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Low-productivity hawaiian volcanism between Kaua‘i and O‘ahu. 2011

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Article Volume 11 23 November 2010 Q0AC08, doi:10.1029/2010GC003233 ISSN: 1525‐2027 Low‐productivity Hawaiian volcanism between Kaua‘i and O‘ahu Andrew R. Greene and Michael O. Garcia Department of Geology and Geophysics, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawaii 96822, USA (agreene@soest.hawaii.edu) Dominique Weis Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada Garrett Ito Department of Geology and Geophysics, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawaii 96822, USA Maia Kuga Laboratoire des Sciences de la Terre, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, F‐69364 Lyon CEDEX 07, France Joel Robinson Volcano Science Center, U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, California 94025, USA Seiko Yamasaki Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake‐cho, Sakyo‐ku, Kyoto 606‐8502, Japan [1] The longest distance between subaerial shield volcanoes in the Hawaiian Islands is between the islands of Kaua‘i and O‘ahu, where a field of submarine volcanic cones formed astride the axis of the Hawaiian chain during a period of low magma productivity. The submarine volcanoes lie ∼25–30 km west of Ka‘ena Ridge that extends ∼80 km from western O‘ahu. These volcanoes were sampled by three Jason2 dives. The cones are flat topped, <400 m high and 0.4–2 km in diameter at water depths between ∼2700 and 4300 m, and consist predominantly of pillowed flows. Ar‐Ar and K‐Ar ages of 11 tholeiitic lavas are between 4.9 and 3.6 Ma. These ages overlap with shield volcanism on Kaua‘i (5.1–4.0 Ma) and Wai‘anae shield basalts (3.9–3.1 Ma) on O‘ahu. Young alkalic lavas (circa 0.37 Ma) sampled southwest of Ka‘ena Ridge are a form of offshore secondary volcanism. Half of the volcanic cones contain high‐SiO2 basalts (51.0–53.5 wt % SiO2). The trends of isotopic compositions of West Ka‘ena tholeiitic lavas diverge from the main Ko‘olau‐ Kea shield binary mixing trend in isotope diagrams and extend to lower 208Pb/204Pb and 206Pb/204Pb than any Hawaiian tholeiitic lava. West Ka‘ena tholeiitic lavas have geochemical and isotopic characteristics similar to volcanoes of the Loa trend. Hence, our results show that the Loa‐type volcanism has persisted for at least 4.9 Myr, beginning prior to the development of the dual, subparallel chain of volcanoes. Several West Ka‘ena samples are similar to higher SiO2, Loa trend lavas of Ko‘olau Makapu‘u stage, Lāna‘i, and Kaho‘olawe; these lavas may have been derived from a pyroxenite source in the mantle. The high Ni contents of olivines in West Ka‘ena lavas also indicate contribution from pyroxenite‐derived melting. Average compositions of Hawaiian shield volcanoes show a clear relation between 206Pb/204Pb and SiO2 within Loa trend volcanoes, which supports a prominent but variable influence of pyroxenite in Copyright 2010 by the American Geophysical Union 1 of 30 the Hawaiian plume source. In addition, both Pb isotopes and volcano volume show a steady increase with time starting from a minimum west of Ka‘ena Ridge. The entrained mafic component in the Hawaiian plume is probably not controlling the increasing magma productivity in the Hawaiian Islands. Components: 18,500 words, 13 figures, 3 tables. Keywords: West Ka‘ena; tholeiitic volcanism; submarine volcanism; Loa‐Kea trends; pyroxenite. Index Terms: 1037 Geochemistry: Magma genesis and partial melting (3619); 3619 Mineralogy and Petrology: Magma genesis and partial melting (1037); 1038 Geochemistry: Mantle processes (3621); 3621 Mineralogy and Petrology: Mantle processes (1038); 1025 Geochemistry: Composition of the mantle. Received 24 May 2010; Revised 7 September 2010; Accepted 14 September 2010; Published 23 November 2010. Greene, A. R., M. O. Garcia, D. Weis, G. Ito, M. Kuga, J. Robinson, and S. Yamasaki (2010), Low‐productivity Hawaiian volcanism between Kaua‘i and O‘ahu, Geochem. Geophys. Geosyst., 11, Q0AC08, doi:10.1029/2010GC003233. Theme: Geochemical Heterogeneities in Oceanic Island Basalt and Mid-ocean Ridge Basalt Sources: Implications for Melting Processes and Mantle Dynamics Guest Editors: C. Beier and P. Asimow 1. Introduction [2] The geographical and temporal distribution of isotopic and geochemical variations of lavas from Hawaiian shield volcanoes indicates that compo- sitional heterogeneity is an intrinsic feature of the Hawaiian plume over a range of spatial scales [Frey and Rhodes, 1993]. The isotopic and geo- chemical variation of Hawaiian lavas has been used to understand the nature of the mantle source (e.g., recycled components, source lithology, scales of heterogeneities), the effect of melting processes, and causes for the evolution of the Hawaiian shield volcanoes over the last 5.5 million years. The recognition of subparallel, isotopically distinct chains of shield volcanoes (Loa and Kea trends) is up to now unique to the Hawaiian volcanic chain and presents a fundamental controversy on what in the structure of the plume stem or the plume source is controlling the emergence of these two trends [Abouchami et al., 2005]. The existence of the Loa and Kea trends has been explained by several competing models involving a concentrically zoned, asymmetrically heterogeneous or vertically zoned mantle plume [Abouchami et al., 2005; Blichert‐Toft et al., 2003; Bryce et al., 2005; Huang and Frey, 2005; Lassiter et al., 1996]. The geochemical differences between Hawaiian shields may also be controlled by the thermal structure of the plume, the Pacific Plate structure and motion, and varying source lithology and scales of hetero- geneities [e.g., Hauri, 1996; Blichert‐Toft et al., 2003; Kurz et al., 2004; Abouchami et al., 2005; Tanaka et al., 2008; Wessel and Kroenke, 2009]. [3] Hawaiian shield volcanoes grow in three main stages (preshield, shield (>95%–98% volume), and postshield) over a period of about 1.5 Myr [Moore et al., 1982; Garcia et al., 2006]. The volcanic growth rate is greatest during the shield‐building stage when the volcano is centered over the melting region of the hot spot and therefore lava compo- sitions are mostly tholeiitic basalt. In contrast, the preshield and postshield lavas form from lower degrees of partial melting and are mostly alkalic [Moore et al., 1982; Frey et al., 1991]. Studies of the main tholeiitic shield‐building stage of Hawaiian volcanoes have examined subaerial [e.g., Frey et al., 1994; Mukhopadhyay et al., 2003; Pietruszka and Garcia, 1999] and subaqueous [e.g., Coombs et al., 2004; Hanyu et al., 2010; Kimura et al., 2006; Ren et al., 2006; Xu et al., 2007] basalts that erupted on a large shield vol- cano and involved magma storage in reservoirs within the crust or volcanic edifice [e.g., Tilling and Dvorak, 1993]. Low‐volume submarine tho- leiitic cones west of Ka‘ena Ridge between the islands of Kaua‘i and O‘ahu (Figure 1) are some of the only known examples of tholeiitic volcanism that occurred astride the axis of the Hawaiian Islands and was not associated with a large sub- aerial shield volcano. This volcanism occurred prior to the formation of the dual Loa and Kea trends. [4] The longest distance between large subaerial shields of the Hawaiian Islands is between the Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 2 of 30 islands of Kaua‘i and O‘ahu, where the volume of lava erupted per unit area is also the smallest (i.e., low productivity compared to shield stage volca- nism). Volcanism in this area formed Ka‘ena Ridge, with two low shields that grew just above sea level [Coombs et al., 2004]. Ka‘ena Ridge is the longest submerged terrace that parallels the axis of the Hawaiian Islands (or volcano propagation vector of ∼293° [Wright and Klein, 2006]). It is flanked by clusters of volcanic cones and a large submarine rift zone (Figure 1). This study exam- ines the petrology, geochemistry and geochronol- ogy of volcanism between Kaua‘i and O‘ahu from submarine cones straddling the axis of the Hawai- ian Islands that formed during a period of low magma productivity and prior to formation of dual Loa and Kea trends. The goals of this study are to evaluate (1) ages and source composition for West Ka‘ena lavas compared to Hawaiian shield tholeiites, (2) pyroxenite versus peridotite source for West Ka‘ena lavas, and (3) the relationship between source composition and lithology to vol- ume flux in the Hawaiian Islands during the last 5 Ma. These new results improve our understand- ing of the evolution of the Hawaiian plume. 2. Geologic Background [5] The volcanic cones and terraces west of Ka‘ena Ridge are uncommon features in the Hawaiian Islands. Most flat‐topped submarine cones in the Hawaiian Islands are found near submarine rift zones or proximal to subaerial shield volcanoes [Clague et al., 2000]. The field of cones west of Ka‘ena Ridge are ∼50 km from the center of the island of Kaua‘i and >100 km from Wai‘anae Volcano, the second largest volcano on the island of O‘ahu. The submarine volcanic cones between Kaua‘i and the base of Ka‘ena Ridge are most abundant within ∼25–30 km of the base of Ka‘ena Ridge (Figure 2). [6] Two giant landslides have modified the original shape of Ka‘ena Ridge on the north and south side Figure 1. Hypsometry and bathymetry around the islands of Kaua‘i and O‘ahu with location of Jason2 dives in black box (shown in Figure 2) and a single dredge site shown with a red cross. Inset shows bathymetry of the Hawaiian Islands and Loa and Kea trend volcanoes based on distribution of Pb isotope ratios in lavas. