the northernmost slabedge in the Cascades (placed at the northernmostlimit of the Explorer plate) is located farther norththan the BSM volcanic centers [Mercier et al.,2009; Audet et al., 2008]. Toroidal mantle flowhas been proposed for the northern Explorer plateedge [Audet et al., 2008] and could be responsiblefor the alkalic basalts of the 500 km long Anahimvolcanic belt, which defines an east-west trendnearly orthogonal to, and north of, the GVB. Thisinterpretation is consistent with that of Thorkelsonet al. [2011] who proposed that Anahim magma-tism is related to mantle upwelling along the ther-mally eroding plate margins of the NorthernCordilleran slab window. However, eruption agesin the Anahim volcanic belt define an easterlytime progression that has been attributed to a hot-spot [Bevier, 1989], and tomographic results areconsistent with either interpretation [Mercier etal., 2009].[49] The BSM alkalic basalts may be related tomantle upwelling at the boundary between theJuan de Fuca and Explorer plates, as illustratedschematically in Figure 14. The northern segmentof the Juan de Fuca plate has had a complex tec-tonic history; about 4 Myr ago, the northernmostportion of the Juan de Fuca plate separated alongthe Nootka fault zone to form the independentExplorer microplate [Riddihough, 1984] (Figure1). Although convergence has ceased at the north-ern edge of the microplate, the southernmost partof the microplate continues to subduct slowly[Braunmiller and Nabelek, 2002], and the entireTable 5. AFC Modeling Parameters and ResultsCompositionAFC 1: Bridge River AFC 2: Mt. MeagerSample Modeled: BRC03?4 Sample Modeled: MM01-1Initial Magma: Assimilant: Initial Magma: Assimilant:BRC09-3a Gabbrob MM08a GabbrobSiO2 (wt %) 45.6 44.6 48.6 44.6TiO2 1.8 1.01 1.4 1.01MgO 15.6 6.57 14.5 6.57Na2O 2.6 1.31 2.9 1.31K2O 0.7 0.40 0.6 0.40Sr (ppm) 400 401 467 401Nd 18.5 5.0 14 5.0Ba 195 153 154 153Zr 127 21 92 21Nb 16.5 1.0 9 1.087Sr/86Sr 0.702986 0.7034 0.703164 0.7040143Nd/144Nd 0.513026 0.51286 0.513030 0.512820ENd ?7.6 ?4.3 ?7.6 ?3.6AFC resultsr c 0.90 0.89FLd 0.83 0.77olive 0.05cpx 0.05 0.10opx 0.05 0.10mt 0.02 0.03aTrace element and isotope data for initial magmas are from Table 1; major element data [from Green and Sinha, 2005] are corrected into equi-librium with Fo90 mantle.bMajor and trace element data for gabbro assimilant are from DeBari et al. [1999] for sample 91-17 of the Westcoast Crystalline Complexexcept Nd (interpolated); isotope ratios selected from within the range defined by the Coast Plutonic Complex [Cui and Russell, 1995b].cmass assimilated/mass crystallized.dfraction liquid remaining.eFraction of each mineral phase removed from magma; sum is equal to (1FL).fAbbreviations: oliv (olivine), cpx (clinopyroxene), opx (orthopyroxene), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913148Explorer region is a zone of strong shear deforma-tion [Dziak, 2006]. The offshore segment of theNootka fault shows left-lateral motion along a rup-ture and the onshore extension of the fault ismarked by thinning and deformation of the sub-ducting plate [Audet et al., 2008]. Seismic dataindicate the Explorer plate currently has a shal-lower dip than the Juan de Fuca plate, which maymanifest itself in a near-vertical gap between theplates (Figure 14). The BSM volcanic centers lieon, or just south of, the Nootka fault zone as ex-trapolated to the northeast (Figure 1a). We suggestthat thinning, deformation, and possible rupture ofthe subducted Explorer plate fragment may pro-vide a pathway for asthenospheric upwellingaccompanied by decompression melting. Farthereast along the projected trace of the Nootka fault,the Wells Gray-Clearwater volcanic field and Chil-cotin basalts have been similarly attributed toenriched asthenosphere upwelling through a gapalong the fault [Madsen et al., 2006; Sluggett,2008; Thorkelson et al., 2011].[50] Seismic tomography is inconclusive as towhether the Nootka fault is ??leaky?? or whethercontinuity is maintained at depth between theExplorer and Juan de Fuca plates [Mercier et al.,2009]. However, as the Explorer plate is situatedat the southern edge of a slab window, it is subjectto progressive thermal and physical degradationthat would facilitate passage of mantle melts frombelow [Thorkelson et al., 2011; Thorkelson andBreitsprecher, 2005]. In an analogous situation inthe Mexican arc, seismic anisotropy measurementsare consistent with plate separation. Faults sepa-rate the subducting Cocos plate into several seg-ments, and each subducts at a different angle,resulting in a scissors-like effect in which gapsbetween the plates allow for mantle upwellingthrough toroidal flow [Stubailo et al., 2012].7. Summary and Conclusions[51] Alkalic basalts at the Bridge River, Salal Gla-cier, and Mt. Meager volcanic centers (BSM vol-canic centers) of the Canadian segment of theCascade arc, known as the Garibaldi volcanic belt,have intraplate characteristics that contrast withtypical calc-alkaline mafic Cascade arc lavas. Newhigh precision Sr-Nd-Hf-Pb isotope ratios andtrace element abundances reveal that the mostprimitive basalts at Salal Glacier and Bridge Riverare essentially free of components derived fromthe subducting slab. The apparent trace element??arc signature?? exhibited by several more evolvedBSM basalts is more likely a consequence ofassimilation of mafic deep crust rather than slabinput. At Mt. Meager, however, primitive basaltsmay include a small amount of fluid derived fromsubducted altered oceanic crust.[52] The mantle source of the BSM basalts isdeeper, hotter, and isotopically distinct from thesource of calc-alkaline basalts from Mt. Baker andthroughout the Cascade arc. The BSM mantlesource is also more enriched in incompatible ele-ments than the depleted mantle wedge tapped bycalc-alkaline Cascade arc basalts, and similar toocean island basalt sources. Similar trace elementabundances among the BSM and Anahim alkalicbasalts, and those in the Cascade-Columbia tran-sect and north of Mt. Rainier (Figures 8 and 9c),indicate mantle sources similarly enriched in in-compatible elements.[53] BSM and Cascade-Columbia intraplate lavashave been previously attributed to enriched mantledomains associated with the base of an accretedterrane [Schmidt et al., 2008]. We consider this hy-pothesis unlikely for the BSM volcanic centers fortwo reasons. First, the accreted terranes beneathFigure 14. Schematic representation of plate configurationat the northern end of the Cascade arc based on a model ofRiddihough [1984]. The Explorer plate detached from theJuan de Fuca Plate along the Nootka fault zone 3 to 4 Myrago as it became younger, hotter, and more buoyant at thetrench. The thick dashed black line indicates the surface traceof the Nootka fault. Convergence of the Explorer plate withNorth America has now nearly ceased. The vertical windowformed between the Explorer and Juan de Fuca plates maypromote upwelling of deep, hot mantle (large orange arrow)at the edge of the currently subducting plate. Decompressionmelting of this mantle accounts for the presence of hot alkalicbasalts essentially free of a subduction signature (red, orange,and yellow triangles along Nootka fault zone for each of theBSM volcanic centers).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913149the BSM centers and the Cascade-Columbia tran-sect are different (Wrangellia and Siletzia, respec-tively), and second, Mt. Baker and the BSM sharethe same accreted terrane at depth yet the state ofmantle source enrichment differs substantially.[54] Although major and trace element datarequire an enriched mantle source for the BSMbasalts, isotopic data provide evidence for long-term mantle depletion. Pb isotope ratios of theBSM basalts are broadly similar to oceanic andintraplate basalts of the northeastern Pacific (Fig-ure 11), indicating that isotopically depleted uppermantle of common origin is regionally wide-spread, albeit with small isotopic heterogeneities.[55] With isotopic data consistent with long-termdepletion, incompatible-element enrichment of theBSM mantle source must have occurred relativelyrecently. Recent mantle enrichment has been pro-posed for numerous other cases of isotopicallydepleted alkalic basalts [Roden and Murthy, 1985,and references therein], including those from theTuzo Wilson volcanic field [Allan et al., 1993](Figure 11) and the Bowie Seamount in the Gulfof Alaska [Cousens, 1988]. The BSM volcaniccenters are located along, and just south of, theprojected trace of the Nootka fault zone, whichseparates the subducting Juan de Fuca plate fromthe Explorer plate fragment. We attribute the BSMbasalts to upwelling asthenosphere through a gapalong the fault, which undergoes decompressionmelting to generate alkalic basalts that are free ofsubduction input yet located in an arc setting.Acknowledgments[56] We thank Bruno Kieffer for assistance with TIMS analy-ses, Vivian Lai for help with trace element analyses, JaneBarling, Kathy Gordon, and Liyan Xing for assistance withMC-ICP-MS analyses, and Ines Nobre Silva for instruction inthe clean laboratory. We are grateful to Marion Carpentier forprocessing and analyzing eight samples for trace elementsand five for isotopes. Derek Thorkelson, Martin Streck, andRichard Carlson provided constructive and thoughtfulreviews. Insightful discussions with Kelly Russell have beenmuch appreciated. We are particularly grateful to StewartMcCallum for detailed reviews, discussions, and advice thathave significantly improved the manuscript. This researchwas funded by an NSERC Discovery Grant to D. 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Humphreys (2008), Toroidal mantle flowthrough the Western U.S. slab window, Geology, 36, 295?298.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913155 18.7018.7518.8018.8518.9018.9519.0018.70 18.80 18.90 19.00206 Pb/204 Pb (leached)206Pb/204Pb (unleached)0.70250.70300.70350.70400.70450.7025 0.7035 0.704587Sr/86Sr(leached)87Sr/86Sr (unleached)0.51270.51290.51310.5127 0.5129 0.5131143 Nd/144 Nd (leached)143Nd/144Nd (unleached)38.0538.1538.2538.3538.4538.5538.05 38.15 38.25 38.35 38.45 38.55208 Pb/204 Pb (leached)208Pb/204Pb (unleached)0.282950.283000.283050.283100.283150.28295 0.28300 0.28305 0.28310 0.28315176 Hf/177Hf (leached)176Hf/177Hf (unleached)15.5415.5615.5815.6015.54 15.56 15.58 15.60207 Pb/204 Pb (leached)207Pb/204Pb (unleached)Mullen and Weis (2013) Figure S1 sample BRC01-3 BRC01-3repa%diffBRC01-3dupb%diffSG10 SG10 dup%diffMM01-1 MM01-1dup%diffmethod c 1 1 1 1 1 2 2ppmLi 7.2 7.2 -0.4 6.2 14 6.6 6.7 -1.1 6.5 6.6 -1.2Cs 0.17 0.17 1.5 0.12 30 0.15 0.16 -4.8 0.10 0.10 4.2Rb 11 10 5.2 7.8 26 16 17 -4.0 8.8 8.8 -0.1Ba 413 413 -0.1 405 1.9 330 330 0.0 676 676 0.0Th 2.0 1.9 6.2 1.8 11.2 1.9 1.9 1.4 1.7 1.7 -2.2U 0.66 0.65 1.8 0.67 -1.2 0.71 0.71 -0.2 0.63 0.61 2.8Nb 14 14 -0.3 14 -0.3 32 32 0.1 12 12 -2.4Ta 0.87 0.82 6.0 0.92 -5.4 1.6 1.5 5.1 0.67 0.78 -17La 24 24 -0.7 23 3.5 22 21 2.5 27 27 0.3Ce 54 54 -0.5 53 1.4 47 48 -1.3 62 62 -0.1Pb 4.2 4.1 1.2 4.2 -1.2 2.2 2.2 -1.0 5.2 5.2 -0.4Pr 6.7 6.8 -0.8 6.7 0.7 6.0 6.1 -1.9 8.4 8.4 0.5Sr 1251 1243 0.6 1149 8.1 585 588 -0.5 1588 1599 -0.7Nd 28 28 -1.1 27 2.5 26 25 3.6 35 35 1.3Sm 5.2 5.2 0.6 5.1 2.5 5.7 5.6 1.5 6.7 6.7 -0.5Zr 127 125 1.7 125 1.7 164 163 0.5 147 147 -0.1Hf 3.4 3.1 9.0 3.2 6.1 3.7 3.9 -4.4 3.4 3.4 0.1Eu 1.6 1.6 -0.8 1.5 5.5 1.9 2.0 -6.9 2.1 2.1 -2.4Gd 4.2 4.4 -5.6 4.2 -0.8 5.1 5.8 -13 5.5 5.4 1.2Tb 0.58 0.60 -2.7 0.59 -1.0 0.78 0.83 -6.2 0.71 0.71 0.5Dy 3.4 3.4 0.2 3.4 0.2 4.4 4.7 -6.0 4.0 4.0 1.0Y 19 18 2.9 16 14 25 24 2.6 21 22 -3.5Ho 0.66 0.60 8.9 0.57 13 0.81 0.82 -1.1 0.77 0.76 0.9Er 1.7 1.8 -4.7 1.7 1.1 2.2 2.2 -0.4 2.0 2.0 2.4Tm 0.24 0.22 7.0 0.21 11 0.30 0.30 1.0Yb 1.4 1.3 10 1.3 10 1.7 1.8 -6.9 1.7 1.7 2.2Lu 0.20 0.21 -4.6 0.18 10 0.26 0.23 9.9 0.25 0.25 1.4Sc 18 17 3.3 14 20 22 22 -0.7 23 22 3.9Zn 85 87 -1.8 129 -51 103 102 1.0 100 98 2.4Cr 48 49 0.7 49 0.7 239 243 0.9 112 109 2.8Ni 44 43 1.2 46 -5.7 130 130 0.2 49 48 2.7V 202 203 -0.3 204 -0.8 227 225 1.1 208 202 2.8Ga 23 23 -1.3 22 3.1 22 23 -3.3Mullen and Weis (2013) Table S1a rep designates replicates (repeat analyses of the same sample solution)b dup designates full procedural duplicates starting with a new sample powder aliquotc Method 1: Thermo Finnigan Element2 HR-ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS Sr-Nd-Hf-Pb isotope and trace element evidencefor the origin of alkalic basalts in the GaribaldiBelt, northern Cascade arcEmily K. Mullen and Dominique WeisPacific Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences,University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, V6T 1Z4, Canada(emullen@eos.ubc.ca)[1] In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier,and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures arenegligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceaniccrust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display traceelement signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation ratherthan primary input from the subducted slab. Primary BSM basalts represent 3?8% melts that segregated fromenriched garnet lherzolite at significantly higher temperatures and pressures (70?105 km) than calc-alkalineCascade arc basalts. The BSM mantle source is significantly more incompatible element-enriched than thedepleted mantle tapped by calc-alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct fromcalc-alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NWNorthAmerica. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent withupwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, whichforms the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate. A gap orattenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low-degreedecompressionmelting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.Components: 17,992 words, 14 figures, 5 tables.Keywords: alkali basalt ; Garibaldi Belt ; cascade arc; Sr-Nd-Pb-Hf isotopes; trace elements.Index Terms: 1040 Radiogenic isotope geochemistry: Geochemistry; 1037 Magma genesis and partial melting: Geo-chemistry; 1031 Subduction zone processes: Geochemistry; 1033 Intra-plate processes: Geochemistry; 1065 Major andtrace element geochemistry: Geochemistry; 3613 Subduction zone processes: Mineralogy and Petrology; 3615 Intra-plateprocesses: Mineralogy and Petrology; 3619 Magma genesis and partial melting: Mineralogy and Petrology; 3060 Subduc-tion zone processes: Marine Geology and Geophysics; 8170 Subduction zone processes: Tectonophysics; 8413 Subductionzone processes: Volcanology; 8415 Intra-plate processes: Volcanology.Received 10 January 2013; Revised 29 May 2013; Accepted 30 May 2013; Published 28 August 2013.Mullen, E. K., and D. Weis (2013), Sr-Nd-Hf-Pb isotope and trace element evidence for the origin of alkalic basalts in theGaribaldi Belt, northern Cascade arc, Geochem. Geophys. Geosyst., 14, 3126?3155, doi:10.1002/ggge.20191.1. Introduction[2] The role of mantle heterogeneity in generatingthe compositional diversity observed in arc magmasis difficult to decipher because it can be masked bycontributions from the subducting slab. There is ageneral consensus that mantle melting in arcs occursin response to input of a hydrous component derived? 2013. American Geophysical Union. All Rights Reserved. 3126ArticleVolume 14, Number 828 August 2013doi: 10.1002/ggge.20191ISSN: 1525-2027from the subducting slab that generates hydrousminerals in the mantle, lowers the mantle solidustemperature, and produces melts by dehydrationmelting or ??flux?? melting [e.g., Kushiro, 1987;Grove et al., 2002]. As a consequence, basaltserupted in subduction zone settings are predomi-nantly aluminous, subalkaline, and bear a slab-derived trace element ??arc signature?? that includesenrichments in large ion lithophile elements (LILEs)and depletions in high field strength elements(HFSEs) [e.g., Gill, 1981; Pearce and Peate, 1995].The Cascade arc, which extends 1300 km fromnorthern California to southwestern British Colum-bia, is an example of an arc in which calc-alkalinelavas dominate [Schmidt et al., 2008; Hildreth,2007; Bacon et al., 1997; Conrey et al., 1997].However, the northernmost segment of the arc is un-usual in that mafic lavas are predominantly alkalic.Alkali olivine basalt and hawaiite occur at the vol-canic fields of Mt. Meager, Salal Glacier, andBridge River Cones (hereinafter referred to as theBSM volcanic centers) [Green and Sinha, 2005].[3] Alkalic lavas are uncommon in arc settings,particularly along the main arc axis, and are attrib-uted to a variety of phenomena that include tearsin the subducting plate, back-arc extension, slabwindows, entrainment of mantle hotspots, accretedenriched mantle, and intraarc rifting [e.g., Naka-mura et al., 1989; Abratis and W?rner, 2001; Fer-rari et al., 2001; Marquez et al., 1999; Turnerand Hawkesworth, 1998; Pearce and Stern, 2006;Thorkelson and Taylor, 1989; Skulski et al., 1991;Righter and Carmichael, 1992; Hughes, 1990;Pearce and Peate, 1995, and references therein].The BSM volcanic centers are located 110 kmabove the subducting plate, broadly similar toother Cascade arc volcanoes [McCrory et al.,2004], suggesting a potential connection to arcmagmatic processes. Elucidating the petrogenesisof the BSM basalts may provide valuable insightsinto mantle and slab processes under the dimin-ished subduction regime at the northern boundaryof the Cascade arc [Harry and Green, 1999].[4] The Cascade magmatic arc has been activesince 40 Ma and is a consequence of subductionof the Juan de Fuca oceanic plate beneath NorthAmerica [Hildreth, 2007]. In northwestern Wash-ington, an abrupt change in the orientation of thearc axis mirrors a bend in the continental margin,subdividing the arc into two major segments (Fig-ure 1a). The High Cascades segment (Mt. Rainierto Lassen Peak) is separated from the GaribaldiVolcanic Belt (GVB) (Glacier Peak to Silver-throne) by a 120 km gap in modern volcanism[Green and Harry, 1999]. Throughout much of theHigh Cascades, the subducting plate is 10 Myrold at the trench with ocean floor isochrons gener-ally parallel to the continental margin [Wilson,2002]. In the GVB, however, isochrons are obliqueto the plate margin and the slab age at the trenchdecreases northward to 6 Ma outboard of theBSM volcanic centers (Figure 1a), making it oneof the youngest and hottest subduction settings inthe world [Syracuse et al., 2010; Harry andGreen, 1999]. Subducted water is expected to belost at shallow depths from a hot slab, leading toreduced hydration of the subarc mantle wedge[Green and Harry, 1999]. A diminished subduc-tion regime may account for why the GVB, incomparison to the High Cascades, has a narrowerwidth, lower magma production rates, and magma-tism mainly restricted to the major volcanic cen-ters [Harry and Green, 1999].[5] In this paper, we assess three hypotheses forthe origin of the BSM alkalic basalts. In the firstmodel, the BSM alkalic basalts are essentiallylower melt fraction ??equivalents?? of more typicalCascade arc calc-alkaline basalts [Green andHarry, 1999; Green and Sinha, 2005]. Reducedhydration of the subarc mantle wedge may reduceits capacity for flux melting, resulting in lowermelt fractions that are enriched in alkali elementsbut display minimal arc signature. A similar modelhas been proposed to account for basalts elsewherein the Cascade arc that are geochemically indistin-guishable from intraplate basalts [Reiners et al.,2000]. Intraplate-type lavas dominate the back-arcSimcoe volcanic field east of Mt. Adams and areinterspersed with other basalt types in a swathextending west 150 km from Simcoe to Portlandthat has been referred to as the Cascades-Columbia transect [Leeman et al., 1990, 2005;Hildreth, 2007; Jicha et al., 2008; Conrey et al.,1997; Bacon et al., 1997]. A few other examplesoccur north of Mt. Rainier [Reiners et al., 2000]and in north-central Oregon [Conrey et al., 1997].[6] Second, a subducted plate boundary or fracturezone may trigger mantle upwelling, inducingdecompression melting that generates low-degree,alkali-rich melts. These processes have been pro-posed for other arcs, including the Mexican arc andLesser Antilles [e.g., Righter et al., 1995; DeLonget al., 1975; Pearce, 2005], and a version of thismodel is mentioned by Lawrence et al. [1984] in thecontext of the Salal Glacier alkalic basalts. The sub-ducted boundary between the Explorer and Juan deFuca plates intersects the BSM volcanic centers andhas been implicated in the origin of the WellsMULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913127Gray-Clearwater volcanic field (3.5 Ma?7.6 ka[Hickson and Souther, 1984]) and Chilcotin Groupbasalts (32?0.8 Ma [Mathews, 1989]) that arelocated in the GVB back-arc region [Madsen et al.,2006; Sluggett, 2008]. Near the triple junctionamong the Explorer, Pacific, and North Americaplates, the Quaternary alkalic seamounts of the TuzoWilson volcanic field are attributed to a ??leakytransform?? in an oceanic setting [Allan et al., 1993].[7] Third, the BSM basalts may reflect one of theother mechanisms proposed to account forMiocene-Holocene intraplate volcanic centers thatoccur in a broad swath extending from southernBritish Columbia to Alaska. The east-west trend-ing Anahim volcanic belt (14.5 Ma?7.2 ka [Bevier,1989]), located immediately north of the GVB,may be related to a hotspot [Bevier, 1989; Char-land et al., 1995], an edge effect of the Juan deFigure 1. (a) Map of the Cascade arc and its tectonic setting. The extents of the Garibaldi volcanic belt andHigh Cascades segments of the arc are indicated with pink arrows. Volcanic and plutonic rocks are shown inyellow and orange shading, respectively. Black triangles denote composite volcanoes. (b) The study area isenclosed by a small bold rectangle and is enlarged. Igneous rock distributions are compiled from Lawrence etal. [1984], Monger [1989], DuBray et al. [2006], Green et al. [1988], Wheeler and McFeely [1991]. Oceanicplate configurations are from Braunmiller and Nabelek [2002], Audet et al. [2008], and Wilson [2002]. Col-ored lines on the oceanic plates are isochrons; accompanying numbers indicate the age of oceanic crust in Ma(from Wilson [2002]). Pseudofaults are shown as thin gray lines. Four heavy gray arrows on the Juan de Fucaand Explorer plates are convergence vectors (mm/yr) obtained from McCrory et al. [2004], Riddihough andHyndman [1991], and Braunmiller and Nabelek [2002] for a reference frame fixed relative to North America.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913128Fuca plate [Stacey, 1974], or ridge subduction[Farrar and Dixon, 1992]. Farther north, theextensive northern Cordilleran volcanic province(20 Ma?200 years B.P.) has been attributed tocrustal extension [Edwards and Russell, 2000].The aforementioned Wells Gray-Clearwater andChilcotin lavas may alternatively be a product ofback-arc extension [Bevier, 1983; Hickson, 1987].More recently, Thorkelson et al. [2011] proposed asingle model in which all of these volcanic provin-ces are related to upwelling of enriched mantlewithin and along the eroding margins of the1500 km long Northern Cordilleran slab window[Thorkelson and Taylor, 1989] that extends nearlyas far south as the BSM volcanic centers.[8] In this study, we investigate the roles of themantle and subducting slab in generating thealkalic compositions of the BSM basalts with highprecision whole-rock Sr-Nd-Pb-Hf isotope ratiosand trace element data. Radiogenic isotope signa-tures of primitive basalts are sensitive indicators ofmantle source heterogeneity [e.g., Hofmann, 2003]and the presence of components derived from sub-ducted oceanic crust and sediment [e.g., Kay et al.,1978]. Green and Sinha [2005] showed that theBSM alkalic basalts record less slab input thancalc-alkaline basalts of the southern GVB, butminimized the possible role of mantle heterogene-ity. However, recent improvements in the precisionof isotopic measurements have revealed mantleheterogeneities that were previously difficult to dis-cern [e.g., Abouchami et al., 2005]. Alkalic basaltsin British Columbia and the Cascade arc are typi-cally ascribed to mantle sources that are moreenriched in incompatible elements than the mantlewedge sampled by calc-alkaline basalts [e.g., Thor-kelson et al., 2011; Sluggett, 2008; Edwards andRussell, 2000; Leeman et al., 1990, 2005; Baconet al., 1997; Borg et al., 1997; Conrey et al.,1997; Schmidt et al., 2008; Jicha, et al. 2008].However, Reiners et al. [2000] proposed that bothbasalt types can be derived from a homogeneousmantle variably fluxed by slab-derived fluids.