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 3 of 30 of the ridge [Coombs et al., 2004]. Giant landslides have reduced Wai‘anae and Ko‘olau on the southwest and northeast of O‘ahu, respectively [Coombs et al., 2004; Garcia et al., 2006; Ozawa et al., 2005], and landslides may have displaced large parts of east Kaua‘i [Moore et al., 1989]. There is no evidence of significant landsliding on the west end of Ka‘ena Ridge. [7] Ka‘ena Ridge is an elongate, relatively flat‐ topped submerged terrace 35–55 km wide that extends ∼80 km from the western edge of O‘ahu. The terrace is bounded by a break in slope that marks a paleoshoreline (1200–1500 m on the north side of Ka‘ena Ridge and ∼1400 m deep on the south side [Coombs et al., 2004]). Two submarine shields, 15 km and 10 km in diameter, rise 200 m above the western part of Ka‘ena Ridge and to within 660–880 m of sea level (approximately 2500 m above the basal seafloor) [Smith, 2002]. A submarine rift zone called Waialu Ridge extends northwest from Ka‘ena Ridge (Figure 1). There are few circular flat‐topped cones closer to Kaua‘i; radiating ridges with elongate sediment‐filled ba- sins and submarine canyons and channels are more common within ∼20 km of Kaua‘i. Young lava flows were recognized southwest of Ka‘ena Ridge by Holcomb and Robinson [2004] from GLORIA side‐scan sonar imagery, but there has been no previous study of the submarine features between Kaua‘i and O‘ahu. In 2007, three dives using the remotely operated vehicle (ROV) Jason2 from Woods Hole Oceanographic Institution aboard R/V Kilo Moana were conducted to survey and sample the volcanic cones and lava flows west of Ka‘ena Ridge for petrologic, geochronological, and geo- chemical studies [Garcia et al., 2008]. In 2009, a Figure 2. Bathymetric map and side‐scan sonar imagery between Kaua‘i and O‘ahu. (a) Volcanic sample locations collected during three Jason2 dives are shown by white circles. (b) Tholeiitic (green), transitional (orange), and alkalic (red) basalts are indicated by colored dots. Area of maps is indicated by black box in Figure 1. See auxiliary material for larger, more detailed bathymetric maps of the three dive areas. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 4 of 30 single dredge haul was collected ∼30 km northeast of Kaua‘i (Figure 1). 3. Description of Dive Areas and Petrology of Samples [8] From north to south, the three dives covered a distance spanning ∼70 km. Dive 305 traversed an elongate cluster of flat‐topped and conical sea- mounts extending northwest from the end of Ka‘ena Ridge (Figure 2a; auxiliary material).1 These cones are <200 m high and <0.4–2 km in diameter with summits at depths of 2700 to 3500 m. Dive 306 sampled 6 cones in a cluster southwest of the end of Ka‘ena Ridge and a terrace and cone at the base of the end of Ka‘ena Ridge. Several cones are disc‐shaped, flat‐topped cones with steep flanks (∼20–30°) and basalt diameters between 1 and 3 km, similar to cones described by Clague et al. [2000]. The cones consist predominantly of pillowed flows and mounds. Thin sheet flows are also present on some of the cones. Dive 296 was 20 km south of the Dive 306 area, over several cone clusters and three young lava fields with high sonar backscatter compared to the surrounding seafloor (Figure 2b). These flows are at water depths greater than 4300 m. The largest flow, 22 km long and 7 km across at its widest point, was previously mapped as a young flow [Holcomb and Robinson, 2004]. The western, higher‐elevation portion of this field appears to emanate from the base of a preexisting submarine tholeiitic cone. A total of 89 samples were collected during Dives 296, 305, and 306 and the dredge haul (sample locations and descriptions are included in the auxiliary material). Sixty‐nine of these samples are basalt or volcanic breccia; eighteen samples are sedimentary rocks (primarily biogenic mudstones and several volcaniclastic rocks collected on the cones). [9] West Ka‘ena lavas are mostly olivine basalt that display cryptocrystalline or intersertal textures with variable amounts of olivine phenocrysts and vesicles. Euhedral to subhedral olivine phenocrysts (<5 mm) make up 5 to 20 vol % of almost all of the lavas and xenocrysts are rare. Chromite inclusions are typically found in the olivine phenocrysts. Clinopyroxene are less common and occurs mostly as microphenocrysts (<1 mm, <5 vol %). The lava groundmass consists of variable amounts of clin- opyroxene, olivine, plagioclase, Fe‐Ti oxides, cryptocrystalline material, and devitrified glass. The lavas typically contain <1 vol % vesicles, but several Figure 3. (a) Latitude versus age (Ma) of West Ka‘ena lavas. Green symbols are Ar‐Ar ages, and yellow symbols are K‐Ar ages. (b) Age (Ma) versus distance fromKīlauea volcano (km) for shield and rejuvenated volcanism for the Hawai- ian Islands. Postshield ages are shown for Kaua‘i. Data from Bianco et al. [2005], Tagami et al. [2003], andOzawa et al. [2005] with newKōloa ages fromHearty et al. [2005], Sano [2006], andGarcia et al. [2010]. Error bars in Figure 3a are ±2s, shown in Table 1. Age ranges for Kaua‘i and Wai‘anae volcanoes are shown by colored fields in Figure 3a. Tick marks within vertical red bar in Figure 3b show ages ofWest Ka‘ena samples from this study from Figure 3a and Table 1. Black bars represent secondary volcanism, gray bars represent shield volcanism, and white bars represent extrapolated shield stage (uniformly 500 ka to estimate the duration of shield volcanism). Symbols for West Ka‘ena lavas are as follows: Dive 306 (squares), Dive 305 (diamonds), and Dive 296 (triangles). 1Auxiliary material data sets are available at ftp://ftp.agu.org/ apend/gc/2010gc003233. Other auxiliary materials are in the HTML. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 5 of 30 T ab le 1. S um m ar y of W ho le ‐R oc k P la te au an d Is oc hr on A ge s fo r W es t K a‘ en a R id ge L av as a S am pl e G ro up L oc at io n T ot al F us io n A ge (M a) ± 2s Is oc hr on A na ly si s N A ge S pe ct ru m (P la te au ) A na ly si s 4 0 A r/ 3 6 A r i ± 2s M S W D A ge (M a) ± 2s 3 9 A r % M S W D A ge (M a) ± 2s D iv e 29 6 29 6‐ 20 al k C on e D 0. 39 3 ± 0. 05 8 29 6. 0 ± 1. 9 0. 15 0. 36 3 ± 0. 10 2 8 of 8 10 0. 0 0. 18 0. 38 9 ± 0. 04 8 0. 32 2 ± 0. 05 9 29 5. 0 ± 0. 9 0. 74 0. 38 4 ± 0. 05 7 9 of 9 97 .7 0. 81 0. 36 0 ± 0. 03 9 0. 37 1 ± 0. 04 0b D iv e 30 5 30 5‐ 02 al k C on e A 1. 38 ± 0. 05 28 7. 7 ± 11 .4 0. 33 1. 47 ± 0. 13 8 of 8 10 0. 0 0. 54 1. 39 ± 0. 05 30 5‐ 04 th ol C on e B 3. 94 ± 0. 13 29 8. 8 ± 8. 8 0. 53 3. 91 ± 0. 22 8 of 8 10 0. 0 0. 54 3. 98 ± 0. 11 30 5‐ 07 th ol L ed ge C 3. 96 ± 0. 21 29 8. 3 ± 7. 0 0. 38 3. 89 ± 0. 18 8 of 8 10 0. 0 0. 42 3. 93 ± 0. 15 30 5‐ 14 th ol C on e E 4. 10 ± 0. 18 29 7. 9 ± 2. 2 1. 01 3. 75 ± 0. 32 8 of 8 10 0. 0 1. 49 4. 05 ± 0. 19 4. 14 ± 0. 21 29 7. 6 ± 2. 1 0. 24 3. 76 ± 0. 30 8 of 8 10 0. 0 0. 74 4. 10 ± 0. 14 4. 02 ± 0. 11 b 30 5‐ 20 th ol C on e G 4. 87 ± 0. 12 29 5. 6 ± 7. 3 1. 08 4. 92 ± 0. 13 8 of 8 10 0. 0 0. 94 4. 92 ± 0. 09 30 5‐ 24 tr an s C on e H 3. 85 ± 0. 08 30 0. 1 ± 58 .8 0. 44 3. 63 ± 0. 63 6 of 10 71 .7 0. 36 3. 68 ± 0. 07 3. 89 ± 0. 12 33 1. 2 ± 43 .6 0. 61 3. 44 ± 0. 45 5 of 10 78 .2 0. 98 3. 80 ± 0. 11 3. 72 ± 0. 06 b D iv e 30 6 30 6‐ 03 th ol C on e A 4. 00 ± 0. 26 29 8. 2 ± 6. 6 0. 10 3. 49 ± 1. 16 10 of 10 10 0. 0 0. 17 3. 98 ± 0. 24 3. 40 ± 1. 63 29 9. 2 ± 10 .2 0. 31 3. 40 ± 1. 63 8 of 8 10 0. 0 0. 34 4. 01 ± 0. 28 4. 00 ± 0. 18 b 30 6‐ 05 th ol C on e B 4. 38 ± 0. 15 29 4. 4 ± 4. 7 0. 82 4. 42 ± 0. 15 8 of 8 10 0. 0 0. 72 4. 40 ± 0. 12 30 6‐ 15 th ol C on e D 3. 61 ± 0. 25 29 7. 9 ± 3. 8 0. 95 3. 52 ± 0. 24 10 of 10 10 0. 0 1. 02 3. 62 ± 0. 19 3. 67 ± 0. 24 29 4. 7 ± 7. 1 0. 33 3. 73 ± 0. 37 7 of 7 10 0. 0 0. 28 3. 69 ± 0. 18 3. 66 ± 0. 13 b 30 6‐ 31 th ol C on e I 3. 81 ± 0. 41 29 1. 5 ± 12 .0 0. 22 4. 13 ± 0. 82 10 of 10 10 0. 0 0. 24 3. 87 ± 0. 31 4. 16 ± 0. 23 28 2. 9 ± 33 .3 0. 38 4. 56 ± 1. 78 5 of 8 62 .2 0. 42 3. 88 ± 0. 28 3. 88 ± 0. 21 b a G ro up is ba se d on ge oc he m is tr y: al k, al ka lic ba sa lt; th ol , th ol ei iti c ba sa lt; tr an s, tr an si tio na l ba sa lt. B ol d ag e is pr ef er re d ag e, w ith er ro r re po rt ed fo r ±2 s. M S W D re fe rs to m ea n su m of th e w ei gh te d de vi at es , w hi ch is a m ea su re of th e sc at te r co m pa re d to th at w hi ch is ex pe ct ed fr om an al yt ic al un ce rt ai nt ie s. P er ce nt ag e of 3 9 A r de ga ss ed us ed in th e pl at ea u ca lc ul at io n. N is nu m be r of st ep s in cl ud ed in th e pl at ea u ca lc ul at io n. A na ly tic al m et ho ds ar e de sc ri be d in A pp en di x A . b W ei gh te d m ea n pl at ea u ag e fr om tw o ex pe ri m en ts . Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 6 of 30 Table 2. Major Element and Trace Element Abundances in Whole‐Rock Samples of West Ka‘ena Ridge Lavasa Sample 296‐25 296‐26 305‐04 305‐07 306‐27 306‐30 305‐24 306‐19 296‐22 305‐02 Group thol basalt thol basalt thol basalt thol basalt thol basalt thol basalt trans basalt trans basalt alk basalt alk basalt Area Cone E Cone F Cone B Ledge C Cone F Cone H Cone H Cone E Cone D Cone A Latitude (deg) 21.641 21.644 22.037 22.021 21.755 21.770 21.955 21.757 21.635 22.049 Longitude (deg) 159.041 159.043 158.920 158.904 158.992 158.957 158.888 159.007 159.040 158.924 Unnormalized Major Element Oxides (wt %) SiO2 50.55 49.17 51.74 50.22 51.32 50.23 48.98 50.03 45.15 41.67 TiO2 2.48 2.63 1.71 1.95 1.46 1.98 2.75 2.68 2.75 2.34 Al2O3 13.29 11.47 12.62 12.22 11.82 12.01 13.43 13.71 14.03 10.72 Fe2O3* 12.00 13.64 11.31 12.07 11.26 12.58 12.43 12.10 13.27 14.40 MnO 0.18 0.20 0.17 0.19 0.17 0.18 0.21 0.16 0.20 0.21 MgO 7.90 11.44 10.78 11.45 13.10 11.54 9.03 8.58 6.16 13.85 CaO 10.57 8.62 8.87 8.83 8.57 9.44 9.11 8.59 12.40 12.05 Na2O 2.19 2.27 2.26 2.18 1.94 1.85 2.81 2.87 3.16 3.03 K2O 0.46 0.48 0.58 0.51 0.28 0.29 1.15 0.95 2.07 0.86 P2O5 0.25 0.28 0.22 0.23 0.15 0.18 0.46 0.47 0.53 0.64 Total 99.88 100.20 100.26 99.86 100.07 100.28 100.36 100.14 99.72 99.77 LOI −0.07 0.08 0.43 0.32 −0.47 0.74 0.33 0.66 2.45 0.67 Trace Elements (ppm) La 12.9 12.7 10.4 9.8 5.7 6.2 24.4 21.4 28.2 43.9 Ce 28.8 29.7 24.0 23.8 14.6 16.1 51.8 48.0 58.0 89.4 Pr 4.08 4.19 3.32 3.35 2.03 2.40 6.92 6.56 7.09 10.4 Nd 19.1 20.2 15.6 16.1 10.1 12.0 32.8 32.0 30.2 41.3 Sm 4.97 5.53 4.12 4.55 2.91 3.62 7.59 7.87 6.71 8.59 Eu 1.60 1.74 1.30 1.55 1.04 1.25 2.28 2.42 2.11 2.73 Gd 5.12 5.60 3.97 4.79 3.17 4.09 6.73 7.39 6.32 7.51 Tb 0.77 0.89 0.69 0.71 0.48 0.71 1.01 1.00 0.88 0.96 Dy 4.85 4.99 3.98 4.30 2.95 4.25 5.54 5.84 5.12 5.09 Ho 0.83 0.94 0.73 0.81 0.57 0.77 0.96 0.99 0.84 0.88 Er 2.20 2.52 2.11 2.09 1.47 2.15 2.43 2.44 2.14 2.03 Yb 1.83 2.05 1.78 1.69 1.30 1.78 1.98 1.95 1.70 1.42 Lu 0.21 0.28 0.25 0.24 0.18 0.25 0.26 0.29 0.19 0.21 V 269 260 228 217 197 242 244 233 285 254 Cr 369 581 588 634 789 857 422 335 72 546 Ni 131 435 332 435 465 413 243 254 68 394 Zn 114 133 106 121 105 135 136 143 130 129 Ga 20 18 17 17 15 17 21 21 22 18 Rb 6.7 7.3 7.3 6.3 4.0 3.8 19.3 14.4 29.3 17.3 Sr 341 301 279 326 213 206 568 525 701 722 Y 24.0 26.1 21.7 22.4 17.0 22.0 26.6 27.3 24.5 23.1 Zr 141 160 121 126 83 100 227 234 148 162 Nb 13.5 14.0 9.1 9.2 5.9 7.7 24.0 18.4 46.9 58.5 Cs 0.07 0.08 0.07 0.03 0.05 0.20 0.16 0.39 0.19 Ba 90 83 91 74 46 47 221 187 560 722 Hf 2.64 3.88 2.85 2.92 1.78 2.48 5.05 5.00 2.61 3.58 Ta 0.52 0.81 1.57 1.00 0.27 0.89 1.67 1.19 1.66 3.33 Pb 0.96 1.01 1.37 0.92 0.60 0.64 2.89 2.04 1.72 2.56 Th 0.71 0.88 0.66 0.62 0.29 0.35 1.62 1.36 3.20 5.52 U 0.21 0.22 0.19 0.18 0.10 0.13 0.51 0.41 0.83 1.26 aSee Data Set S1 in the auxiliary material for complete data. Abbreviations for groups are as follows: thol basalt, tholeiitic basalt; trans basalt, transitional basalt; alk basalt, alkalic basalt. XRF analyses were performed at University of Massachusetts XRF Laboratory. Fe2O3* is total iron expressed as Fe2O3. LOI is loss on ignition. Totals have not been resummed using the LOI value. Elements by XRF are as follows: V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, and Ba. ICP‐MS analyses were performed at the PCIGR at University of British Columbia. Elements by ICP‐MS are as follows: REE, Cs, Hf, Ta, Pb, Th, and U. Analytical methods are described in Appendix A. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 7 of 30 samples have >10 vol % small, round vesicles. The lavas have weak to moderate levels of alteration. The secondary minerals include iddingsite partially replacing olivine, clay partly replacing glassy and cryptocrystalline material, and zeolites and calcite infilling some vesicles. 4. Age of West Ka‘ena Lavas [10] Eleven samples from Dives 296, 305 and 306 were dated using the Ar‐Ar method (Table 1); six samples from Dive 296 (4 tholeiitic, 2 alkalic) were dated by the K‐Ar technique (auxiliary material; see Appendix A for analytical methods). Seven of the eleven lavas (tholeiitic basalts, and one transi- tional basalt) from Dives 305 and 306 range in age from 4.00 ± 0.18 to 3.66 ± 0.13 Ma (Figure 3a and Table 1; all errors are ±2s). Two additional sam- ples have Ar‐Ar ages of 4.40 ± 0.12 and 4.92 ± 0.09 Ma. Six duplicate measurements yielded reproducible ages within analytical error and the total fusion ages for all samples match the plateau ages within error. The plateau ages of these sam- ples represent a high percentage of the 39Ar (100% for 13 of 17 analyses) and have low MSWD values (Table 1). Three of the four unaltered tholeiitic basalts from Dive 296 dated by the K‐Ar method range in age from 4.70 ± 0.64 to 4.31 ± 0.46 Ma (auxiliary material). The K‐Ar age for the fourth sample (296‐30) is 2.85 ± 1.08 Ma. A higher LOI value of 0.84 wt % and air contamination of 94.1% for this sample indicate Ar loss, therefore the obtained age is likely younger than the real age. The Ar‐Ar and K‐Ar ages for West Ka‘ena lavas overlap and fall between the current estimates for the age of shield stage volcanism on Kaua‘i (5.1– 4.0 Ma [Garcia et al., 2010]) and Wai‘anae vol- cano (3.9–3.1 Ma [Guillou et al., 2000]) on O‘ahu (Figures 3a and 3b). There is no apparent rela- tionship between cone location and age, except that the three K‐Ar ages for Dive 296 lavas are slightly older than the average Ar‐Ar age for Dives 305 and 306 to the north. [11] Two alkalic samples, one collected from a cone adjacent to the large young lava flow from Dive 296 (296‐20; Figure 2) and one from Dive 305 (305‐02), yielded Ar‐Ar ages of 0.37 ± 0.04 and 1.39 ± 0.05 Ma, respectively (Figure 3b and Table 1). Mass fractionation corrected K‐Ar ages for two samples from the young lava flow from Dive 296 (296‐20, 296‐21) are 0.24 ± 0.16 and 0.47 ± 0.24 Ma (see Appendix A and auxiliary Figure 4. Total alkalis versus silica variation diagram for whole‐rock samples from West Ka‘ena submarine cones. Alkalic‐tholeiitic boundary from MacDonald and Katsura [1964]. “Kaua‘i” late shield and rejuvenated (Kōloa Volcanics) data from Garcia et al. [2010]. Wai‘anae shield field (black outline) based on data from MacDonald and Katsura [1964] and Sherrod et al. [2007]. Kaua‘i shield data from Mukhopadhyay et al. [2003]. Ko‘olau early middle shield field (blue curve) from Haskins and Garcia [2004]; Ko‘olau late shield data (Makapu‘u stage) from Frey et al. [1994]. Ka‘ena Ridge data from Coombs et al. [2004]. Three transitional basalts from Dive 306 (Cone E; 306‐19, 306‐22, and 306‐25) are highly vesicular and have the high alkali contents. Dive 305‐24 is highly altered and falls slightly above the alkalic‐tholeiitic divide but has similar trace element and isotopic composition to the transitional basalts. Symbols for West Ka‘ena lavas are as follows: Dive 306 (squares), Dive 305 (diamonds), and Dive 296 (triangles). Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 8 of 30 material). The ages for these alkalic lavas overlap current estimates for the age of rejuvenated volca- nism on Kaua‘i (2.6–0.15 Ma [Garcia et al., 2010]) and O‘ahu (0.80–0.1 Ma [Ozawa et al., 2005]) (Figure 3b). 5. Whole‐Rock Chemistry 5.1. Major and Trace Element Compositions [12] Most of the analyzed lavas (38 of 52) from cones west of Ka‘ena Ridge are tholeiitic basalts with large ranges in MgO (7–13 wt %) and SiO2 (49–53 wt %) (Table 2; see Appendix A for ana- lytical methods). Eight lavas from Dives 305 and 306 are transitional basalts with 48–50 wt % SiO2, 8–13 wt % MgO, and 3–4 wt % Na2O + K2O that plot near the alkalic‐tholeiitic boundary (Figures 4 and 5). Of the tholeiitic and transitional basalts, 15 samples have >51 wt % SiO2 (referred to as high‐ SiO2 lavas). Six lavas are alkalic basalts with 42– 48 wt % SiO2, 10–14 wt % MgO, and 3.5–5.3 wt % Na2O + K2O, similar in composition to rejuvenated lavas from Kaua‘i (3 of the 6 alkalic basalts are from flows <0.5 Ma in age, sampled during Dive 296, except 305‐02 is from the most northerly cone from Dive 305). [13] The tholeiitic basalts form a broad composi- tional field in which K2O mostly increases and CaO/Al2O3 (0.6–0.8) shows a narrow range with decreasing MgO (Figure 5). All but two of the tholeiitic lavas have >9 wt % MgO, so they are not likely to show inflections in CaO/Al2O3 as a result of clinopyroxene and plagioclase fractionation. The transitional basalts have higher TiO2 (2.7–2.8 wt %) and lower CaO/TiO2 than most tholeiitic basalts (1.3–2.6 wt % TiO2). The trace element abun- dances of the tholeiitic basalts (La/SmN = 1.0–1.6; Dy/YbN = 1.4–1.7) are lower than the transitional basalts (La/SmN = 1.7–2.1 Dy/YbN = 1.8–2.0) except for the heavy rare earth elements (HREE; Figure 6a). There are only a few crossing REE patterns. One transitional basalt and one alkalic basalt also show different variations in Pb and K than other West Ka‘ena lavas. The alkalic basalts are highly enriched in incompatible trace elements. Almost all the tholeiitic, transitional, and alkalic basalts are depleted in Cs, Th and U relative to Ba and K, and in Pb relative to Ce and Pr. Hf in 6 of 15 Figure 5. Whole‐rock major element variation diagrams for West Ka‘ena lavas. (a) K2O versus MgO. (b) TiO2 ver- sus MgO. (c) CaO/Al2O3 versus MgO. (d) CaO/TiO2 versus MgO. Data sources listed in Figure 4. Symbols for West Ka‘ena lavas are the same as in Figure 4. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 9 of 30 analyzed tholeiitic lavas shows depletion relative to Ti and Zr in primitive mantle‐normalized trace element patterns (Figure 6b). [14] In addition to distinguishing among the tho- leiitic, transitional, and alkali basalts, the trace element ratios also reveal melting relationships and source differences between the different types of basalts. The tholeiitic basalts have similar Nb/Y and Zr/Y to Hawaiian shield stage lavas and the transitional basalts have slightly higher ratios Figure 6. Whole‐rock REE and other incompatible element concentrations for West Ka‘ena lavas. Normalization values are from McDonough and Sun [1995]. NMORB average from Salters and Stracke [2004]. Symbols are the same as in Figure 4. Figure 7. Whole‐rock trace element ratio variation diagrams for West Ka‘ena lavas. (a) Nb/Y versus Zr/Y. Plot orig- inated from Fitton et al. [1997]. (b) TiO2/Yb versus Nb/Yb. Plot, fields, and melt model originated from Pearce [2008]. Melting model shown in Figure 7a with red curve marked with percent melting is for tholeiitic basalts, not alkalic basalts. The model assumes incongruent dynamic melting [Zou and Reid, 2001] and coefficients for melting reactions based on lherzolite melting experiments [e.g., Kinzler and Grove, 1992; Walter, 1998; Longhi, 2002]. The model simulates garnet lherzolite melting because garnet is required in the source to produce variations of HREE observed in West Ka‘ena lavas. One percent of the total residue is residual melt. Partition coefficients from Salters and Stracke [2004] and Shaw [2000] were kept constant. Source mineralogy for primitive mantle garnet lherzolite is 0.2cpx:0.25opx:0.5ol:0.05gt. Data sources for Figure 7a are as follows: Kōloa Volcanics [Garcia et al., 2010], Honolulu Volcanics [Yang et al., 2003], Ko‘olau Makapu‘u [Frey et al., 1994], Ko‘olau shield Kalihi stage [Haskins and Garcia, 2004], Kaua‘i shield [Mukhopadhyay et al., 2003], and Kīlauea‐Pu‘u ’Ō‘ō [Garcia et al., 1992, 1996, 2000]. PM, primitive mantle. PM composition from McDonough and Sun [1995]. Parameters for melting model shown in Figure 7b are described by Pearce [2008]. Estimates for degree of partial melting for HSDP lavas range from 8% to 15% [Feigenson et al., 2003]. Inset in Figure 7a shows only lavas from west of Ka‘ena Ridge. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 10 of 30 (Figure 7a). The tholeiitic and transitional lavas overlap shield lavas from Hawaiian Islands and form a trend that corresponds with an incongruent dynamic melting model (Figure 7a). The alkalic basalts overlap rejuvenated lavas from Kaua‘i and O‘ahu. In a plot of TiO2/Yb versus Nb/Yb [e.g., Pearce, 2008], the tholeiitic and transitional basalts lie along a predicted melting trend, with the tran- sitional basalts indicating deeper, lower‐degree melting than the tholeiitic basalts (Figure 7b). The tholeiitic and transitional basalts have overlapping Nb/La (not shown). Figure 8. Sr‐Nd‐Hf isotopic compositions for West Ka‘ena lavas compared to other Hawaiian shield volcanoes. (a) Plot of 143Nd/144Nd versus 87Sr/86Sr. (b) Plot of 176Hf/177Hf versus 143Nd/144Nd. References for data sources are listed in the auxiliary material (Text S1). All isotope data from Hawaiian shield volcanoes are normalized to the same set of standards. Ocean island basalt (OIB) array from Vervoort et al. [1999]. West Ka‘ena lavas are outlined with black circles. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 11 of 30 5.2. Sr‐Nd‐Hf‐Pb Isotopic Compositions [15] Tholeiitic and transitional basalts from sub- marine cones west of Ka‘ena Ridge have over- lapping Sr, Nd, Hf, and Pb isotopic compositions that are significantly different from those of the alkalic basalts (Figures 8 and 9; see Appendix A for analytical methods). Except for three anomalous lavas with high SiO2 (305‐04, 306‐09, 306‐11), the tholeiitic and transitional basalts have a restricted range of Sr, Nd, and Hf isotopic compositions (87Sr/86Sr = 0.70373–0.70393; 143Nd/144Nd = 0.51284–0.51297; 176Hf/177Hf = 0.28303–0.28308; Table 3 and Figure 8). Pb isotopic compositions of the tholeiitic and transitional lavas span a greater compositional range than Sr, Nd, and Hf, relative to the whole isotopic range of Hawaiian lavas, and form two groups (except for 306‐09 and 306‐11); one group clusters at intermediate Pb isotopic compositions and overlaps with the Mauna Loa field, and the other group extends to very low 208Pb/204Pb and 206Pb/204Pb (Figure 9). The two samples (306‐09 and 306‐11, having high‐SiO2, low CaO and K2O, and low Zr/Y) that lie away from the two main groups have also lower Hf and Nd and higher Sr isotopic compositions, and higher 208Pb/204Pb with respect to 206Pb/204Pb. Sample Figure 9. Sr‐Nd‐Hf‐Pb isotopic compositions for West Ka‘ena lavas compared to other Hawaiian shield volcanoes. (a) Plot of 208Pb/204Pb versus 206Pb/204Pb. (b) Plot of 87Sr/86Sr versus 206Pb/204Pb. (c) Plot of 143Nd/144Nd versus 206Pb/204Pb. (d) Plot of 176Hf/177Hf versus 206Pb/204Pb. References for data sources are listed in the auxiliary material (Text S1). Bold line in Figure 9a marks the Loa‐Kea divide from Abouchami et al. [2005]. All isotope data from Hawaiian shield volcanoes are normalized to the same set of standards. West Ka‘ena lavas are outlined with black circles. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 12 of 30 305‐04 extends to the lowest 206Pb/204Pb and 143Nd/144Nd, and is the only sample displaced sig- nificantly above the Hf‐Nd OIB array plotting among the field defined by Lana’i shield tholeiites [Abouchami et al., 2005; Gaffney et al., 2005] (Figure 8b). [16] The alkalic lavas have high 143Nd/144Nd (0.51306–0.51308) and 176Hf/177Hf (0.28315– 0.28317) and low 87Sr/86Sr (0.70305–0.70328) isotopic ratios, similar to rejuvenated lavas from Kaua‘i and O‘ahu (Table 3 and Figure 8). The alkalic lavas have low 208Pb/204Pb compared to tholeiitic basalts with similar 206Pb/204Pb (Figure 9). 6. Discussion 6.1. Volcanism West of Ka‘ena Ridge [17] The clusters of submarine volcanic cones west of Ka‘ena Ridge are products of a low‐volume tholeiitic phase of volcanism that occurred between Kaua‘i and O‘ahu circa 4.9 to 3.6 Ma. The timing of this volcanism overlaps and falls between tho- leiitic shield‐building volcanism on Kaua‘i (5.1– 4.0 Ma) and Wai‘anae Volcano (3.9–3.1 Ma). The closest shield‐like feature to these volcanic cones is a low‐relief shield (20–35 km east) on the western end of Ka‘ena Ridge that rises ∼200 m above the surrounding submerged platform (Figures 1 and 2). West Ka‘ena cones may be the products of distal eruptions from this volcanic edifice. The volcanism west of Ka‘ena Ridge is an uncommon type of tholeiitic volcanism in the Hawaiian Islands in that it formed astride the main axis of the Hawaiian chain but was not associated with a large sub- aerial shield volcano (i.e., near a rift zone or the submarine flank of a subaerial shield volcano). The submarine cones retain their primary morphology and in this study volcano volume of the Hawaiian Islands is estimated along axis of the Hawaiian Chain rather than by age. Table 3. Sr, Nd, Hf, and Pb Isotopic Geochemistry of Whole‐Rock Samples of West Ka‘ena Lavasa Sample 87Sr/86Sr 2SE 143Nd/144Nd 2SE "Nd 176Hf/177Hf 2SE "Hf 206Pb/204Pb 2SE 207Pb/204Pb 2SE 208Pb/204Pb 2SE Tholeiitic Basalts 296‐25 0.703733 8 0.512949 7 6.1 0.283049 9 9.8 18.1722 4 15.4658 8 37.923 20 296‐26 0.703755 9 0.512932 4 5.7 0.283071 5 10.6 18.1086 5 15.4542 10 37.896 24 296‐32 0.703788 9 0.512893 5 5.0 0.283048 6 9.8 17.9496 5 15.4525 10 37.780 26 296‐32 dup 0.703795 9 0.512904 5 5.2 0.283039 5 9.4 17.9481 6 15.4528 12 37.781 31 305‐04 0.703920 8 0.512840 6 3.9 0.283033 6 9.2 17.7954 10 15.4361 7 37.671 25 305‐04 dup 0.703926 9 0.512862 7 4.4 0.283040 15 9.5 17.7968 8 15.4377 7 37.675 17 305‐07 0.703716 8 0.512922 5 5.5 0.283057 3 10.1 17.9242 8 15.4483 7 37.747 16 305‐09 0.703710 8 0.512926 7 5.6 0.283046 5 9.7 17.9323 7 15.4512 8 37.762 15 305‐14 0.703816 9 0.512881 8 4.7 0.283037 4 9.4 18.0572 7 15.4550 6 37.876 19 306‐01 0.703709 9 0.512911 6 5.3 0.283061 4 10.2 17.8603 8 15.4424 7 37.706 19 306‐05 0.703723 8 0.512925 6 5.6 0.283053 4 9.9 17.8718 7 15.4427 7 37.709 18 306‐09 0.704119 8 0.512844 6 4.0 0.282976 6 7.2 18.0279 7 15.4480 6 37.966 21 306‐11 0.704137 10 0.512844 6 4.0 0.282980 6 7.4 18.0255 19 15.4577 24 37.963 77 306‐15 0.703798 8 0.512913 8 5.4 0.283060 6 10.2 17.8267 6 15.4412 6 37.647 16 306‐26 0.703740 7 0.512894 7 5.0 0.283037 5 9.4 17.8727 7 15.4398 8 37.704 19 306‐27 0.703743 8 0.512893 8 5.0 0.283042 5 9.5 17.8785 21 15.4516 22 37.720 65 306‐30 0.703759 8 0.512975 6 6.6 0.283080 5 10.9 18.1155 7 15.4504 6 37.878 19 EK‐1 0.703607 8 0.512973 8 6.5 0.283117 4 12.2 18.3074 8 15.4680 7 38.002 23 EK‐2 0.703816 8 0.512944 9 6.0 0.283079 6 10.9 18.1885 9 15.4570 9 37.953 27 Transitional Basalts 305‐20 0.703920 7 0.512878 6 4.7 0.283016 6 8.6 17.9075 6 15.4455 6 37.738 17 305‐24 0.703744 9 0.512922 6 5.5 0.283058 5 10.1 17.9396 8 15.4534 7 37.714 17 306‐19 0.703837 8 0.512920 6 5.5 0.283033 6 9.2 17.9018 6 15.4447 5 37.709 15 Alkalic Basalts 296‐01 0.703220 8 0.513062 7 8.3 0.283161 5 13.8 18.1844 5 15.4561 9 37.871 27 296‐11 0.703071 8 0.513077 6 8.6 0.283153 4 13.5 18.0771 8 15.4345 7 37.759 20 296‐22 0.703276 10 0.513058 6 8.2 0.283167 5 14.0 18.1557 14 15.4559 13 37.824 37 305‐02 0.703054 9 0.513073 5 8.5 0.283170 5 14.1 18.1519 7 15.4472 7 37.796 19 aAbbreviations are as follows: thol, tholeiitic basalt; trans, transitional basalt; alk, alkalic basalt. dup indicates complete chemistry duplicate. All trace element and isotopic analyses were carried out at the PCIGR. The analytical methods are described in Appendix A. Two SE (standard error) is expressed as 106 for Sr, Nd, and Hf and times 104 for Pb. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 13 of 30 [18] Submarine volcanism west of Ka‘ena Ridge represents a period of minimal magma production in the Hawaiian Islands, in terms of volcano volume per area, and therefore is an important reference point for sampling the evolution of the Hawaiian plume. New geochemical data of lavas collected from these cones reveal they consist primarily of tholeiitic and transitional basalts with major and trace element compositions that overlap those of shield stage basalts along the Hawaiian chain. Although the West Ka‘ena tholeiitic cones are very small in volume compared to adjacent Hawaiian shield volcanoes, the lavas span a considerable range of the shield stage compositions and define a group of tholeiitic basalts with distinct trace element ratios (Sr/Nb, La/Nb) and Pb isotopic compositions. Trace element melting models indicate a moder- ately high degree of melting (8%–15%) comparable to Hawaiian shield volcanoes despite the low pro- ductivity of volcanism (Figure 7a). [19] The flat‐topped and conical submarine cones west of Ka‘ena Ridge formed isolated volcanic centers through steady centralized effusive erup- tions. These cones probably formed as overflowing submarine lava ponds from a levee surrounding a seafloor vent [Clague et al., 2000]. The formation of submarine volcanic cones of similar size and shape is attributed to effusive eruption of lava with low volatile contents, high hydrostatic pressure at moderate to great ocean depth, long‐lived steady eruption (years to decades), moderate effusion rates (assuming 0.1 km3/year), and eruption on low‐ degree slopes [Clague et al., 2000]. [20] All indicators (geology, geochronology, major and trace element, and isotopic compositions) show that the alkalic lavas, which mostly erupted as part of young lava fields sampled during Dive 296, are similar to the rejuvenated Kōloa Volcanics from Kaua‘i [Garcia et al., 2010]. Their young ages (1.39–0.24 Ma) confirm that these alkalic lavas are a form of late stage volcanism that was secondary to the main tholeiitic phase. Contrasting with onshore rejuvenated lavas, such as the Kōloa Volcanics, this form of secondary volcanism was well offshore and could be analogous to the sub- marine volcanic field west of Ni`ihau [Clague et al., 2000] and other rejuvenated stage lavas that occur offshore of Hawaiian shield volcanoes [Clague et al., 2003]. The alkalic basalts have major elements and trace element ratios similar to Kaua‘i rejuvenated lavas, which were modeled to result from deep (3.5–4.0 GPa) very low degree (0.02%–2.6%) partial melting of a peridotite source [Garcia et al., 2010]. The tholeiitic, transitional, and alkalic lavas from west of Ka‘ena Ridge together provide an opportunity to examine volca- nism spanning greater than 4.5 Myr along a section of the Hawaiian chain between subaerial shield volcanoes. [21] The goals of sections 6.2–6.4 are to evaluate (1) source composition for West Ka‘ena tholeiitic and transitional lavas compared to Hawaiian shield tholeiites, (2) pyroxenite versus peridotite source lithology for West Ka‘ena lavas, and (3) the rela- tionship between source composition and lithology to volume flux in the Hawaiian Islands. 