[9] We compare the BSM basalts to calc-alkalinebasalts from Mt. Baker, a stratovolcano located inthe ??cooler?? southern GVB and representative ofmore typical Cascade arc basalts, and to pub-lished data for other intraplate alkalic basaltsfrom British Columbia. Our new isotope and traceelement data show minimal subduction influenceon the source of the most primitive basalts atSalal Glacier and Bridge River. The BSM basaltsalso have a mantle source that is isotopically dis-tinct from, and more incompatible elementenriched, than the mantle underlying much of theCascade arc. These results have important impli-cations for the physical configuration of the sub-ducting slab and mantle flow patterns in northernCascadia.2. Geology of the Bridge River, SalalGlacier, and Mt. Meager VolcanicCenters[10] The Bridge River Cones, Salal Glacier, andMt. Meager are located 150 km north of Van-couver, British Columbia (Figure 1b). At the twonorthernmost centers (Salal Glacier and BridgeRiver), lavas are almost exclusively mafic. TheMt. Meager volcanic field includes basalt throughrhyolite but is dominated by intermediate compo-sitions, and Mt. Meager proper is a compositeandesitic stratovolcano [Ke, 1992].[11] The Salal Glacier volcanic field includes pil-low lavas, tuffs, and variably palagonitized andbrecciated flow remnants that survived continentalice sheet advances as high-altitude nunataks. Atlower altitudes, severe glacial erosion has revealedrhyolite and andesite dikes. Age dates for an alkalibasalt and overlying hawaiite are 0.97 and 0.59Ma (K-Ar), respectively [Lawrence, 1979].[12] Lavas at the Bridge River Cones are exclu-sively alkalic [Roddick and Souther, 1987]. Theterm ??cones?? is a misnomer because none of thedeposits is a true volcanic cone; rather, glacialerosion has produced cone-like forms. Columnarlavas of the Sham Hill plug and Tuber Hill expo-sure are dated at 1 Ma and 600 ka (K-Ar), respec-tively [Roddick and Souther, 1987].[13] At Mt. Meager, intermediate to silicic lavasspan the alkalic-subalkalic boundary [Stasiuk et al.,1994]. Mafic lavas are exclusively alkalic,however, and occur as four flow remnantscollectively known as the Mosaic Assemblage[Stasiuk and Russell, 1989; Stasiuk et al., 1994].Two of the basalts are dated at 90 and 140 ka (K-Ar) [Anderson, 1975; Woodsworth, 1977].Evidence for recent involvement of mafic magmain the form of mafic enclaves and banded pumicesis preserved in the 2360 years B.P. thatexplosively released 10 km3 of dacite [Clague etal., 1995; Michol et al., 2008]. Banded pumicesand mafic enclaves indicate that the intrusion of abasaltic magma may have triggered the eruption[Stasiuk et al., 1994].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131293. Major Element Compositions andPetrography[14] Samples analyzed for this study are from theBridge River, Salal Glacier, Mt. Meager, and Mt.Baker sample suites of Green and Sinha [2005]and from the Mt. Baker sample suite of E. K.Mullen and I. S. McCallum (Origin of basalts in ahot subduction setting: Petrologic and geochemi-cal insights from Mt. Baker, northern Cascade arc,submitted to Journal of Petrology, 2013, herein-after referred to as Mullen and McCallum, submit-ted manuscript, 2013). Major element data anddetailed petrographic descriptions for all samplesare provided in those references.[15] The BSM basalts are alkalic [Macdonald,1968] and nepheline normative with Na2O>K2O(Figure 2a). The basalts have distinctly lower SiO2(Figure 2) and Al2O3 than the calc-alkaline, hyper-sthene-normative Mt. Baker basalts.[16] Bridge River alkali olivine basalts and hawai-ites encompass the largest range of compositionaldiversity (Figure 2). Molar Mg/(Mg?Fe2?) valuesrange from 0.44 to 0.62; we consider two sampleswith Mg/(Mg?Fe2?)> 0.6 as primitive. Phenocrystand microphenocryst minerals are limited to olivine(1?2%) and rare plagioclase. Except for one sam-ple with a brown glass matrix, the basalts have hol-ocrystalline groundmasses containing olivine,plagioclase, titanaugite, magnetite, and ilmenite.The most primitive basalt (BRC10) contains biotiteand amphibole in the groundmass. Two of the moreevolved samples contain quartz xenocrysts andgranodiorite xenoliths (BRC03?4, BRC01?3).[17] Mt. Meager alkali basalts and hawaiites con-tain <1% microphenocrysts of olivine, clinopyr-oxene, and plagioclase. The groundmass containsglass and magnetite and, in the least primitivesample (MM01-1), biotite and amphibole. Mg/(Mg?Fe2?) values are 0.59?0.63.[18] Salal Glacier samples are the most primitiveamong the BSM basalts with Mg/(Mg?Fe2?)? 0.58to 0.66 and have the highest normative nephelinecontents. The most primitive samples are glassy andvesicular with phenocryst assemblages including<15% olivine, <1% plagioclase, <1% clinopyrox-ene, and rare orthopyroxene xenocrysts. Lessrimitive samples contain orthopyroxene phenocrystsand more abundant plagioclase, and olivine is eitherrimmed by clinopyroxene or absent.[19] At Mt. Baker, the most mafic lavas includemedium-K calc-alkaline basalt, high-Mg basalticandesite, and low-K olivine tholeiite, with Mg/(Mg? Fe2?)? 0.56?0.70. All samples contain oli-vine and plagioclase phenocrysts and some alsohave clinopyroxene phenocrysts (Mullen andMcCallum, submitted manuscript, 2013).4. Analytical Methods[20] Trace element abundances and Sr-Nd-Hf-Pbisotope ratios were measured on 19 BSM basalts,using splits of sample powders analyzed by Greenand Sinha [2005] for major and trace elements andSr isotope ratios. Larger symbols in Figure 2 des-ignate samples analyzed for the present study. HfFigure 2. Major element variation diagrams for the basaltsof Bridge River (red), Salal Glacier (orange), Mt. Meager(yellow), and Mt. Baker (lavender). (a) wt % Na2O?K2Oversus SiO2 with discriminant line of Macdonald [1968] andfields of Le Bas et al. [1986]. (b) Miyashiro diagram (FeO/MgO versus SiO2) with discriminant line of Miyashiro[1974]. Bridge River, Salal Glacier, and Mt. Meager data arefrom Green and Sinha [2005]. Mt. Baker data are fromMullen and McCallum (submitted manuscript, 2013) exceptLib21 from Green and Sinha [2005]. Large symbols (circlesand diamonds) indicate samples analyzed in this study fortrace elements and isotope ratios; diamonds are accompaniedby sample numbers and are the Suite 2 samples discussed inthe text. Small circles indicate samples of Green and Sinha[2005] not analyzed for the present study.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913130Figure 3. (a) 208Pb/204Pb versus 206Pb/204Pb, (b) 207Pb/204Pb versus 206Pb/204Pb. Insets show the same datafor the BSM and Mt. Baker basalts but on an expanded scale. Symbols for the BSM and Mt. Baker samplesgiven in legend; circles are used for primitive basalts. The more evolved basalts are subdivided into Suite 1(circles; isotopically similar to the primitive basalts) and Suite 2 (diamonds accompanied by sample numbers;isotopically distinct from the primitive basalts). 2 error bars (external reproducibilities) are smaller thansymbols in all plots. NHRL is the Northern Hemisphere Reference Line of Hart [1984]. Cascade arc basaltdata (pink crosses; only those with >8 wt % MgO are included) are from Conrey et al. [1997], Jicha et al.[2008], Bacon et al. [1994, 1997], Leeman et al. [1990, 2005], Baker et al. [1991],Magna et al. [2006], Groveet al. [2002], and Borg et al. [1997, 2000]. Northern Juan de Fuca MORB data (dark blue ?) are from Cou-sens et al. [1995]. Explorer MORB data (dark gray filled squares) are from B. Cousens (unpublished data2007). N. Gorda MORB (black ?) are from Allan et al. [1993]. Northern Cascadia sediment data (orangecircles) are from ODP sites 1027 and 888 [Carpentier et al., 2010, 2013]. Note that all isotope data are nor-malized to the same isotope standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913131isotope ratios were also measured on splits of pow-ders of three Mt. Baker basalts previously ana-lyzed for major and trace elements and Sr-Nd-Pbisotopes [Mullen and McCallum, submitted manu-script, 2013; Green and Sinha, 2005].[21] All chemical separations and mass spectro-metric analyses were carried out in Class 100 and10,000 clean laboratories, respectively, at the Pa-cific Centre for Isotopic and GeochemicalResearch at the University of British Columbia.Rock powders (100 mg) were digested in sub-boiled concentrated HF?HNO3 in 15 mL screw-top Savillex beakers on a hotplate for 48 h at130C. Samples were dried down on a hotplateand brought up in subboiled 6 N HCl and fluxedon a hotplate for at least 24 h. Sample aliquots of5?10% were diluted 5000X with an HNO3?HFsolution for analysis on a Thermo Finnigan Ele-ment2 HR-ICP-MS or an Agilent 7700 quadrupoleICP-MS. Sr isotope ratios were measured on aThermo Finnigan Triton TIMS and Pb, Nd, and Hfisotope ratios on a Nu Instruments MC-ICP-MS(Nu 021) following the procedures of Weis et al.[2006]. Pb, Sr, Hf, and Nd were separated fromsingle powder dissolutions by sequential ionexchange column chemistry as described in Weiset al. [2006, 2007]. All solutions were passedtwice through Pb exchange columns to ensure Pbpurification. Although thin sections of the ana-lyzed samples indicate little or no alteration in allsamples (minor iddingsite in olivine) and LOIvalues are low (<1%), even minimally alteredsamples can yield isotopic compositions that arenot representative of magmatic isotopic signatures,particularly in the case of Sr and Pb isotopes[Hanano et al., 2009; Nobre Silva et al., 2009].Therefore, we measured isotope ratios on bothunleached and leached powders of some samples.Leaching was conducted prior to powder dissolu-tion following the procedures of Nobre Silva et al.[2009, 2010]. Leached samples gave isotope ratioswithin analytical error of respective unleachedsamples for Sr and Nd (Figure S1).1 Hf isotoperatios are also within analytical error except forone sample (BRC10) that gave a higher value inthe leached sample. 207Pb/204Pb in leached sam-ples is systematically lower than in unleachedsamples while 208Pb/204Pb and 206Pb/204Pb arewithin error of unleached samples, with the excep-tion of one Mt. Baker sample (Lib21) (FigureS1).1 All isotope plots in the main text show dataobtained on leached samples except for cases inwhich only unleached samples were analyzed.Blank contributions to isotope ratios were negligi-ble with total procedural blanks of 50, 400, 90,and 15 pg for Pb, Sr, Nd, and Hf, respectively.5. Results5.1 Isotopes[22] Isotope ratios are reported in Table 1 and plot-ted in Figures 3?5. 87Sr/86Sr values measured in theBSM basalts are systematically lower than reportedby Green and Sinha [2005] for the same samplesand lie outside their reported uncertainties (Figure4a inset). For direct comparison among datasets, allliterature data are normalized to standard values of87Sr/86Sr? 0.710248 for SRM987 and 0.708028 forEimer and Amend; 143Nd/144Nd? 0.511973for Rennes, 0.511858 for La Jolla, 0.512633 forBCR-1, and 0.512130 for Ames [Weis et al., 2006,2007]; 176Hf/177Hf? 0.282160 for JMC 475[Vervoort and Blichert-Toft, 1999]; and208Pb/204Pb? 36.7219, 207Pb/204Pb? 15.4963,206Pb/204Pb? 16.9405 for SRM981 [Galer andAbouchami, 1998].[23] Primitive BSM basalts (Mg/Mg?Fe2?> 0.60)form an isotopic cluster (Figures 3 and 4) with anarrow range of 87Sr/86Sr? 0.70299?0.70314,ENd??7.1 to ?7.7, EHf??8.3 to ?10.0,208Pb/204Pb? 38.075?38.172, 207Pb/204Pb? 15.541?15.557, 206Pb/204Pb? 18.690?18.774. The primitiveBSM basalts overlap in 208Pb/204Pb and 206Pb/204Pbwith N. Juan de Fuca MORB [Cousens et al., 1995]and Explorer MORB (B. Cousens, unpublished data2007) (Figure 3a) but have slightly higher207Pb/204Pb and 87Sr/86Sr and lower ENd (Figures 3band 4a). Primitive BSM basalts plot near thedepleted end of the Sr-Nd-Pb isotopic arrays definedby other Cascade arc basalts (Figures 3 and 4).Along with Mt. Baker, primitive BSM basalts haveamong the highest ENd values reported for the Cas-cade arc. Mt. Baker basalts have slightly higher208Pb/204Pb and 206Pb/204Pb than primitive BSMsamples, but overlap in 207Pb/204Pb. Mt. Baker hassignificantly higher EHf (?11.1 to? 12.1) andslightly higher 87Sr/86Sr. In EHf-ENd isotopic space(Figure 4b), primitive BSM and Mt. Baker basaltsoverlap with only one outlier among data previouslypublished for the Cascade arc (Lassen Peak [Borg etal., 2002] and Mt. Adams [Jicha et al., 2008]). Mt.Adams EHf values cluster between primitive BSMbasalts and Mt. Baker. Together, the BSM, Mt.Baker, and Mt. Adams basalts define an EHf range1Additional supporting information may be found in the onlineversion of this article.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913132Table 1. Sr, Nd, Hf, and Pb Isotope Ratios87Sr/86Srb 143Nd/144NdcSample#Lat(N)Long(W)SiO2(wt%)MgNumbera Leached 2SEh Unleached 2SE Leached 2SE"NdLeachedf Unleached 2SE"NdUnleachedfBridge River ConesBRC01-3 50.93 123.45 50.51 56.5 0.703212 9BRC02 50.91 123.45 49.60 52.8BRC03-4 50.93 123.45 50.07 54.4 0.703219 6 0.512966 7 6.4BRC04 50.93 123.45 49.48 52.4 0.703184 9BRC05-1 50.92 123.45 46.66 48.3 0.703119 9BRC06 50.90 123.45 49.35 53.5 0.703175 8BRC07-2 50.92 123.45 47.24 47.7 0.703098 9 0.703124 7 0.513012 5 7.3 0.513012 6 7.3BRC09-3 50.92 123.41 48.02 59.6 0.702985 7 0.703012 9 0.513024 7 7.5BRC10 50.92 123.38 45.10 61.2 0.703052 7 0.703054 7 0.513031 7 7.7dupi 0.703055 7 0.703056 8Salal GlacierSG01-2 50.81 123.45 46.64 65.1 0.703140 7 0.513021 6 7.5SG01-3 50.81 123.45 46.62 64.5 0.703143 8 0.703149 9 0.513001 7 7.1SG10 50.78 123.39 46.04 65.7 0.703122 9dup 0.703117 8SG12 50.77 123.40 46.71 66.5 0.703065 7 0.703067 8 0.513013 6 7.3SG16 50.77 123.39 46.59 65.9 0.703101 10Mt. MeagerMM01-1 50.65 123.59 48.84 58.8 0.703764 8 0.703758 9 0.512930 5 5.7 0.512926 7 5.7dup 0.703762 6 0.703763 7 0.512936 5 5.8 0.512941 6 5.6MM02 50.69 123.57 48.64 63.0 0.703132MM04 50.69 123.57 48.94 60.8 0.703144 6 0.703146 0.513030 9 7.6MM08 50.55 123.53 49.68 60.6 0.703164Mount BakerLIB-21 48.67 121.74 51.04 63.5 0.703964 9 0.703970 7 0.512834 6 3.8 0.512834 6 3.802-MB-5 48.72 121.85 53.69 69.7 0.703109 7 0.513001 6 7.107-MB-112 48.66 121.70 52.56 56.6 0.703240 7 0.513034 7 7.702-MB-1 48.72 121.85 53.30 56.0 0.703513 8 0.512899 5.106-MB-82 48.72 121.85 50.57 65.1 0.703156 7 0.512986 6.807-MB-114 48.64 121.73 52.06 61.3 0.703213 7 0.513037 7.806-MB-97 48.78 121.88 54.45 49.0 0.703173 10 0.512993 6.9176Hf/177HfdSample # Leached 2SE "Hf Leachedg Unleached 2SE "Hf UnleachedgBridge River ConesBRC01?3 0.283067 8 10.4BRC02BRC03?4 0.283052 5 9.9BRC04 0.283026 7 9.0BRC05-1 0.283040 7 9.5BRC06 0.283018 6 8.7BRC07-2 0.283027 4 9.0 0.283030 5 9.1BRC09-3 0.283007 6 8.3 0.283030 5 9.1BRC10 0.283025 6 8.9 0.282985 4 7.5Salal GlacierSG01?2 0.283017 5 8.7SG01?3 0.283021 5 8.8 0.283025 24 8.9SG10 0.283012 9 8.5SG12SG16 0.283023 7 8.9Mt. MeagerMM01-1 0.283063 9 10.3 0.283064 6 10.3dup 0.283050 8 9.9 0.283056 7 10.3MM02 0.283050 6 9.8MM04 0.283056 5 10.1MM08 0.283022 5 8.9Mount BakerLIB-21 0.283084 5 11.0 0.283094 5 11.402-MB-5 0.283100 5 11.607-MB-112 0.283114 4 12.1MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913133that is similar to Explorer MORB, but with slightlylower ENd.[24] More evolved BSM basalts define twogroups: Suite 1 has isotopic ratios that overlapwith primitive basalts from the same volcanic cen-ter, whereas Suite 2 has substantially lower ENdand higher Sr and Pb isotope ratios than otherbasalts from their respective volcanic centers (Fig-ures 3 and 4a). Suite 2 includes three BSM basalts(BRC03?4, BRC01?3 at Bridge River; MM01-1at Mt. Meager), all of which have Pb isotope ratiosTable 1. (continued)208Pb/204Pbe 207Pb/204Pbe 206Pb/204PbeSample # Leached 2SE Unleached 2SE Leached 2SE Unleached 2SE Leached 2SE Unleached 2SEBridge River ConesBRC01-3 38.2441 22 15.5615 8 18.7878 8dup 38.2433 24 15.5604 9 18.7863 10BRC02 38.1901 21 15.5554 8 18.7481 10BRC03-4 38.2671 18 15.5584 7 18.8334 6dup 38.2674 18 15.5585 6 18.8337 8BRC04 38.1961 27 15.5622 9 18.7379 13BRC05-1 38.1274 17 15.5481 7 18.7591 8BRC06 38.1828 28 15.5530 12 18.7463 14BRC07-2 38.1215 19 38.1260 26 15.5450 7 15.5484 9 18.7626 8 18.7539 10BRC09-3 38.1245 18 38.1432 40 15.5462 6 15.5561 15 18.7432 8 18.7340 13BRC10 38.1496 22 38.1580 20 15.5566 10 15.5610 7 18.7738 10 18.7754 8dup 38.1601 36 15.5620 14 18.7750 17Salal GlacierSG01-2 38.1087 20 15.5419 8 18.7465 9SG01-3 38.1088 24 38.1250 31 15.5407 8 15.5508 7 18.7536 10 18.7536 7SG10 38.1285 24 15.5492 9 18.7363 12SG12 38.1721 22 38.1492 38 15.5521 7 15.5621 14 18.7739 9 18.6997 14SG16 38.1585 33 15.5527 12 18.7591 12Mt. MeagerMM01-1 38.2664 21 38.2734 19 15.5692 8 15.5731 7 18.8033 9 18.8090 7dup 38.2697 18 38.2772 19 15.5703 7 15.5738 7 18.8102 8 18.8115 7MM02 38.1005 35 15.5470 13 18.7034 15MM04 38.1046 14 38.1370 36 15.5470 4 15.5580 12 18.7082 6 18.7271 13MM08 38.0747 25 15.5550 9 18.6896 15Mount BakerLIB-21 38.4708 24 38.5285 20 15.5851 9 15.5931 7 18.9235 11 18.9726 802-MB-5 38.2495 22 15.5505 8 18.7975 907-MB-112 38.3077 21 15.5575 7 18.8385 802-MB-1 38.3598 15.5645 18.851506-MB-82 38.2661 15.5518 18.828607-MB-114 38.2753 15.5560 18.846606-MB-97 38.2597 15.5529 18.8356aCalculated as 100Mg/Mg?Fe2? (molar), using major element data from Green and Sinha [2005] and Mullen and McCallum (submittedmanuscript, 2013) and assuming Fe3?/Fe? 0.15.bReported Sr isotope ratios are corrected for mass fractionation using 86Sr/88Sr? 0.1194. Repeat analysis of the Sr SRM987 standard yielded amean (6 2) of 87Sr/86Sr? 0.7102486 2 (n? 7), identical to the accepted value [Weis et al., 2006].cReported Nd isotope ratios are corrected for mass fractionation using 146Nd/144Nd? 0.7219 and are normalized to 143Nd/144Nd? 0.511973 forthe Rennes reference material [Chauvel and Blichert-Toft, 2001] using the daily average method. The Rennes standard was analyzed every twosamples and over the course of analysis gave a mean (6 2) value of 143Nd/144Nd? 0.5119806 65 (n? 16). On a per session basis, reproducibil-ity was significantly better with a maximum daily 2 value of627 (53 ppm).dReported Hf isotope ratios are corrected for mass bias using 179Hf/177Hf? 0.7325 [Patchett and Tatsumoto, 1981] and normalized to176Hf/177Hf? 0.282160 for the ULB-JMC 475 reference material [Vervoort and Blichert-Toft, 1999] using the daily average of standard analyses.JMC 475 was analyzed every two samples and over the course of analysis gave a mean (6 2) of 176Hf/177Hf? 0.28217326 24 (86 ppm)(n? 24). On a per session basis, reproducibility was significantly better with daily 2 values ranging from 34 to 61 ppm.eReported Pb isotope ratios were corrected for mass bias by Tl doping [White et al., 2000] and are normalized to 208Pb/204Pb? 36.7219,207Pb/204Pb ?15.4963, 206Pb/204Pb? 16.9405 for the SRM981 standard [Galer and Abouchami, 1998] by sample-standard bracketing. Replicateanalysis of SRM981 over the course of analysis yielded in-run mean6 2 values of 208Pb/204Pb? 36.71986 91 (247 ppm),207Pb/204Pb? 15.49986 34 (220 ppm), and 206Pb/204Pb? 16.94346 29 (168 ppm) (n? 43). On a per session basis, reproducibility was signifi-cantly better with daily 2 values ranging from 74 to 186 ppm, 62 to 176 ppm, and 45 to 139 ppm for 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb,respectively.fENd calculated using a CHUR value of143Nd/144Nd? 0.512638 [Jacobsen and Wasserburg, 1980].gEHf calculated using CHUR value of176Hf/177Hf? 0.282772 [Blichert-Toft and Albarede, 1997].h2SE values (twice the standard errors) apply to the last decimal place(s) and are the internal absolute errors values for individual sample analyses.idup designates full procedural duplicates starting with a new sample powder aliquot; reproducibilities are similar to, or better than, the reprodu-cibilities determined through repeat standard analysis (values listed above).Data in italics are from Mullen and McCallum (submitted manuscript, 2013).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913134Figure 4. (a) ENd versus87Sr/86Sr; (b) EHf versus ENd. Symbols and data references as in Figure 3, plus Las-sen and Adams data in Figure 4b from Borg et al. [2002] and Jicha et al. [2008]. Note that all isotope data arenormalized to the same isotope standard values as described in main text. For the BSM and Mt. Baker data,2 error bars (external reproducibilities) are smaller than symbols. Inset in Figure 4a compares 87Sr/86Srmeasured in the present study (2 error bars? 20 ppm; smaller than symbol size) to 87Sr/86Sr measured onthe same samples by Green and Sinha [2005] (2 error bars? 100 ppm). The mantle array in EHf versus ENdspace is from Chauvel et al. [2008]. Orange and blue curves show the effect of adding 2% bulk sediment(blue curve with long dashes), 2% sediment fluid (blue curves with short dashes), 2% sediment melt (solidblue curves), 10% metabasalt fluid (orange dashed curve), and 2% metabasalt melt (orange solid curve) to themantle prior to 5% equilibrium partial melting of a primitive mantle composition [Sun and McDonough,1989]. Each curve has two tick marks indicating 1% and 2% addition, except for the metabasalt fluid curve(ticks at 5% and 10% addition). Slab fluid and melt compositions calculated using equilibrium melting/dehy-dration equations with FL?0.05. Trace element compositions for sediment and metabasalt are from Carpent-ier et al. [2013] (average of bulk ODP sites 888 and 1027) and Becker et al. [2000] (900C eclogite),respectively. Sediment isotope composition is the average of ODP sites 888 and 1027 from Carpentier et al.[2010]. Metabasalt Sr and Nd isotope ratios are from Staudigel et al. [1995] and the Hf isotope ratio is the av-erage of Explorer MORB shown here. Partition coefficients from Kessel et al. [2005] at 700C, 4 GPa (all flu-ids); 1000C, 4 GPa (metabasalt melt and sediment melt 1), and Hermann and Rubatto [2009] at 1050C, 4.5GPa (sediment melt 2).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913135similar to Mt. Baker basalts, plotting within theCascade arc array and closer to the field definedby subducting sediment (Figure 3). Suite 2 alsoincludes two Mt. Baker basalts that are isotopi-cally anomalous (in Sr and Nd) relative to moreprimitive Mt. Baker basalts ; Cathedral Crag(MB1) and one Sulphur Creek sample (Lib21) plotcloser to the field defined by subducting sediment(Figure 4a). As a group, Suite 2 has lower MgOand generally higher SiO2 than other basalts fromtheir respective volcanic centers (Figure 5).5.2 Trace Elements[25] Trace element abundances are reported inTable 2 and plotted in Figures 6?9. With theexception of Suite 2 (discussed later), the BSMbasalts have LILE and Pb abundances similar toMt. Baker basalts, but much higher HFSE (Figures6 and 8a). BSM basalts have substantially lowerZr/Nb and Ba/Nb than Mt. Baker basalts (Figure8b) and higher (La/Yb)N and (Dy/Yb)N (Figure 7).Among the BSM basalts, Salal Glacier has thehighest (La/Yb)N and lowest Yb and displays theleast variability among samples (Figure 7a).Bridge River and Mt. Meager have variable REEabundances, and the lowest (La/Yb)N occurs atMt. Meager (Figure 7b). However, Mt. Baker sam-ples extend to lower (La/Yb)N and higher Yb val-ues than the BSM basalts (Figure 7). Salal Glacierand Bridge River have no Nb anomalies whilesmall negative anomalies occur at Mt. Meager andprominent ones at Mt. Baker (Figure 9a). Ba/Lavalues are lowest at Salal Glacier and BridgeRiver, intermediate at Mt. Meager, and highest atMt. Baker (Figure 9b). Abundances of all traceelements in Bridge River and Salal Glacier primi-tive basalts are similar to samples from nonsub-duction settings, including Hawaiian postshieldalkalic basalts [Hanano et al., 2010], and overlapwith alkalic basalts from the Anahim volcanic belt[Charland et al., 1995], Cascade-Columbia tran-sect [Leeman et al., 2005; Jones, 2002], andDalles Lakes north of Mt. Rainier [Reiners et al.,2000] (Figures 8, 9a, and 9c).