6.2. Source Composition for Tholeiitic and Transitional West Ka‘ena Lavas Compared to Hawaiian Shield Lavas [22] The tholeiitic and transitional basalts from West Ka‘ena are distinct in combined Pb‐Sr‐Nd‐Hf isotopic systems from any lavas sampled in the Hawaiian Islands. Isotopic compositions of West Ka‘ena lavas extend to extremely low 206Pb/204Pb and to a lower 208Pb/204Pb ratio than any Hawaiian tholeiitic lava (Figure 9). Several West Ka‘ena samples have similar 206Pb/204Pb to SiO2‐enriched lavas of Ko‘olau Makapu‘u, Lāna‘i, and Kaho‘olawe [e.g., Frey et al., 1994; Huang et al., 2005]. How- ever, compared to these lavas, the West Ka‘ena lavas have higher Hf and Nd and lower Sr isotopic compositions but do not have elevated 208Pb/204Pb ratios at a given 206Pb/204Pb (Figure 9). West Ka‘ena lavas have a high degree of variability in Pb isotope compositions compared to most Hawaiian shields, and their low volume and eruption from isolated cones may be an important factor. The relatively wide range in 208Pb/204Pb of West Ka‘ena lavas is unmatched by any Hawaiian shield volcano. [23] Large‐scale intershield isotopic heterogeneity in the Hawaiian Islands has been defined by the distinction between the southern Loa trend and northern Kea trend volcanoes [Tatsumoto, 1978] (Figure 1). The spatial distribution of this large‐ scale bilateral asymmetry of Hawaiian shield vol- canoes has been explained by (1) a concentrically zoned model [e.g., Lassiter et al., 1996], (2) radi- ally zoned plume model [Bryce et al., 2005], (3) a bilaterally zoned plume model [Tatsumoto, 1978; Abouchami et al., 2005], (4) a partly ordered zonation model [Herzberg, 2005], (5) a randomly distributed heterogeneity model [Frey and Rhodes, 1993; Huang and Frey, 2005; Ren et al., 2006], and (6) vertically stacked heterogeneities [Blichert‐ Toft et al., 2003]. The Loa trend is defined by Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 14 of 30 lower Pb, Nd, and Hf and higher Sr isotopic ratios, whereas lavas of the Kea trend typically have higher Pb, Nd, and Hf and lower Sr isotopic ratios [Abouchami et al., 2005] (Figures 8 and 9). Loa‐ type lavas have been recognized among the southern volcanoes between Mauna Loa and Ko‘olau, and Kea‐type lavas are common among the northern subchain of volcanoes between Kīlauea and Kaua‘i, as well as throughout the Hawaiian‐Emperor chain [Tanaka et al., 2008]. West Ka‘ena lavas have similar Sr, Nd, and Hf isotopic compositions to lavas from Loa trend volcanoes but a group of lavas have lower 208Pb/204Pb and 206Pb/204Pb (Figure 9). Thus, our results demonstrate that Loa‐type magmatism extends beyond Ko‘olau volcano to circa 4.9 Ma and that it occurred prior to the development of the dual chain of volcanoes. [24] The source of the Loa and Kea geochemical lines are thought to consist of small‐scale hetero- geneities [e.g., Frey and Rhodes, 1993; Rhodes and Hart, 1995; Marske et al., 2007, 2008]. Isotope variation of Hawaiian basalts is usually attributed to melting of different proportions of hetero- geneities of recycled material in the plume [e.g., Ren et al., 2009]. West Ka‘ena lavas may be influenced by these local‐scale heterogeneities. Many of the West Ka‘ena tholeiitic and transitional lavas have compositions that are displaced from the main Hawaiian shield array, similar to postshield lavas from Hualālai volcano [Hanano et al., 2010]. The samples with the least radiogenic Pb compo- sitions deviate from the main shield array toward less radiogenic Sr and Pb isotopic compositions, and more radiogenic Nd. These deviations indicate that the West Ka‘ena tholeiitic lavas cannot be explained by mixing of the Ko‘olau and Kea end‐ members [e.g., Blichert‐Toft et al., 1999]. Instead, the group of West Ka‘ena lavas with low Pb iso- tope compositions sampled a distinct component that is not regularly sampled by Hawaiian shield lavas or define an isolated compositional hetero- geneity. However, the group of West Ka‘ena lavas with higher Pb isotopes overlap the main field of Loa volcanism (Figure 9). The low Pb component cannot be Pacific oceanic crust because the Pb isotopic composition of Pacific crust is too radio- genic and Pb‐Pb mixing lines do not intersect Pacific MORB compositions (Figure 9). This combination of evidence suggests that the lavas west of Ka‘ena Ridge formed from recycled material within the Hawaiian plume and these extreme Pb isotope compositions may be present in distinct heterogeneities or sampled preferentially under specific melting conditions or area of the plume. [25] Trace element ratios combined with isotope compositions provide insights on the nature of heterogeneities (lithologic versus isotopic hetero- geneities, type of recycled material). West Ka‘ena lavas, compared to Hawaiian shield lavas, highlight the differences between Loa and Kea trend lavas (Figure 10). Sr/Nb and La/Nb of West Ka‘ena lavas correlate with 206Pb/204Pb and West Ka‘ena vol- canics are the only individual suite of lavas that form a clear trend in these plots. The tholeiitic basalts from Hawaiian shields as a whole form a broad overall trend in these trace element ratios and Pb and Sr isotope ratios (Figure 10). Compared to Kea‐type lavas, Loa‐type volcanism generally has lower Pb and higher Sr isotope ratios, higher Sr/Nb and La/Nb, and lower Th/La than Kea‐type lavas. West Ka‘ena samples exemplify these character- istics. The correlation of trace element ratios and Pb isotope ratios in West Ka‘ena lavas indicates this relationship is related to the composition of the source and is not solely controlled by partial melting effects or degree of crystal fractionation. Correlation of trace element and isotope ratios between Hawaiian shields reflects tapping of mul- tiple, distinct source components or differences in the proportion of source components [Frey and Rhodes, 1993]. Variation of Sr/Nb and La/Nb and isotopes in Hawaiian lavas has been attributed to (1) melting of garnet pyroxenite that originated from a recycled plagioclase‐rich gabbroic compo- nent and (2) a small sedimentary component derived from ancient phosphate‐bearing carbonate [Huang and Frey, 2005]. Huang and Frey [2005] suggested that high Sr/Nb and La/Nb, and low Th/La in Ko‘olau lavas are characteristic of enrichment in marine sediments, which also typi- cally possess lower 238U/204Pb than primitive mantle which would lead to low 206Pb/204Pb ratios. Low Th/La ratios, negative Th‐U anomalies, and positive Sr anomalies have been identified with a recycled gabbroic component based on the parti- tioning of these elements in plagioclase in oceanic crust [Hofmann and Jochum, 1996; Sobolev et al., 2000]. The correlation between intershield differ- ences in some incompatible element ratios (e.g., Sr/ Nb, La/Nb) and isotope ratios, indicates that each shield has been formed from a compositionally distinct source with small‐scale heterogeneities [Putirka, 1999]. [26] Several lines of evidence preclude a significant role for recycled marine sediments in the source of West Ka‘ena lavas, assuming the composition of Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 15 of 30 sediments has not been significantly modified during recycling. A sedimentary component with high Rb/Sr would lead to high Sr isotope ratios and a positive correlation between Rb/Sr and 87Sr/86Sr, as suggested by Huang et al. [2009] for Mahukona lavas; there is no correlation between Rb/Sr and 87Sr/86Sr in West Ka‘ena lavas. A positive corre- lation is apparent between Zr/Nb and Sr/Nb (not shown), which is not expected from a recycled carbonate, sediment source. Marine sediments usually have higher 232Th/238U and lower 238U/204Pb than primitive mantle and therefore re- cycled sedimentary material is characterized by high 208Pb/204Pb relative to 206Pb/204Pb; West Ka‘ena lavas do not have high 208Pb/204Pb relative to 206Pb/204Pb as observed in Ko‘olau lavas. Radiogenic ingrowth of sediments with high Lu/Hf leads to high 176Hf/177Hf relative to 143Nd/144Nd; this relationship is not evident in West Ka‘ena la- vas (with the exception of the one sample, 305‐04). High 176Hf/177Hf for this sample may reflect greater contribution from a depleted lithosphere component [Salters et al., 2006]. There is also no apparent correlation between Ce/Pb and La/Nb or Th/La, which is anticipated by low‐Ce/Pb sedi- ments (although Ce/Pb of whole rocks may be Figure 10. Trace element ratios versus Pb and Sr isotope compositions for Hawaiian shield volcanoes. (a) Sr/Nb versus 206Pb/204Pb. (b) Sr/Nb versus 87Sr/86Sr. (c) La/Nb versus 206Pb/204Pb. (d) Th/La versus 87Sr/86Sr. References for data sources are listed in the auxiliary material (Text S1). Green line is a best fit line for West Ka‘ena tholeiitic basalts. Symbols for each Hawaiian shield volcano are labeled in Figures 10a and 10b. Symbols for Loa trend volcanoes are in cooler (bluish) colors, and Kea trend volcanoes are in hotter (reddish) colors. All isotope data from Hawaiian shield volcanoes are normalized to the same set of standards. Alkalic lavas from west of Ka‘ena Ridge and other rejuvenated lavas are not shown in these plots. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 16 of 30 affected by alteration [Hofmann, 2003; Huang and Frey, 2005]). West Ka‘ena lavas have Loa‐type trace element and isotopic compositions that are distinct from the extreme Loa‐type lavas from Makapu‘u stage Ko‘olau and Lāna‘i (e.g., they lack comparably high Sr/Nb, La/Nb, and 87Sr/86Sr) that has been attributed to a recycled sedimentary component. 