[26] The five basalts comprising Suite 2(BRC03?4, BRC01?3, MM01-1, MB1, andLib21) have trace element abundances that con-trast with other basalts at their respective vol-canic centers, including significantly higherLILE (La/Yb)N and Ba/Nb, and lower HFSE(Figures 6b?6e). These samples are excludedfrom the following discussion of mantle sourcecharacteristics but are revisited later in the contextof crustal assimilation.6. Discussion6.1. Mantle Source Characteristics6.1.1. Temperatures and Pressures[27] The BSM basalts segregated from their man-tle source at significantly higher pressures andtemperatures than the Mt. Baker basalts (Figure10). Liquidus pressures and temperatures (i.e.,mantle potential temperatures) were calculated forthe two most primitive basalts at each BSM vol-canic center (Table 3) using the olivine-liquid geo-thermometer and silica activity geobarometer ofPutirka [2008]. Whole-rock data [Green andSinha, 2005] were first adjusted into equilibriumwith Fo90 mantle by incremental olivine additionFigure 5. 208Pb/204Pb versus (a) wt % SiO2, and (b) wt % MgO. Note reversed scale for MgO.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913136Table 2. Trace Element AbundancesSample Number BRC01?3 BRC02 BRC03?4 BRC04 BRC05-1 BRC06 BRC07-2 BRC09-3 BRC10 SG01-1Methoda 1 1 2 1 1 1 2 1 1 1Concentrationb (ppm)Li 7.2 8.8 10 8 7.7 8.0 8.2 6.1 6.4 4.4Cs 0.17 0.22 0.46 0.17 0.14 0.18 0.13 0.07 0.09 0.19Rb 11 15 16 13 11 14 12 4.0 7.7 12Ba 413 310 450 303 269 288 286 195 176 269Th 2.0 1.6 2.2 1.5 1.5 1.5 1.8 0.94 0.87 1.7U 0.66 0.64 0.65 0.41 0.57 0.60 0.63 0.34 0.35 0.63Nb 14 27 16 25 26 26 28 17 15 29Ta 0.87 1.9 0.92 1.4 1.7 1.8 1.6 1.2 1.1 1.8La 24 20 24 18 20 17 22 14 12 20Ce 54 45 53 43 48 42 50 30 26 43Pb 4.2 2.8 3.9 3.0 2.2 2.6 2.3 1.7 1.3 2.1Pr 6.7 5.7 6.8 5.6 6.3 5.6 6.7 4.2 3.6 5.5Sr 1251 626 1213 577 587 650 628 399 388 545Nd 28 26 28 25 28 24 29 18 16 24Sm 5.2 6.0 5.5 5.9 6.7 5.9 6.8 4.7 4.3 5.5Zr 127 174 143 168 200 168 213 127 108 154Hf 3.4 4.1 3.3 4.1 4.6 3.9 4.8 3.2 2.9 3.7Eu 1.6 2.0 1.8 1.9 2.2 1.8 2.3 1.7 1.5 1.7Gd 4.2 6.2 4.8 6.0 6.5 5.8 6.6 5.1 4.7 5.2Tb 0.58 0.89 0.68 0.86 0.98 0.85 0.96 0.75 0.74 0.73Dy 3.4 4.7 3.9 4.9 5.6 4.9 5.7 4.3 4.0 4.0Y 19 25 21 27 31 26 30 25 23 22Ho 0.66 0.91 0.74 0.90 1.0 0.84 1.1 0.82 0.75 0.69Er 1.7 2.3 2.0 2.3 2.8 2.4 2.9 2.2 1.9 1.9Tm 0.24 0.32 0.31 0.39 0.31 0.31 0.28 0.27Yb 1.4 1.9 1.7 1.8 2.3 1.9 2.4 1.8 1.5 1.5Lu 0.20 0.27 0.23 0.26 0.33 0.24 0.34 0.26 0.22 0.22Sc 18 20 19 20 24 20 23 23 22 20Zn 85 101 98 111 122 106 126 98 107 100Cr 48 98 47 100 44 115 34 325 310 321Ni 44 50 41 51 36 56 32 134 228 281V 202 211 216 207 284 207 282 196 200 199Ga 23 24 24 27 24 21 22 21Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Concentration (ppm)Li 7.1 6.5 6.6 5.6 6.4 6.5 7.5 7.7 5.4 8.3Cs 0.20 0.17 0.15 0.14 0.18 0.10 0.08 0.13 0.08 0.44Rb 18 17 16 12 17 8.8 7.6 12 8.2 15Ba 339 330 330 247 264 676 249 265 154 659Th 2.2 2.1 1.9 1.7 1.7 1.7 1.1 1.2 0.65 2.4U 0.73 0.76 0.71 0.54 0.66 0.63 0.71 0.54 0.34 0.79Nb 31 31 32 26 26 12 17 19 9.1 6.3Ta 1.7 1.5 1.6 1.9 1.4 0.7 1.1 1.3 0.63 0.31La 23 24 22 18 18 27 13 15 8.4 24Ce 49 49 47 40 43 62 30 31 21 53Pb 2.2 2.1 2.2 2.3 2.0 5.2 1.9 2.0 1.4 3.8Pr 6.2 6.1 6.0 5.2 5.6 8.4 4.0 4.2 3.0 7.1Sr 658 589 585 508 624 1588 479 545 467 1455Nd 26 25 26 23 24 35 17 18 14 30Sm 5.5 5.4 5.7 5.1 5.3 6.7 4.2 4.4 4.0 5.8Zr 171 161 164 141 150 147 112 122 92 93Hf 3.6 3.8 3.7 3.3 3.5 3.4 2.9 3.0 2.7 2.4Eu 1.8 1.9 1.9 1.6 1.8 2.1 1.5 1.6 1.4 1.8Gd 5.1 5.3 5.1 5.0 5.2 5.5 4.2 4.8 4.3 4.7Tb 0.72 0.76 0.78 0.68 0.76 0.71 0.65 0.68 0.64 0.61Dy 4.1 4.0 4.4 3.7 4.3 4.0 3.8 3.8 3.9 3.4Y 22 23 25 21 21 21 20 22 21 18Ho 0.77 0.77 0.81 0.69 0.73 0.77 0.69 0.74 0.69 0.65Er 2.0 2.0 2.2 1.7 2.0 2.0 1.8 1.9 1.9 1.8Tm 0.29 0.30 0.24 0.26 0.24 0.27 0.25MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913137assuming Fe3?/PFe? 0.15. Amounts of olivineadded range from 11.5% (SG16) to 17.7%(BRC09). Primary biotite and amphibole in thegroundmass of some primitive BSM samples attestto the presence of water, so pressures and tempera-tures were calculated for several possible H2Ocontents (listed in Table 3). The plagioclase-liquidhygrometer of Lange et al. [2009] applied to themost primitive Salal Glacier basalt gives 1 wt %H2O at the liquidus assuming plagioclase satura-tion at P? 100 MPa and maximum An60 in plagio-clase cores [Lawrence, 1979]. At this watercontent, pressures calculated for primitive BSMbasalts correspond to depths ranging from 70 km(Mt. Meager) to 105 km (Bridge River) (Figure10). Decreasing melt SiO2 with increasing pres-sure [Longhi, 2002] is consistent with the P-Tdata. Calculated mantle potential temperatures are100?200C higher than predicted for the Cas-cade subarc mantle wedge [Syracuse et al., 2010],and similar to those of average MORB (1454C)[Putirka, 2008]. Intraplate basalts of the westernBasin and Range province give a broadly similarP-T range (60?90 km, 1350C1450C) as theBSM basalts [Lee et al., 2009]. Intraplate basaltsin the Cascade-Columbia transect have lower max-imum segregation depths (75?80 km) but similarmaximum temperatures (1460C), although an-hydrous conditions were assumed [Leeman et al.,2005]. Simcoe intraplate basalts record P-T condi-tions similar to the BSM basalts (max 100 km,1500C) [Leeman et al., 2005].[28] For Mt. Baker basalts, liquidus water contentsare 1.5 to 3.7 wt % (Mullen and McCallum, sub-mitted manuscript, 2013) and mantle potentialtemperatures are 1273C to 134C (Figure 10,Table 3), within the range for the subarc mantlewedge [Syracuse et al., 2010]. Mantle segregationdepths are 35 to 52 km, i.e., ranging from theMoho to just above the hot core of the mantlewedge. The shallower depths recorded by the Mt.Baker basalts are consistent with trace elementmodeling (below) that indicates residual garnet forthe BSM basalts but not Mt. Baker.6.1.2. Mantle Isotopic Characteristics[29] Primitive BSM basalts (Mg/[Mg?Fe2?]> 0.6)have isotope ratios that define a narrow range, con-sistent with a common mantle source and differen-tiation dominated by fractional crystallization. Pbisotope ratios overlap with Explorer and northernJuan de Fuca MORB [Cousens et al., 1995; B.Cousens, unpublished data 2007], Chilcotin plateaubasalts [Bevier, 1983], and the least radiogenic sam-ples from the Anahim volcanic belt [Bevier, 1989](Figure 11). The isotopic similarity among thesevolcanic provinces confirms that the northwesternmargin of North America is underlain by uppermantle that is relatively depleted and generally sim-ilar to northeastern Pacific mantle [Cousens andBevier, 1995; Bevier, 1989].[30] Although the BSM basalts have 208Pb/204Pbsimilar to local MORBs at a given 206Pb/204Pb,207Pb/204Pb is slightly higher (Figure 3).Relatively high 207Pb/204Pb could be interpreted asreflecting subducting sediment input, but thisshould increase 208Pb/204Pb along with207Pb/204Pb, and the BSM basalts overlap withMORB in 208Pb/204Pb.[31] High 207Pb/204Pb relative to 208Pb/204Pb mayinstead indicate a higher time-integrated U/Th inthe BSM source than in the MORB sources.Table 2. (continued)Sample # SG01?2 SG01?3 SG10 SG12 SG16 MM01-1 MM02 MM04 MM08 LIB-21Method 2 1 1 1 1 2 1 1 1 2Yb 1.7 1.7 1.7 1.4 1.5 1.7 1.5 1.5 1.6 1.5Lu 0.24 0.23 0.26 0.21 0.22 0.25 0.21 0.22 0.22 0.22Sc 20 20 22 19 20 23 20 22 21 25Zn 102 93 103 95 106 100 106 102 106 91Cr 248 255 239 332 330 112 274 271 255 55Ni 227 216 130 301 285 49 188 146 156 24V 198 188 227 157 212 208 176 185 164 199Ga 20 22 17 21 20 21 22aMethod 1: Thermo Finnigan Element2 HR-ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS.bData were corrected for blank contributions and by sample-standard bracketing to published values for the USGS BCR2 reference material[Raczek et al., 2001] analyzed every eight samples (method 1), or the USGS AGV1 reference material [Chauvel et al., 2011] analyzed every sixsamples (method 2). Repeat analysis of the USGS BHVO2 standard gave RSD values of <5% and concentrations within 5% (relative) of pub-lished values (as compiled by Carpentier et al. [2013] from GeoRem) for most elements. The average BHVO2 values obtained during analyticalsessions are reported in Table S1. Duplicate analyses gave reproducibilities better than 5% (Table S1). Total procedural blanks (Table S1) werenegligible relative to analyzed sample concentrations.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913138Enrichment of the BSM mantle source in U relativeto Th at some time in the past could be accom-plished through addition of fluid or melt compo-nents derived from subducting sediment and/oroceanic crust, since U is slightly more incompatiblethan Th during dehydration and melting [Brenan etal., 1995; Kessel et al., 2005; Hermann andRubatto, 2009]. However, this situation wouldresult in the presence of a subduction signature inthe BSM mantle source, which is not observed.[32] A more plausible explanation may be meltingof the BSM mantle source in the presence of resid-ual garnet at some time in the past. Unlike othertypical mantle minerals, which do not fractionateU and Th appreciably, U is more compatible ingarnet than Th [Beattie, 1993; LaTourrette et al.,1993]. The BSM basalts also plot at the lower mar-gin of the Hf-Nd mantle array (Figure 4b), consist-ent with the isotopic evolution of mantle thatproduced melts within the garnet stability field[Carlson and Nowell, 2001].[33] Because BSM basalts have lower EHf (3 ep-silon units) than Mt. Baker but similar ENd (Figure4b), two distinct mantle sources are required. Par-titioning experiments show that Hf is preferen-tially retained in the subducting slab relative toNd, most effectively during slab dehydration butalso during slab melting [e.g., Kessel et al., 2005;Hermann and Rubatto, 2009]. High Nd/Hf in thesubduction component is further enhanced by theFigure 6. (a?d) Extended N-MORB normalized [Sun and McDonough, 1989] trace element diagrams, sub-divided by volcanic center. (d) Light gray field in each panel encompasses the range defined by Mt. Bakerbasalts. The darkest colors (with sample numbers) signify Suite 2 samples discussed in the text. (e) All of theSuite 2 samples are plotted together for comparison to Mt. Baker basalts.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913139preexisting negative HFSE anomalies that charac-terize Cascadia sediment [Carpentier et al., 2013;Prytulak et al., 2006]. As a consequence, additionof a sediment component to the BSM mantlesource generates mixing curves that extendtowards lower ENd values but with smaller changesin EHf. Most importantly, mixing trajectoriesextend away from the Mt. Baker data. Thus sub-duction input cannot account for the Hf isotopicdistinction between the BSM and Mt. Bakerbasalts; the difference is instead a primary featureof their respective mantle sources. Since mantleHf isotope ratios can be affected by both fluidsand melts derived from the slab (Figure 4b), Hfisotopes do not always directly record the isotopiccomposition of the mantle as is commonlyassumed.6.1.3. Mantle Source Fertility[34] Zr/Nb in basalts provides a useful indicator ofmantle source fertility because this ratio is mini-mally affected by subduction input or fractionalcrystallization (Figure 8b). Although Zr/Nb is con-trolled to some extent by melt fraction, the Zr/Nbrange defined by melts of average depleted mantledoes not overlap the range for melts of moreenriched mantle tapped by ocean island basalts.Mt. Baker basalts have Zr/Nb consistent with10% partial melting of average depleted mantlewith an additional slab component (Figure 8b). Zr/Nb is too low in the primitive BSM basalts to beproduced from the same mantle source as Mt.Baker, requiring a more incompatible elementenriched mantle source. The relatively high Nbcontents of the BSM basalts also indicate a sourcerelatively enriched in incompatible elements (Fig-ure 8a), as do high Na2O and TiO2 [Prytulak andElliott, 2007].6.1.4. Assessment of Subduction Input[35] For primitive samples at Salal Glacier andBridge River, Ba/Nb values lie within the range ofHawaiian basalts and coincide with melting curvesfor enriched mantle (at 2?5% partial melt) (Fig-ure 8), pointing to the likelihood that a slab-derived component was not present in the mantlesource. The absence of slab input is supported bythe absence of negative Ta-Nb anomalies (FigureFigure 7. (a?d) Chondrite-normalized [McDonough and Sun, 1995] rare-earth element diagrams subdividedby volcanic center.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913140Figure 8. (a) Ba (ppm) versus Nb (ppm); (b) Zr/Nb versus Ba/Nb. Except for Suite 2 samples (diamonds),plus MM08 from Mt. Meager, the BSM basalts are similar to basalts from MORB and OIB settings, i.e.,essentially no subduction component. MORB data (gray filled circles) and Hawaiian basalt data (shield andpostshield lavas shown as filled green and pink circles, respectively) were obtained from the PETDB (http://www.earthchem.org/petdb) and GEOROC databases (georoc.mpchmainz.gwdg.de/georoc), respectively,accessed in May 2012. Average OIB composition [Sun and McDonough, 1989] is shown as a black asterisk inFigure 8b. Black arrows in lower right corner of Figure 8a and upper right corner of Figure 8b show the effectof 15% fractionation of each mineral indicated, calculated using the Rayleigh equation, the starting composi-tion of BRC09-3, and partition coefficients listed in Table 4 plus ilmenite from McCallum and Charette[1978]. Only ilmenite and magnetite vectors are shown in Figure 8b because the other phases shown in Figure8a have a negligible effect. Orange and blue curves are for addition of subduction components to a depletedmantle source (average depleted MORB mantle of Salters and Stracke, 2004) prior to 10% partial melting,calculated as described in Fig. 4 caption. Most subduction components increase Ba at a given Nb, and Ba/Nbat a given Zr/Nb. The heavy black curves are the compositions of DM melts at 1 GPa and PM melts at 3 GPa,respectively (tick marks indicate % partial melt). Melt compositions were calculated using the equilibriummelting equation with mineral/melt partition coefficients from Table 4 and residual mantle mineral assemb-lages determined by BATCH modeling [Longhi, 2002] of starting compositions of Wasylenki et al. [2003]and Kinzler [1997]. Filled black squares with white (cross) and (plus) symbol are for DM (depleted MORBmantle) of Salters and Stracke [2004] and PM (primitive mantle) of Sun and McDonough [1989], respec-tively. Inset diagrams include data for alkalic basalts (molar Mg/(Mg? Fe2?)> 0.60, assuming Fe3?/PFe? 0.15) from the Anahim volcanic belt (dark blue squares) [Charland et al., 1995], Cascade-Columbiatransect (light blue squares) [Leeman et al., 2005; Jones, 2002], and Dalles Lakes north of Mt. Rainier (purplesquares) [Reiners et al., 2000]. Abbreviations: ol (olivine), opx (orthopyroxene), cpx (clinopyroxene), plag(plagioclase), ilm (ilmenite), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131419a) that are observed at Mt. Baker and in othercalc-alkaline Cascade arc basalts [e.g., Schmidt etal., 2008]. The absence of slab input is further sup-ported by the similarity of Salal Glacier andBridge River primitive basalts to Mauna Kea post-shield alkalic basalts [Hanano et al., 2010], whichsample the same mantle source as shield lavas,that is, a composition comparable to the PREMA,or C, mantle component [Nobre Silva et al., 2013](Figure 9c). The only major difference is in Pb,which is deficient in Hawaii (a ubiquitous featureof oceanic basalts [Hofmann, 1997]) but showssmall positive spikes at Bridge River and SalalGlacier. The Pb spikes are successfully modeledwithout subduction input (see later). However, wecannot definitively rule out the presence of a verysmall subduction component in the mantle source.An ancient subduction component may have beenadded to the source in the past, or the primaryalkalic magmas may have acquired a small sub-duction component during migration through themantle.Figure 9. (a) EHf versus Nb/Nb (niobium anomaly) for BSM and Mt. Baker basalts, compared to the rangesdefined by Hawaiian shield and postshield basalts (green and pink lines, respectively) with >8 wt % MgO.Nb/Nb calculated as 2(Nbsample/NbPM)/(Basample/BaPM?Lasample/LaPM) [Verma, 2009] where PM refers toPrimitive Mantle. Hawaii data were obtained from the GEOROC database accessed in May 2012(georoc.mpchmainz.gwdg.de/georoc). Also shown are values for average depleted mantle (DM, black square)[Salters and Stracke, 2004] and average N-MORB (gray square) and Primitive Mantle (PM, black square withwhite ?) of Hofmann et al. [1988]. (b) Ba/La versus 208Pb/204Pb for BSM and Mt. Baker basalts. (c) N-MORB normalized [Sun and McDonough, 1989] extended trace element diagram comparing Salal Glacier(orange) and Bridge River (red) primitive basalts (molar Mg/[Mg?Fe2?]> 0.60) to alkalic postshield basaltsfrom Mauna Kea (light blue) [Hanano et al., 2010], Simcoe volcanic field (dark blue) [Battleground Lakesample of Jones, 2000], and Anahim volcanic belt (green) [sample 2278 of Charland et al., 1995].MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913142[36] In contrast to Salal Glacier and Bridge River,even the most primitive Mt. Meager basalts haveslightly elevated Ba and Ba/Nb relative to mantlemelting curves and Hawaiian basalts (Figure 8), aswell as small negative Nb anomalies (Figure 9a),all of which point to subduction input (althoughsignificantly less than at Mt. Baker). Zr/Nb in thetwo of the three primitive Mt. Meager basalts isthe same as at Salal Glacier and Bridge River,indicating similar mantle sources. In the other Mt.Meager sample (MM08), higher Zr/Nb and lowerNb is consistent with a mantle source that is transi-tional between the Mt. Baker mantle source andthat of the other BSM basalts. An intermediate Hfisotopic composition for MM08 (Figure 4b) sup-ports this conclusion.[37] Mt. Meager basalts have the lowest206Pb/204Pb and 208Pb/204Pb of all the BSM vol-canic centers; sample MM08 has the lowest208Pb/204Pb, 206Pb/204Pb, and (La/Yb)N, coupledwith the highest Ba/La (Figure 9b). These charac-teristics are not consistent with addition of a sub-ducting sediment component to the mantle sourceand may instead reflect the influence of a fluidderived from altered oceanic crust (AOC). AOCfluid input can increase LILE in the mantle sourcewithout affecting LREE [Kessel et al., 2005], andsince recent AOC has MORB-like Pb isotoperatios, it is capable of ??pulling?? Pb isotope ratiosof the mantle source to lower values. Mt. Meageralso has similar ENd to Bridge River and Salal Gla-cier but slightly higher 87Sr/86Sr (Figure 4a), con-sistent with the involvement of AOC that acquireshigh 87Sr/86Sr with minimal change in ENd duringprogressive seafloor alteration [Staudigel et al.,1995].6.1.5. Trace Element Modeling[38] Mantle melt fractions and residual mantle min-eral assemblages were determined by modeling theabundances of 28 trace elements in the two mostprimitive basalts at each BSM center (BRC09 andBRC10 at Bridge River, MM04 and MM02 at Mt.Meager, SG10 and SG16 at Salal Glacier). We alsomodeled MM08 at Mt. Meager because it may havea slightly more depleted mantle source.[39] The model is based upon the mass balanceequation for equilibrium melting, CLi = C0i ?Figure 10. Pressure versus temperature plot illustrating theconditions at which the BSM and Mt. Baker magmas segre-gated from the mantle. P and T (from Table 3) were calcu-lated using the silica activity geobarometer and olivine-liquidgeothermometer calibrations of Putirka [2008] for 1% and2% dissolved water (BSM basalts) or for the specific H2Ocontent given (Mt. Baker basalts). Standard estimates of errorare 43C and 0.29 GPa [Putirka, 2008].Table 3. Liquidus pressures and temperaturesH2O (wt.%)0.0 1.0 2.0P(GPa) T(C) P(GPa) T(C) P(GPa) T(C)Bridge RiverBRC10 3.49 1576 3.19 1535 2.89 1496BRC09 2.68 1505 2.50 1470 2.31 1437Salal GlacierSG10 2.82 1514 2.61 1478 2.41 1443SG16 2.67 1502 2.48 1467 2.29 1433Mt. MeagerMM02 2.04 1441 1.91 1409 1.79 1379MM04 2.15 1460 2.02 1427 1.86 1395a Mt. Baker P (GPa) T (C) H2O (wt.%)MB5 0.90 1273 2.7MB82 1.4 1326 2.1MB1 1.1 1274 3.7MB97 1.2 1309 2.1MB114 1.4 1326 15MB112 1.5 1350 1.5aP-T data for Mt. Baker (from Mullen and McCallum, submitted manuscript, 2013) are calculated only at the specific water content listed foreach sample.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.2019131431= FL 1 Di ? Di [Shaw, 1970], where CLi isthe concentration of trace element (i) in the liquid(L), C0i is the initial concentration of trace element(i), FL is melt fraction, and Di is the bulk partitioncoefficient, defined as crystalline assemblage/melt.The model does not require a priori knowledge ofinitial mantle mineral assemblages but does requireinitial trace element abundances. The primitivemantle composition of Sun and McDonough [1989]was used as the source for all BSM samples, and amixture of 50% primitive mantle and 50% depletedmantle [Salters and Stracke, 2004] was also testedfor MM08. Distribution coefficients used in the cal-culations are listed in Table 4. Least-squares mini-mization was used to generate best fit models forthe basalts by varying the mantle mineral modesand melt fractions (see the caption of Figure 12).Note that the substitution of fractional melting inour model results in negligible change to modeloutcomes. Melt fractions are within 0.5% and resid-ual mantle modal abundances change by less than afew percent, with overall residual mineral assemb-lages remaining identical.6.1.5.1. Modeling Results[40] Representative best fit trace element solutionsare shown in Figure 12. Melt fractions are 2?4%for Salal Glacier, 4?5% for Mt. Meager, and 7?8%for Bridge River, all with residual garnet lherzo-lite. Lower melt fractions for Salal Glacier basaltsare consistent with their higher alkali elementabundances. For Mt. Meager sample MM08, aprimitive mantle source indicates 8% partial meltand the mixed PM-DM source gives 4%. The latterresult is preferred because it is more consistentwith the results for other Mt. Meager samples. Re-sidual garnet in all samples is consistent with thepressures of melting (2?3 GPa) calculated for theBSM basalts, as garnet is stable at the solidus ofhydrous mantle at pressures above 1.6 GPa [Gae-tani and Grove, 1998].[41] Results of similar modeling for Mt. Bakerbasalts, using a depleted mantle source, indicate5?12% partial melting of depleted lherzolite orharzburgite. Best fit solutions require overprintingby a subduction component consisting of AOCfluid, AOC melt, and sediment melt (Mullen andMcCallum, submitted manuscript, 2013). No re-sidual garnet is present in the Mt. Baker source,consistent with calculated melt segregation pres-sures (1?1.5 GPa) and with the lower (Dy/Yb)N,and higher Yb and Sc contents of the Mt. Bakersamples (24?33 ppm Sc) (Mullen and McCallum,Figure 11. Plot of 208Pb/204Pb versus 206Pb/204Pb comparing Pb isotope ratios for the BSM and Mt. Bakerbasalts (symbols as in Figure 9) to other basalts from the northeastern Pacific and southwestern British Colum-bia: Anahim volcanic belt (purple diamonds; Bevier [1989]); Chilcotin Plateau (black diamonds; Bevier[1983]); Wells Gray-Clearwater volcanic field (blue diamonds; Hickson [1987]); and Tuzo Wilson volcanicfield [Allan et al., 1993]. Other data and references as in Figure 3. Note that all isotope data are normalized tothe same standard values as described in main text.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913144submitted manuscript, 2013) as compared with theBSM basalts (18?24 ppm Sc; Table 2).6.4. Nonprimitive BSM Basalts: CrustalContamination or Subduction Input?[42] Although we make the case above that theprimitive Salal Glacier and Bridge River basaltsessentially lack a subduction component, some ofthe nonprimitive BSM basalts have geochemicalcharacteristics that could be interpreted as an ??arcsignature.?? Do these reflect subduction input thatis not displayed by more primitive samples?