6.3. Pyroxenite Versus Peridotite Source for West Ka‘ena Lavas [27] Several key geochemical and seismological observations support that the Hawaiian hot spot originates as a deep‐seated, high‐temperature plume from the lower mantle [e.g., DePaolo and Manga, 2003; Montelli et al., 2004; Wolfe et al., 2009]. Geochemistry of Hawaiian shield lavas suggests the Hawaiian plume consists partly of recycled oceanic crust [Blichert‐Toft et al., 1999; Gaffney et al., 2005; Hauri, 1996; Herzberg, 2006; Lassiter and Hauri, 1998; Sobolev et al., 2000, 2005, 2007]. Oxygen isotopes of olivines from Hawaiian shield lavas also indicate a component of hydrothermally altered rocks, perhaps derived from Cretaceous oceanic crust beneath Hawai‘i or earlier erupted Hawaiian lavas, in the source of Hawaiian shield basalts [e.g., Eiler et al., 1996]. One of the fundamental questions from geochemistry is the importance and nature of mafic lithologies in the mantle source of Hawaiian basalts. [28] The Hawaiian plume is thought to consist of a peridotite matrix with finely distributed streaks or parcels of recycled eclogite or pyroxenite, perhaps superimposed on the larger‐scale, plume‐wide Loa‐Kea asymmetry [e.g., Kogiso et al., 2004; Reiners, 2002; Ren et al., 2009]. Pyroxenite and peridotite undergo different degrees of partial melting and these melts may react with surrounding plume material and mix in at shallower levels during ascent [Pertermann and Hirschmann, 2003; Reiners, 2002; Sobolev et al., 2005]. The emer- gence of different proportions of pyroxenite‐ and Figure 11. NiO (wt %) versus Fo content (%) for olivines from West Ka‘ena lavas compared to olivines from tho- leiitic basalts from several Hawaiian shield volcanoes (Ko‘olau, Mauna Loa, Mauna Kea, and Lō‘ihi). Olivine com- positions from Hawaiian shield volcanoes are from Sobolev et al. [2007]. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 17 of 30 peridotite‐derived melts may be controlled by the thermal structure of the plume and spatial distri- bution of lithologic heterogeneities [Ren et al., 2009], which may be a function of proximity to the central axis of the plume, plume flux, and/or plate thickness or motion [Sobolev et al., 2007; Vidal and Bonneville, 2004; White, 1993]. [29] A pyroxenite source model for Hawaiian tho- leiites contends that the Ni concentrations of oli- vines in these lavas should be a reflection of the Ni concentration and the presence of olivine in the source [Sobolev et al., 2005, 2007]. Therefore, olivines crystallizing in melts derived from Ni‐rich peridotite should be lower in NiO (as a result of the high compatibility of Ni in olivine). Melts from a pyroxenite source and their crystallizing olivines should be enriched in NiO, because olivine is not controlling the partitioning of Ni. Sobolev et al. [2005] demonstrated that olivines and calculated parental magmas of some shield stage Hawaiian tholeiites have high Ni contents compared to other oceanic islands, MORB, and Gorgona komatiites. They proposed a model of melting of eclogite and reaction of this melt with peridotite to produce an olivine‐free pyroxenite source for Hawaiian mag- mas, which dictates excess NiO in some lavas. Lavas erupted on Ko‘olau and Mauna Loa volca- noes have olivines with the highest NiO at a given Fo content and are proposed to involve the greatest contribution of pyroxenite‐derived melt [Sobolev et al., 2005]. This interpretation is consistent with the presence of high‐SiO2 lavas in these and other Loa‐type volcanoes, which is expected for melting of a recycled eclogite [Kogiso et al., 2003]. Ni contents in olivine in primitive (11–13 wt % MgO) West Ka‘ena lavas provides a test for evaluating the contribution of pyroxenite‐derived melts, based on the method of Sobolev et al. [2005]. [30] Olivine phenocrysts in five West Ka‘ena tho- leiitic lavas were analyzed using analytical condi- tions similar to the ones used by Sobolev et al. [2005] (the results are compared to the large data- base of olivine compositions from Hawaiian shield basalts presented by Sobolev et al. [2007] (Figure 11; see Appendix A for summary of analytical meth- ods)). Olivine phenocrysts form two broad bands in NiO (wt %) versus Fo (%). Olivines from three West Ka‘ena lavas (305‐04, 305‐07, 306‐05) have similar NiO to Ko‘olau lavas at a given Fo con- tent, whereas olivines from two samples overlie compositions of olivines for the main field of Hawaiian shield basalts from Mauna Loa, Mauna Kea, and early/middle Ko‘olau. Sample 305‐04 is of particular interest because it has olivines with the highest NiO, and whole‐rock composition with the lowest 206Pb/204Pb and high SiO2 (51.7 wt %) [this sample has multiple olivine populations sug- gesting it picked up olivine, probably from the cumulate pile, as found on other Hawaiian volca- noes (Kīlauea [Clague et al., 1995]; Mauna Loa [Garcia et al., 1995]; and Ko‘olau [Garcia, 2002]). These are characteristics similar to the Ko‘olau Makapu‘u stage (late stage) lavas, proposed to have a significant contribution of pyroxenite‐derived melt [Hauri, 1996; Sobolev et al., 2005]. Interest- ingly, sample 305‐04 is also the only West Ka‘ena sample that lies well above the Hawaiian Hf‐Nd isotope array (Figure 8), indicating contribution from a depleted lithosphere component [Salters et al., 2006] or marine sedimentary component [Blichert‐Toft et al., 1999]. Whole‐rock trace ele- ment and isotope compositions do not support contribution from recycled marine sediments (see previous discussion). Sample 306‐05 has olivines that fall on both trends. Based on the high Ni contents (and high Fe/Mn and low CaO) of some olivines in West Ka‘ena lavas, in combination with the whole‐rock trace element and isotope compo- sitions compared to Hawaiian shield basalts, West Ka‘ena lavas likely involved contribution from melting of pyroxenite. [31] Another origin for high‐Ni olivines has been proposed Wang and Gaetani [2008] based on experimental data on partitioning of Ni between olivine and siliceous eclogite partial melts. Wang and Gaetani [2008] suggested that Ni is more Figure 12. CaO versus MgO of West Ka‘ena lavas compared to lavas from Hawaiian shield volcanoes. Symbols are the same as in Figures 8 and 9. Lavas with CaO contents lower than the black line are potential pyroxenite partial melts, and compositions above the black line are similar to lavas that form from peridotite source melts [Herzberg and Asimow, 2008]. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 18 of 30 Figure 13. Estimated volume of the Hawaiian Islands compared to Pb isotope compositions and SiO2 of Hawaiian shield stage basalts. (a) Volume (106 × km3) at the center of a series of 50 km wide bins shown in Figure 13b versus distance from Kīlauea (km). Inset shows volume flux (m3/s) for the Hawaiian Ridge from Vidal and Bonneville [2004], with dashed line at 550 km indicating the start of volume calculations in this study. (b) Map of the Hawaiian Islands showing area used for volume calculations and 50 km wide bins approximately perpendicular to the axis of the islands. Volume estimates for pink and blue colored bins are indicated by pink and blue symbols in Figure 13a, plotted at a distance from Kīlauea at the right edge of the bin, where the small white numbers are for the bin. (c) Plot of 208Pb/204Pb versus distance from Kīlauea (km) with averages shown for individual shields. Linear regressions are shown for Kea (red) and Loa (blue) trend volcanoes. Dashed blue regression line does not include Kaua‘i. (d) Average 206Pb/204Pb versus SiO2 for individual shield volcanoes. Bars represent one standard deviation. Most of the lavas in this compilation have been filtered for alteration (e.g., K2O/P2O5 > 1). References for data sources for isotope and major element data are listed in the auxiliary material (Text S1). All isotope data from Hawaiian shield volcanoes are normalized to the same set of standards. Alkalic lavas from west of Ka‘ena Ridge and other rejuvenated lavas are not shown in these plots. Kaua‘i and West Moloka‘i are divided into Loa and Kea compositions in Figures 13c and 13d based on Garcia et al. [2010] and Xu et al. [2007], respectively. Volcano volume is calculated by assuming the base of the volcano is the surface of the preexisting Pacific seafloor. This surface was determined by first extrapolating the surface of the seismically determined, base of the preexisting oceanic crust (i.e., Moho) [Watts and ten Brink, 1989] and then shifting it upward by 6 km, to account for the assumed uniform crustal thickness. We then calculated the volume between the top of the preexisting Pacific seafloor and the high‐resolution bathymetry and subaerial topography (see Robinson and Eakins [2006] for bathymetry and topog- raphy data sources and further details of volume calculations). The volume of the compacted moat sediments was computed by multiplying their assumed average thickness of 500 m by the area of the edifices; this volume was then subtracted from the total. Volumes are minimum estimates because they also ignore the large debris avalanches deposited on the archipelagic aprons. Volume calculations are shown in 50 km bins perpendicular to the axis of the islands, rather than by volcano, because of the difficulty in delineating the boundary of each volcano below the surface [see Robinson and Eakins, 2006]. Subsidence, landsliding, sedimentation, and intrusion between Kaua‘i and O‘ahu will not significantly effect volume calculations for West Ka‘ena or obscure the increasing volume along the Hawaiian Islands. Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 19 of 30 compatible in olivine crystallizing from siliceous melts than in basaltic melts, and that Hawaiian lavas with high Ni (e.g., Ko‘olau Makapu‘u stage lavas) may have formed from mixing of reacted high‐SiO2 eclogite partial melt with primitive basalt. However, based on careful evaluation of partition coefficients and Fe/Mn ratios from Wang and Gaetani [2008], Gurenko et al. [2009] con- cluded that the experimental data failed to explain Fe/Mn ratios. Thus, Gurenko et al. [2009] sug- gested variations of Ni in olivines from Canary Island shield lavas are likely due to varying pyroxenite/peridotite in the source rather than from mixing and crystallization of SiO2‐rich melts as suggested by Wang and Gaetani [2008]. Using the equations of Gurenko et al. [2009], the proportion of pyroxenite‐derived melt relative to peridotite‐ derived melt in West Ka‘ena lavas is estimated between 0.65 to 0.77. [32] Major element compositions of lavas also help constrain source lithology. Hawaiian shield lavas have low CaO contents compared to model CaO contents of partial melts of peridotite and similar CaO to many experiments of pyroxenite melting (Figure 12) [Herzberg and Asimow, 2008]. Partial melts of pyroxenite have low CaO because of the strong influence of residual clinopyroxene in the source [Herzberg, 2006]. West Ka‘ena lavas have similar whole‐rock CaO contents to other shield stage lavas in Hawai‘i (Figure 12). The low CaO of West Ka‘ena lavas and Hawaiian shield lavas suggest a significant role for melting of pyroxenite in the Hawaiian plume. Whereas, the alkalic lavas from west of Ka‘ena Ridge and rejuvenated lavas from the Hawaiian Islands have high CaO contents that are similar to melts derived from peridotite sources. West Ka‘ena lavas are some of the earliest examples of Loa‐type volcanism in the Hawaiian Islands (circa 4.9 Ma) that indicate melting of a pyroxenite source, and thus provide insights on the evolution of the Hawaiian Islands. 6.4. What Is the Relationship of Source Composition and Lithology to Volume Flux in the Hawaiian Islands? [33] The Hawaiian‐Emperor Chain is the longest example of an island chain on Earth, both in time and space, active for 80 Myr and extending almost 6000 km. One of the distinctive features of the Hawaiian‐Emperor Chain is the dramatic and rapid variation in magma flux rate along the Hawaiian Ridge (<30 Ma), and particularly along the Hawaiian Islands. The activity of the Hawaiian hot spot has increased by ∼300% in the last 30 Myr and this increase has been most dramatic over the last 3 Myr [Vidal and Bonneville, 2004] (Figure 13a, inset). An opposite trend of decreasing melt flux with age is observed for the Louisville chain [Lonsdale, 1988] and Walvis and St. Helena hot spots [Adam et al., 2007]. It has been suggested that the Loa component only recently emerged in the Hawaiian chain (<3 Ma) [Abouchami et al., 2005] and is related to a pyroxenite source [Hauri, 1996], which in most cases melts at a lower temperature and to higher extent than peridotite [Stracke and Bourdon, 2009]. These observations were combined to suggest that the recent appear- ance of pyroxenite is the cause of the large magma flux for the Hawaiian Islands [Tanaka et al., 2008]. However, our new data indicate the presence of Loa‐type isotopic characteristics in low‐volume, >3 Myr old lavas from West Ka‘ena and melting of pyroxenitic source material (Figures 13c and 13d). [34] Determining the relationship between mantle source composition, lithology and volume flux in the Hawaiian Islands can impact our thinking about the evolution of the Hawaiian hot spot and provides constraints on the mechanisms of large‐scale melt flux variations [White, 1993]. Proposed mechan- isms to account for melt flux variations involve shallow (lithosphere) or deep (plume) processes. Lithosphere‐related explanations include (1) plate thickness variations or lithospheric features (e.g., fracture zones) [Hieronymus and Bercovici, 1999; Regelous et al., 2003;White, 1993] and (2) changes in plate stresses resulting from reconfigurations of plate motion [Wessel and Kroenke, 2009]. Plume‐ related causes include (1) southward migration of the Hawaiian hot spot [Tarduno et al., 2003; Wright and Klein, 2006], (2) periodic oscillation in the plume conduit [e.g., Ito, 2001; Sleep, 1992; van Keken and Gable, 1995], and (3) changes in source composition and/or melting conditions [Lee et al., 2009; Sobolev et al., 2007]. Here we evaluate changes in source composition and volume flux in the Hawaiian Islands. [35] Hawaiian shield volcanoes show dramatic changes in their source composition [Tatsumoto, 1978; Stille et al., 1983; Staudigel et al., 1984; Abouchami et al., 2005]. Our new volume cal- culations for the Hawaiian Islands reveal a dra- matic increase in magma production rate since circa 4.5–4.0 Ma (Figures 13a and 13b; the new volume calculations are described in the caption of Figure 13). The excellent geochemical coverage for the Hawaiian Islands, along with the new isotopic analyses between Kaua‘i and O‘ahu, has allowed Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 20 of 30 us to identify a correlation between Pb isotopic composition, major element composition, and vol- ume flux in the Hawaiian Islands. Average 208Pb/204Pb ratios for Hawaiian shield volcanoes show a resolvable systematic increase with decreasing age (or distance from Kīlauea) for both Kea and Loa trend volcanoes (Figure 13c). West Ka‘ena lavas have the lowest average 208Pb/204Pb and form the lowest volume along the axis of the Hawaiian Islands. Both volume and 208Pb/204Pb increase from a local minimum near West Ka‘ena toward a peak centered on Mauna Loa and flanked by Mauna Kea and Kīlauea. Although trends in 208Pb/204Pb versus age are observed within the Loa and Kea volcano trends individually, Loa trend volcanoes have systematically lower 206Pb/204Pb ratios than Kea trend volcanoes. Average 206Pb/204Pb correlates with SiO2 for Loa trend volcanoes (Figure 13d). Kea trend volcanoes do not show this systematic variation between Pb iso- topes and SiO2. The variation in Pb isotopes in the Loa trend volcanoes is also about 1.5 times that of the Kea trend volcanoes. There is no cor- relation between the individual volcano volumes, from Robinson and Eakins [2006], and average 206Pb/204Pb or SiO2 for individual volcanoes (not shown). [36] There are several possible explanations for the correlation between increasing volume flux and major element and isotopic composition in Hawaiian lavas: (1) increasing pyroxenite in the source, (2) changing composition of pyroxenite in the source, (3) increasing temperature of the plume, (4) increasing plume flux (i.e., material supplied by the plume), (5) migration or tilting of the plume, and (6) change in plate motion. Our results provide constraints on pyroxenite in the source based on lava composition and volcano volumes along the Hawaiian chain. If the low Pb isotopic ratios are representative of the pyroxenite source of Hawaiian lavas (West Ka‘ena, Ko‘olau Makapu‘u, Lāna‘i, Kaho‘olawe) and pyroxenite is more fusible than the higher 208Pb/204Pb‐206Pb/204Pb peridotite source material, then in the simplest case, an increasing volcano volume should correlate with decreasing, not increasing Pb isotope ratios toward Kīlauea. Therefore, if the increase in volcano vol- ume is related to an increase in pyroxenite content in the source, the observed compositional trend would require the radiogenic Pb content of the pyroxenite source to change with time. The positive relationship between radiogenic Pb and volcano volume suggests an additional factor besides the proportion of pyroxenite in the source is changing with time. Another possible interpretation is that the temperature of plume or plume flux has been increasing and the amount of pyroxenite in the source has not significantly increased. However, our study does not provide constraints on mantle temperature or plate motion, so we cannot evaluate these possibilities. [37] The Hawaiian Islands are rare among ocean island basalts in that they display evidence for a silica‐rich pyroxenite source, perhaps from melting beneath relatively thick lithosphere (∼100 km thick beneath the Big Island [Li et al., 2004]) compared to Iceland and Galāpagos [Jackson and Dasgupta, 2008; Sobolev et al., 2007]. Our new data and previous work [e.g., Hauri, 1996] suggest pyrox- enite (recycled oceanic crust) sampled at different phases of the Hawaiian plume (e.g., West Ka‘ena, Ko‘olau) has low time‐integrated U/Pb ratios. This is not always the case for recycled oceanic crust (e.g., HIMU in the Canary Islands) [Jackson and Dasgupta, 2008; Gurenko et al., 2009]. Loa‐type magmatism, commonly attributed to mafic lithol- ogies in the source [Hauri, 1996], occurs at times of the highest (Mauna Loa) and lowest (West Ka‘ena) productivity in the Hawaiian Islands. This also suggests the increasing volume of the Hawaiian Islands may not be controlled primarily by the amount of entrained mafic lithologies in the Hawaiian plume. The appearance of Loa‐type volcanism may not be independently related to increase in volume flux. The emergence of greater proportions of the pyroxenite component in Loa‐ type magmatism has been proposed to be related to the thermal structure of the Hawaiian plume, dis- tribution of heterogeneities, and the solidus tem- perature of pyroxenite heterogeneities [Reiners, 2002; Ren et al., 2009]. Initiation of pyroxenite melting can be considerably deeper than peridotite (depending on the composition of the pyroxenite) and preservation and abundance of pyroxenite‐ derived melt depends on (1) extent of pyroxenite melting prior to peridotite melting, (2) relative masses of pyroxenite and peridotite, and (3) how pyroxenite and peridotite melts mix and interact with surrounding mantle during ascent [Stracke and Bourdon, 2009]. Pyroxenite melts may become increasingly diluted as the extent of peridotite melting increases and therefore melts extracted from greater depth may contain greater proportion of pyroxenite‐derived melt (assuming melts do not react at high pressure to precipitate pyroxenite) [Kogiso et al., 2004; Herzberg and Asimow, 2008]. For lavas west of Ka‘ena Ridge, melts may have formed along the margin of the Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 21 of 30 central axis of the plume and perhaps avoided substantial mixing with melts from shallower peridotite sources. This is due to less upwelling away from the center of the plume axis, as plume material is transported laterally [e.g., Ribe and Christensen, 