[43] BSM samples with Mg/(Mg? Fe2?)< 0.60are subdivided into two suites based upon isotopicand trace element compositions: Suite 1 has iso-tope and trace element ratios similar to primitiveBSM basalts, indicating minimal crustal contami-nation and differentiation processes dominated byfractional crystallization. Relative to the mostprimitive basalts, Suite 2 has high Sr-Pb isotoperatios, (La/Yb)N, and incompatible element abun-dances, coupled with low Nd-Hf isotope ratios andHFSE abundances.[44] Pearce element ratio diagrams [Russell andNicholls, 1988] show that the Suite 1 basalts areconsistent with fractionation of olivine? plagio-clase (6 minor clinopyroxene) from parental mag-mas that were similar to the most primitive basaltsat each volcanic center. The crystallizing assemb-lages are consistent with the presence of the sameminerals as phenocryst phases.[45] For Suite 2 samples, trace element abundan-ces and Pb isotope ratios are nearly indistinguish-able from the most primitive Mt. Baker basalts(Figures 3, 6e, and 8), pointing to the possibilitythat they may record input from the subductingslab as does Mt. Baker. However, Suite 2 has sig-nificantly higher 87Sr/86Sr and lower ENd than themost primitive Mt. Baker and BSM basalts (Figure4a). The lower MgO contents of Suite 2 lavas rela-tive to the most primitive lavas are consistent withTable 4. Partition Coefficients (Mineral/Melt)acpx opx oliv sp gar amph phlog mt plagCs 0.000201 0.00091 0.00004525 0.000625 0.00011 0.02325 2.261 0.00121 0.00623Rb 0.000603 0.00381 0.0000453 0.000625 0.00022 0.0232 1.702 0.00121 0.01823Ba 0.000683 0.00361 0.0000433 0.00067 0.000072 0.012 1.502 0.00121 0.3223Th 0.0124 0.00051 0.000053 0.0107 0.00212 0.00102 0.000201 0.002420 0.1923U 0.0134 0.00071 0.000053 0.0147 0.00947 0.00122 0.000201 0.01220 0.3423Nb 0.0051 0.00071 0.000413 0.0217 0.00315 0.082 0.0551 0.8620 0.00823Ta 0.0211 0.00081 0.00021 0.0217 0.0199 0.0831 0.0621 0.9520 0.02723K 0.00725 0.00012 0.000022 0.00125 0.0132 0.222 1.502 0.00121 0.09721La 0.0546 0.00061 0.000053 0.0119 0.001615 0.05524 0.0002525 0.001220 0.1123Ce 0.08625 0.00171 0.000063 0.0119 0.00515 0.09625 0.000301 0.001920 0.08525Pb 0.0104 0.00011 0.0000712 0.00057 0.000315 0.0424 0.091 0.02221 0.108523Pr 0.1425 0.002625 0.0001325 0.0119 0.02925 0.1316 0.000425 0.002320 0.06525Sr 0.04811 0.0093 0.0002512 0.00477 0.002515 0.3015 0.161 0.003020 1.9423Nd 0.1925 0.0041 0.000203 0.0119 0.05215 0.18716 0.000551 0.00425 0.05223Sm 0.276 0.0111 0.000603 0.0119 0.2515 0.3216 0.000701 0.007020 0.04123Zr 0.061 0.0133 0.0006810 0.00817 0.6614 0.1816 0.0111 0.5620 0.003923Hf 0.121 0.0133 0.001110 0.00307 0.6814 0.6316 0.0161 0.6520 0.001523Eu 0.4525 0.01625 0.0008025 0.0119 0.4015 0.4316 0.000725 0.01025 1.4223Ti 0.308 0.06110 0.00221 0.04819 0.2915 0.952 0.791 2021 0.04723Gd 0.5025 0.02225 0.000993 0.0119 0.9025 0.5416 0.000725 0.01620 0.03521Tb 0.5625 0.0301 0.0023 0.0119 1.415 0.6025 0.00071 0.02325 0.03121Dy 0.6125 0.03825 0.0043 0.0119 2.215 0.6325 0.000825 0.03325 0.02621Y 0.6525 0.0461 0.0073 0.00207 3.115 0.5215 0.0031 0.0520 0.02621Ho 0.656 0.0481 0.0063 0.0119 2.815 0.6224 0.00091 0.0525 0.01821Er 0.6925 0.05825 0.00873 0.0119 3.6 15 0.5724 0.001025 0.0725 0.014521Tm 0.7225 0.0711 0.01325 0.0119 3.725 0.5325 0.00141 0.01125 0.01221Yb 0.7425 0.0771 0.0173 0.0119 3.94 0.4825 0.001625 0.1725 0.009721Lu 0.756 0.0901 0.0203 0.0119 3.84 0.4324 0.00171 0.2820 0.00821aAbbreviations: cpx (clinopyroxene), opx (orthopyroxene), oliv (olivine), sp (spinel), gar (garnet), amph (amphibole), phl (phlogopite), mt(magnetite), plag (plagioclase).Data sources: 1Adam and Green [2006]; 2Halliday et al. [1995] compilation; 3Donnelly et al. [2004] compilation; 4Hauri et al. [1994]; 5Hartand Dunn [1993]; 6Gaetani [2004]; 7Elkins et al. [2008]; 8McDade et al. [2003]; 9Green et al. [2000]; 10Kennedy et al. [1993]; 11Beattie[1993];12Beattie [1994]; 13Canil and Fedortchouk [2001]; 14Salters and Longhi [1999]; 15Abraham et al. [2005] compilation; 16Chazot et al.[1996]; 17Horn et al. [1994]; 18Nagasawa et al. [1980]; 19McKenzie and O?Nions [1991]; 20Klemme et al. [2006]; 21Claeson and Meurer [2004]compilation; 22Dunn and Sen [1994]; 23Tepley et al. [2010]; 24LaTourrette et al. [1995]; 25Interpolated from neighboring elements.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913145crustal contamination (Figure 5b). Furthermore,Suite 2 contains the only two BSM basalts withxenocrysts (BRC01?3, BRC03?4).[46] The GVB crustal basement is a collage of Pa-leozoic and Mesozoic accreted terranes [Mongeret al., 1982]. At depths greater than 10 km, theGVB is underlain by the composite Wrangellia?Harrison terranes [Mullen, 2011; Miller et al.,2009; Monger and Price, 2000]. With the excep-tion of the Mt. Baker region, the terranes areintruded extensively by Jurassic to Cretaceousgranitoids of the Coast Plutonic Complex, the larg-est composite batholith in North America [Barkerand Arth, 1984; Friedman et al., 1995; Cui andRussell, 1995a, 1995b]. Because the crust is rela-tively young and Cascadia subducting sediment ismainly terrigenous [Carpentier et al., 2013; Pry-tulak et al., 2006], the isotopic effects of crustalassimilation are similar in many respects to theeffects of subducting sediment input. However,sediment input cannot account for the 87Sr/86Srversus Sr systematics of the Suite 2 lavas (Figure13a). Assimilation-fractional crystallization (AFC)modeling [DePaolo, 1981] using a granodioriticassimilant from the Coast Plutonic Complex canreproduce the Suite 2 trace element and isotopicdata, but the volume of assimilant required(>20%) would increase the SiO2 content beyondthe range of the Suite 2 samples (Figure 5a).Assimilation that takes place in the deep crust,Figure 12. Best fit trace element solutions for four of the most primitive BSM basalts, shown on N-MORBnormalized extended element diagrams and accompanying inset chondrite-normalized REE diagrams. Actualdata are shown with colored lines and symbols; modeling solutions shown as heavy black lines with blacksquares. Each BSM sample has been adjusted into equilibrium with Fo90 mantle using olivine/melt partitioncoefficients from Table 4. The mantle source composition used in the model for all BSM basalts (PM of Sunand McDonough [1989]) is shown as a thin black line in each panel; DM (source used for Mt. Baker) is alsoshown for reference [Salters and Stracke, 2004]. The Generalized Reduced Gradient (GRG2) nonlinear opti-mization code in Microsoft Excel Solver was used to obtain the best fit for each basalt by minimizing the sumof squares of residuals for 28 trace elements, i.e.,Pi Cliqi calc? ?  Cliqi obs? ?h i=Cliqi ?obs?n o2. The denomina-tor in the equation normalizes the concentrations of the elements so that each trace element has an equivalentimpact on the solution regardless of its absolute concentration. Best fit melt fractions (FL) and residual mantlemineral modes are given in lower right corner of each N-MORB-normalized panel. Abbreviations: ol (oli-vine), opx (orthopyroxene), cpx (clinopyroxene), gar (garnet).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913146where the country rock is mafic, can minimizechanges to the major element abundances of theoriginal basaltic magma [Reiners et al., 1995,1996]. AFC models in which the assimilant is agabbro from the lower crustal section of the Bo-nanza arc (Westcoast crystalline complex) ofWrangellia [DeBari et al., 1999] provide good fitsto Suite 2 trace element and isotope data with15% gabbro assimilated at Bridge River and21% at Mt. Meager (Figure 13). Modeling pa-rameters and results are provided in Table 5. Frac-tionating mineral phases (olivine, clinopyroxene,orthopyroxene, and minor magnetite) are consist-ent with experimental results for partial melting ofmafic compositions under lower crustal conditions[Rapp, 1995; Rapp and Watson, 1995]. Becausethe assimilant has a low SiO2 content (45 wt %)and the fractionating mineral assemblages havebulk SiO2 contents similar to the basalts, the finalmagmas maintain an overall basaltic compositionin the magmas.6.5. Relationship Between Tectonics andVolcanism[47] An incompatible element-enriched, garnet-bearing mantle source essentially free of subduc-tion input, coupled with relatively high mantlemelting temperatures and pressures, is consistentwith decompression melting of an upwelling as-thenosphere source for the primitive BSM basalts.Upwelling mantle is potentially consistent with aslab edge effect as proposed by Lawrence et al.[1984] for Salal Glacier basalts. Seismic anisot-ropy measurements reveal toroidal mantle flowaround the descending edges of subducted platesthat are undergoing rollback, thereby drawingexternal mantle (subslab) into the mantle wedgeFigure 13. AFC modeling results for (a) 87Sr/86Sr versus Sr; (b) ENd versus87Sr/86Sr; (c) Zr/Nb versus Ba/Nb. Heavy green curves with triangles and squares (AFC1 and AFC2, respectively) are the best fitassimilation-fractional crystallization pathways for Suite 2 samples BRC03?4 and MM01-1, respectively,using a gabbroic assimilant (from Table 5). The large green triangle and square are the gabbro compositionsused as assimilants in AFC 1 and 2, respectively. Orange and blue curves in Figure 13a are slab fluid/meltaddition curves, calculated as described in Fig. 4 caption. Heavy black curves in Figure 13c are from Figure8b. Data for the Coast Plutonic Complex shown as small blue triangles and light blue field [Friedman et al.,1995; Cui and Russell, 1995a, 1995b]. Sources of other data shown are given in Figure 4a caption.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913147[Long and Silver, 2008]. In other arcs, influx ofexternal mantle has been implicated in the genesisof lavas that are atypical for an arc setting [e.g.,Leat et al., 2004; Smith et al., 2001; Ferrari et al.,2001]. Slab rollback is occurring in the Cascadearc [Schellart, 2007], but the limited mantle ani-sotropy measurements in the GVB are inconclu-sive as to mantle wedge flow patterns [Currie etal., 2004]. Toroidal mantle flow has been docu-mented at the southern Juan de Fuca plate edge[Zandt and Humphreys, 2008], yet alkalic basaltsare not present [Hildreth, 2007] indicating that thetwo phenomena are not necessarily interrelated.[48] A slab edge origin may be improbable for theBSM volcanic centers in light of recent seismic to-mography, which indicates the northernmost slabedge in the Cascades (placed at the northernmostlimit of the Explorer plate) is located farther norththan the BSM volcanic centers [Mercier et al.,2009; Audet et al., 2008]. Toroidal mantle flowhas been proposed for the northern Explorer plateedge [Audet et al., 2008] and could be responsiblefor the alkalic basalts of the 500 km long Anahimvolcanic belt, which defines an east-west trendnearly orthogonal to, and north of, the GVB. Thisinterpretation is consistent with that of Thorkelsonet al. [2011] who proposed that Anahim magma-tism is related to mantle upwelling along the ther-mally eroding plate margins of the NorthernCordilleran slab window. However, eruption agesin the Anahim volcanic belt define an easterlytime progression that has been attributed to a hot-spot [Bevier, 1989], and tomographic results areconsistent with either interpretation [Mercier etal., 2009].[49] The BSM alkalic basalts may be related tomantle upwelling at the boundary between theJuan de Fuca and Explorer plates, as illustratedschematically in Figure 14. The northern segmentof the Juan de Fuca plate has had a complex tec-tonic history; about 4 Myr ago, the northernmostportion of the Juan de Fuca plate separated alongthe Nootka fault zone to form the independentExplorer microplate [Riddihough, 1984] (Figure1). Although convergence has ceased at the north-ern edge of the microplate, the southernmost partof the microplate continues to subduct slowly[Braunmiller and Nabelek, 2002], and the entireTable 5. AFC Modeling Parameters and ResultsCompositionAFC 1: Bridge River AFC 2: Mt. MeagerSample Modeled: BRC03?4 Sample Modeled: MM01-1Initial Magma: Assimilant: Initial Magma: Assimilant:BRC09-3a Gabbrob MM08a GabbrobSiO2 (wt %) 45.6 44.6 48.6 44.6TiO2 1.8 1.01 1.4 1.01MgO 15.6 6.57 14.5 6.57Na2O 2.6 1.31 2.9 1.31K2O 0.7 0.40 0.6 0.40Sr (ppm) 400 401 467 401Nd 18.5 5.0 14 5.0Ba 195 153 154 153Zr 127 21 92 21Nb 16.5 1.0 9 1.087Sr/86Sr 0.702986 0.7034 0.703164 0.7040143Nd/144Nd 0.513026 0.51286 0.513030 0.512820ENd ?7.6 ?4.3 ?7.6 ?3.6AFC resultsr c 0.90 0.89FLd 0.83 0.77olive 0.05cpx 0.05 0.10opx 0.05 0.10mt 0.02 0.03aTrace element and isotope data for initial magmas are from Table 1; major element data [from Green and Sinha, 2005] are corrected into equi-librium with Fo90 mantle.bMajor and trace element data for gabbro assimilant are from DeBari et al. [1999] for sample 91-17 of the Westcoast Crystalline Complexexcept Nd (interpolated); isotope ratios selected from within the range defined by the Coast Plutonic Complex [Cui and Russell, 1995b].cmass assimilated/mass crystallized.dfraction liquid remaining.eFraction of each mineral phase removed from magma; sum is equal to (1FL).fAbbreviations: oliv (olivine), cpx (clinopyroxene), opx (orthopyroxene), mt (magnetite).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913148Explorer region is a zone of strong shear deforma-tion [Dziak, 2006]. The offshore segment of theNootka fault shows left-lateral motion along a rup-ture and the onshore extension of the fault ismarked by thinning and deformation of the sub-ducting plate [Audet et al., 2008]. Seismic dataindicate the Explorer plate currently has a shal-lower dip than the Juan de Fuca plate, which maymanifest itself in a near-vertical gap between theplates (Figure 14). The BSM volcanic centers lieon, or just south of, the Nootka fault zone as ex-trapolated to the northeast (Figure 1a). We suggestthat thinning, deformation, and possible rupture ofthe subducted Explorer plate fragment may pro-vide a pathway for asthenospheric upwellingaccompanied by decompression melting. Farthereast along the projected trace of the Nootka fault,the Wells Gray-Clearwater volcanic field and Chil-cotin basalts have been similarly attributed toenriched asthenosphere upwelling through a gapalong the fault [Madsen et al., 2006; Sluggett,2008; Thorkelson et al., 2011].[50] Seismic tomography is inconclusive as towhether the Nootka fault is ??leaky?? or whethercontinuity is maintained at depth between theExplorer and Juan de Fuca plates [Mercier et al.,2009]. However, as the Explorer plate is situatedat the southern edge of a slab window, it is subjectto progressive thermal and physical degradationthat would facilitate passage of mantle melts frombelow [Thorkelson et al., 2011; Thorkelson andBreitsprecher, 2005]. In an analogous situation inthe Mexican arc, seismic anisotropy measurementsare consistent with plate separation. Faults sepa-rate the subducting Cocos plate into several seg-ments, and each subducts at a different angle,resulting in a scissors-like effect in which gapsbetween the plates allow for mantle upwellingthrough toroidal flow [Stubailo et al., 2012].7. Summary and Conclusions[51] Alkalic basalts at the Bridge River, Salal Gla-cier, and Mt. Meager volcanic centers (BSM vol-canic centers) of the Canadian segment of theCascade arc, known as the Garibaldi volcanic belt,have intraplate characteristics that contrast withtypical calc-alkaline mafic Cascade arc lavas. Newhigh precision Sr-Nd-Hf-Pb isotope ratios andtrace element abundances reveal that the mostprimitive basalts at Salal Glacier and Bridge Riverare essentially free of components derived fromthe subducting slab. The apparent trace element??arc signature?? exhibited by several more evolvedBSM basalts is more likely a consequence ofassimilation of mafic deep crust rather than slabinput. At Mt. Meager, however, primitive basaltsmay include a small amount of fluid derived fromsubducted altered oceanic crust.[52] The mantle source of the BSM basalts isdeeper, hotter, and isotopically distinct from thesource of calc-alkaline basalts from Mt. Baker andthroughout the Cascade arc. The BSM mantlesource is also more enriched in incompatible ele-ments than the depleted mantle wedge tapped bycalc-alkaline Cascade arc basalts, and similar toocean island basalt sources. Similar trace elementabundances among the BSM and Anahim alkalicbasalts, and those in the Cascade-Columbia tran-sect and north of Mt. Rainier (Figures 8 and 9c),indicate mantle sources similarly enriched in in-compatible elements.[53] BSM and Cascade-Columbia intraplate lavashave been previously attributed to enriched mantledomains associated with the base of an accretedterrane [Schmidt et al., 2008]. We consider this hy-pothesis unlikely for the BSM volcanic centers fortwo reasons. First, the accreted terranes beneathFigure 14. Schematic representation of plate configurationat the northern end of the Cascade arc based on a model ofRiddihough [1984]. The Explorer plate detached from theJuan de Fuca Plate along the Nootka fault zone 3 to 4 Myrago as it became younger, hotter, and more buoyant at thetrench. The thick dashed black line indicates the surface traceof the Nootka fault. Convergence of the Explorer plate withNorth America has now nearly ceased. The vertical windowformed between the Explorer and Juan de Fuca plates maypromote upwelling of deep, hot mantle (large orange arrow)at the edge of the currently subducting plate. Decompressionmelting of this mantle accounts for the presence of hot alkalicbasalts essentially free of a subduction signature (red, orange,and yellow triangles along Nootka fault zone for each of theBSM volcanic centers).MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913149the BSM centers and the Cascade-Columbia tran-sect are different (Wrangellia and Siletzia, respec-tively), and second, Mt. Baker and the BSM sharethe same accreted terrane at depth yet the state ofmantle source enrichment differs substantially.[54] Although major and trace element datarequire an enriched mantle source for the BSMbasalts, isotopic data provide evidence for long-term mantle depletion. Pb isotope ratios of theBSM basalts are broadly similar to oceanic andintraplate basalts of the northeastern Pacific (Fig-ure 11), indicating that isotopically depleted uppermantle of common origin is regionally wide-spread, albeit with small isotopic heterogeneities.[55] With isotopic data consistent with long-termdepletion, incompatible-element enrichment of theBSM mantle source must have occurred relativelyrecently. Recent mantle enrichment has been pro-posed for numerous other cases of isotopicallydepleted alkalic basalts [Roden and Murthy, 1985,and references therein], including those from theTuzo Wilson volcanic field [Allan et al., 1993](Figure 11) and the Bowie Seamount in the Gulfof Alaska [Cousens, 1988]. The BSM volcaniccenters are located along, and just south of, theprojected trace of the Nootka fault zone, whichseparates the subducting Juan de Fuca plate fromthe Explorer plate fragment. We attribute the BSMbasalts to upwelling asthenosphere through a gapalong the fault, which undergoes decompressionmelting to generate alkalic basalts that are free ofsubduction input yet located in an arc setting.Acknowledgments[56] We thank Bruno Kieffer for assistance with TIMS analy-ses, Vivian Lai for help with trace element analyses, JaneBarling, Kathy Gordon, and Liyan Xing for assistance withMC-ICP-MS analyses, and Ines Nobre Silva for instruction inthe clean laboratory. We are grateful to Marion Carpentier forprocessing and analyzing eight samples for trace elementsand five for isotopes. Derek Thorkelson, Martin Streck, andRichard Carlson provided constructive and thoughtfulreviews. Insightful discussions with Kelly Russell have beenmuch appreciated. We are particularly grateful to StewartMcCallum for detailed reviews, discussions, and advice thathave significantly improved the manuscript. This researchwas funded by an NSERC Discovery Grant to D. 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Humphreys (2008), Toroidal mantle flowthrough the Western U.S. slab window, Geology, 36, 295?298.MULLEN AND WEIS: ALKALIC BASALTS, NORTHERN CASCADE ARC 10.1002/ggge.201913155 18.7018.7518.8018.8518.9018.9519.0018.70 18.80 18.90 19.00206 Pb/204 Pb (leached)206Pb/204Pb (unleached)0.70250.70300.70350.70400.70450.7025 0.7035 0.704587Sr/86Sr(leached)87Sr/86Sr (unleached)0.51270.51290.51310.5127 0.5129 0.5131143 Nd/144 Nd (leached)143Nd/144Nd (unleached)38.0538.1538.2538.3538.4538.5538.05 38.15 38.25 38.35 38.45 38.55208 Pb/204 Pb (leached)208Pb/204Pb (unleached)0.282950.283000.283050.283100.283150.28295 0.28300 0.28305 0.28310 0.28315176 Hf/177Hf (leached)176Hf/177Hf (unleached)15.5415.5615.5815.6015.54 15.56 15.58 15.60207 Pb/204 Pb (leached)207Pb/204Pb (unleached)Mullen and Weis (2013) Figure S1 sample BRC01-3 BRC01-3repa%diffBRC01-3dupb%diffSG10 SG10 dup%diffMM01-1 MM01-1dup%diffmethod c 1 1 1 1 1 2 2ppmLi 7.2 7.2 -0.4 6.2 14 6.6 6.7 -1.1 6.5 6.6 -1.2Cs 0.17 0.17 1.5 0.12 30 0.15 0.16 -4.8 0.10 0.10 4.2Rb 11 10 5.2 7.8 26 16 17 -4.0 8.8 8.8 -0.1Ba 413 413 -0.1 405 1.9 330 330 0.0 676 676 0.0Th 2.0 1.9 6.2 1.8 11.2 1.9 1.9 1.4 1.7 1.7 -2.2U 0.66 0.65 1.8 0.67 -1.2 0.71 0.71 -0.2 0.63 0.61 2.8Nb 14 14 -0.3 14 -0.3 32 32 0.1 12 12 -2.4Ta 0.87 0.82 6.0 0.92 -5.4 1.6 1.5 5.1 0.67 0.78 -17La 24 24 -0.7 23 3.5 22 21 2.5 27 27 0.3Ce 54 54 -0.5 53 1.4 47 48 -1.3 62 62 -0.1Pb 4.2 4.1 1.2 4.2 -1.2 2.2 2.2 -1.0 5.2 5.2 -0.4Pr 6.7 6.8 -0.8 6.7 0.7 6.0 6.1 -1.9 8.4 8.4 0.5Sr 1251 1243 0.6 1149 8.1 585 588 -0.5 1588 1599 -0.7Nd 28 28 -1.1 27 2.5 26 25 3.6 35 35 1.3Sm 5.2 5.2 0.6 5.1 2.5 5.7 5.6 1.5 6.7 6.7 -0.5Zr 127 125 1.7 125 1.7 164 163 0.5 147 147 -0.1Hf 3.4 3.1 9.0 3.2 6.1 3.7 3.9 -4.4 3.4 3.4 0.1Eu 1.6 1.6 -0.8 1.5 5.5 1.9 2.0 -6.9 2.1 2.1 -2.4Gd 4.2 4.4 -5.6 4.2 -0.8 5.1 5.8 -13 5.5 5.4 1.2Tb 0.58 0.60 -2.7 0.59 -1.0 0.78 0.83 -6.2 0.71 0.71 0.5Dy 3.4 3.4 0.2 3.4 0.2 4.4 4.7 -6.0 4.0 4.0 1.0Y 19 18 2.9 16 14 25 24 2.6 21 22 -3.5Ho 0.66 0.60 8.9 0.57 13 0.81 0.82 -1.1 0.77 0.76 0.9Er 1.7 1.8 -4.7 1.7 1.1 2.2 2.2 -0.4 2.0 2.0 2.4Tm 0.24 0.22 7.0 0.21 11 0.30 0.30 1.0Yb 1.4 1.3 10 1.3 10 1.7 1.8 -6.9 1.7 1.7 2.2Lu 0.20 0.21 -4.6 0.18 10 0.26 0.23 9.9 0.25 0.25 1.4Sc 18 17 3.3 14 20 22 22 -0.7 23 22 3.9Zn 85 87 -1.8 129 -51 103 102 1.0 100 98 2.4Cr 48 49 0.7 49 0.7 239 243 0.9 112 109 2.8Ni 44 43 1.2 46 -5.7 130 130 0.2 49 48 2.7V 202 203 -0.3 204 -0.8 227 225 1.1 208 202 2.8Ga 23 23 -1.3 22 3.1 22 23 -3.3Mullen and Weis (2013) Table S1a rep designates replicates (repeat analyses of the same sample solution)b dup designates full procedural duplicates starting with a new sample powder aliquotc Method 1: Thermo Finnigan Element2 HR-ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS sample digestionblank 1digestionblank 2BHVO2average(n=8)BHVO2average(n=5)BCR2average(n=8)BCR2average(n=3)AGV1average(n=5)method a 1 1 1 2 1 2 2ppmLi 0.00 0.00 4.4 4.8 8.4 9.7 10.9Cs 0.00 0.00 0.09 0.10 1.1 1.2 1.3Rb 0.00 0.00 8.6 9.1 44 47 67Ba 0.01 0.06 126 132 642 697 1230Th 0.00 0.00 1.1 1.2 6.1 6.0 6.3U 0.00 0.00 0.42 0.40 1.8 1.6 1.8Nb 0.00 0.00 17 18 12 12 14Ta 0.00 0.00 1.3 1.1 0.74 0.75 0.82La 0.00 0.00 15 15 26 25 38Ce 0.00 0.00 38 38 54 53 69Pb 0.00 0.00 3.2 3.2 11 10 36Pr 0.00 0.00 5.2 5.4 6.6 6.9 8Sr 0.00 0.02 378 397 356 341 657Nd 0.00 0.00 25 25 29 29 32Sm 0.00 0.00 6.1 6.1 6.7 6.6 5.8Zr 0.00 0.02 169 176 181 192 237Hf 0.00 0.00 4.4 4.3 4.7 4.8 5.0Eu 0.00 0.00 2.1 2.1 1.9 2.0 1.6Gd 0.00 0.00 6.2 6.3 6.4 6.7 4.7Tb 0.00 0.00 0.94 0.91 1.0 1.0 0.63Dy 0.00 0.00 5.2 5.3 6.0 6.4 3.5Y 0.00 0.00 28 27 37 37 20Ho 0.00 0.00 1.0 0.98 1.2 1.3 0.67Er 0.00 0.00 2.5 2.5 3.5 3.7 1.8Tm 0.00 0.00 0.35 0.54Yb 0.00 0.00 2.0 2.0 3.3 3.4 1.6Lu 0.00 0.00 0.27 0.27 0.49 0.49 0.24Sc 0.00 0.00 31 30 33 30 117Zn 0.00 0.08 96 97 115 124 92Cr 0.02 0.11 303 265 15 13 8.0Ni 0.01 0.06 122 111 11 10 14V 0.01 0.03 309 296 391 372 8.0Ga 0.00 0.00 22 21 10.9Mullen and Weis (2013) Table S2a Method 1: Thermo Finnigan Element2 HR ICP-MS; Method 2: Agilent 7700 quadrupole ICP-MS

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