1999]. This may have lead to a melting column with a deep zone of largely melted pyroxenite and a restricted amount of small‐degree peridotite melt from shallower levels [Reiners, 2002]. Another possibility is these melts may have formed later than the main phase of volcanism that created the low shields on Ka‘ena Ridge and the melts may have been extracted from increasing depths, which increased the proportion of pyroxe- nite melts and decreased contribution from peri- dotite melting. Also, the nature of the eruptions that formed the submarine cones west of Ka‘ena Ridge, at isolated volcanic centers through short‐lived effusive eruptions (similar to rejuvenated volca- nism), may have decreased the degree of mixing and homogenization between shallow and deeper melts in a centralized magma reservoir system. [38] Lithospheric thickness and location of fracture zones may play a role in controlling the spacing of the volcanoes and the formation of parallel chains (Loa‐Kea trends) [e.g., Hieronymus and Bercovici, 1999]. However, they do not appear to be respon- sible for the dramatic volume variation along the Hawaiian Islands. There is no correlation between magma production rate and lithosphere thickness or fracture zones [White, 1993; Van Ark and Lin, 2004; Vidal and Bonneville, 2004]. Periodic oscil- lation in the plume conduit [e.g., Ito, 2001] may cause perturbations in magma flux, but evaluation of a greater part of the Hawaiian chain is necessary to assess the role of solitary waves. Changes in Pacific plate motion due to plate boundary inter- actions affect the horizontal stresses in the litho- sphere [e.g., Sager and Keating, 1984; Koppers and Staudigel, 2004; Wessel and Kroenke, 2007]. Depending on the direction of these changes and the location on the plate, a region may experience an increase in tensional or compressional stresses; these stresses may modulate the amount of melt that reaches the surface [Hieronymus and Bercovici, 1999]. 7. Conclusions [39] A field of tholeiitic submarine cones west of Ka‘ena Ridge formed astride the axis of the Hawaiian Islands between 4.9 to 3.6 Ma. Their eruptions were contemporaneous with shield‐ building volcanism on the island of Kaua‘i and Wai‘anae Volcano on O‘ahu. The longest distance between large subaerial shields of the Hawaiian Islands is between the islands of Kaua‘i and O‘ahu, where the volume of the islands is also the smallest. These cones sampled volcanism across the axis of the Hawaiian Islands (spanning ∼70 km) during a time of minimal magma productivity and prior to formation of dual Loa and Kea geographical trends of volcanoes. The cones may be distal eruptions related to the low shields on Ka‘ena Ridge. The lavas west of Ka‘ena Ridge have distinct trace element and isotopic compositions compared to those of Hawaiian shield basalts, with the lowest Pb isotopic ratios in the Hawaiian Islands. Half of the volcanic cones contain high‐SiO2 basalts (51.0–53.5 wt % SiO2), similar to Loa trend lavas of Ko‘olau Makapu‘u, Lāna‘i, and Kaho‘olawe. Samples with the lowest time‐integrated U/Pb ratios reveal a contribution from pyroxenite‐ derived melts, based on olivines in several West Ka‘ena lavas with high Ni contents compared to most Hawaiian shield basalts. The low Pb isotope compositions may have formed from pyroxenite under specific melting conditions, from local‐scale heterogeneities, or possibly in the periphery of the plume without mixing with melts from shallower peridotite sources. Young lava fields (circa 0.37 Ma) sampled southwest of Ka‘ena Ridge are related to secondary volcanism derived from a peridotite source. [40] Tholeiitic volcanism west of Ka‘ena Ridge provides insights about Loa‐type volcanism and the evolution of the Hawaiian Islands. 1. Loa‐type volcanism has persisted for at least 4.9 Myr along the Hawaiian Archipelago. 2. Volcanism between Kaua‘i and O‘ahu sampled Loa‐type components without the complementary Kea component, prior to formation of a dual chain of volcanoes; this suggests vertical heterogeneity in the Hawaiian plume. 3. There is a resolvable correlation between vol- ume flux and Pb isotope ratios in Hawaiian tho- leiitic basalts, and a correlation between major elements and isotopes in Loa trend volcanism. The correlation between Pb isotope ratios and SiO2 in the Loa trend volcanoes is evidence for variable influence of pyroxenite in these volcanoes. If the increase in volume flux in the Hawaiian Islands is due to increasing pyroxenite in the source, and unradiogenic Pb isotope ratios are representative of the pyroxenite source of Hawaiian lavas, then Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 22 of 30 increasing volcano volume should correlate with a trend of decreasing radiogenic Pb toward Kīlauea. It does not. Therefore, other factors besides the proportion of pyroxenite in the source are influ- encing the increase in volume flux (e.g., temper- ature, plume migration or oscillation in plume flux, or plate stresses). 4. The presence of Loa magmatism at times of the highest and lowest volume flux in the Hawaiian Islands suggests Loa‐type volcanism may not be independently related to increase in volume flux or entrained mafic component in the Hawaiian plume (although further work along the older part of the Hawaiian Ridge (>6 Ma) is necessary to test this relationship). Appendix A: Sample Preparation and Analytical Methods A1. University of Wisconsin 40Ar/39Ar Dating Method [41] The 40Ar/39Ar incremental heating experi- ments were undertaken on eleven West Ka‘ena lava lavas. Holocrystalline groundmass separates were prepared from porphyritic lava samples by crushing, sieving to 250–350 mm, magnetic sorting, density separation using methylene iodide, and ultimately hand picking under a binocular microscope. Purified groundmass separates were weighed and then wrapped in 99.99% copper foil packets placed into Al disks with the 28.201 Ma Fish Canyon sanidine [Kuiper et al., 2008], which monitors neutron fluence. The Al disks were irra- diated for 3 h at the Oregon State University TRIGA reactor in the Cadmium‐Lined In‐Core Irradiation Tube (CLICIT) where they a received fast neutron dose of ∼1.5 × 1015 n/cm2. Based on previous experiments, corrections for undesirable nucleogenic reactions on 40K and 40Ca are: [40Ar/39Ar]K = 0.00086; [ 36Ar/37Ar]Ca = 0.000264; [39Ar/37Ar]Ca = 0.000673. J values were uniform within analytical error across individual Al disks, and the precision of the J values averaged ±0.10% (2s). The age uncertainty determined for each sample is the 2s analytical error, unless otherwise noted. [42] At the University of Wisconsin Rare Gas Geochronology Laboratory, ∼20–25 mg of ground- mass was placed in a 3 mm × 20 mm copper trough and incrementally heated using a 25 Watt CO2 laser. Prior to each incremental heating experiment, samples were degassed at 2% power to potentially remove large amounts of water and atmospheric argon. Fully automated experiments consisted of 8–10 steps; each step included a scan across the trough at 150 mm/s at a given laser power, followed by an additional 15 min for gas cleanup. The gas was cleaned during and after the heating period with two SAES C50 getters. Argon isotope anal- yses were done using a MAP 215–50, and the isotope data was reduced using ArArCalc software version 2.4 [Koppers, 2002]. [43] Obtaining precise 40Ar/39Ar age determina- tions for Pleistocene volcanics requires careful characterization of blank levels in the analytical system and mass discrimination of the instrument. Blanks were measured after every two steps during the incremental heating experiment and interpo- lated. Mass discrimination was monitored daily via an automated air pipette and averaged 1.0000 ± 0.04% per atomic mass unit (a.m.u.) during the analytical period. Replicate experiments were per- formed on several samples to check for accuracy and improve precision. Isochron regressions [York, 1969] agree with plateau ages and do not reveal evidence that excess argon is present in any of the lavas, therefore, we consider the plateau ages to give the best estimate of the time elapsed since eruption (Table 1). All ages were calculated using the decay constants of Steiger and Jäger [1977]. For each analysis the uncertainties include esti- mates of the analytical precision on peak signals, the system blank, spectrometer mass discrimina- tion, and reactor corrections. Criteria used to determine whether an incremental heating experi- ment gave meaningful results and to calculate plateau and isochron ages were (1) plateaus are defined by at least four contiguous steps all con- cordant in age at the 95% confidence level and comprising >50% of the 39Ar released, (2) a well‐ defined isochron exists for the plateau points as defined by the F variate statistic SUMS/(N‐2) [York, 1969], (3) the plateau and isochron ages are concordant at the 95% confidence level, and (4) the 40Ar/36Ar intercept for the isochron does not differ from the atmospheric value of 295.5 at the 95% confidence level. A2. Kyoto University Unspiked K‐Ar Dating [44] Six of the freshest West Ka‘ena lavas samples were selected for unspiked K‐Ar dating, the pre- ferred method for dating young samples with high atmospheric contamination [Matsumoto et al., 1989]. K‐Ar dating was done at Kyoto University Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 23 of 30 starting with 80–100 g of rock crushed in a stain- less steel pestle. Samples were sieved to 180– 250 mm, and washed with deionized water and then acetone in an ultrasonic bath. Phenocrysts and xenoliths were carefully removed from all samples using a Frantz isodynamic separator to minimize the presence of extraneous argon. Argon isotope ratios were measured using a VG Isotech© VG3600 mass spectrometer operated in the static mode, connected to extraction and purification lines. Sensitivity of the mass spectrometer was determined by analyzing a known amount of the air standard, whichwas generally ∼1.2 × 107 V/cm3STP. Mass discrimination in the mass spectrometer was corrected assuming 40Ar/36Ar and 38Ar/36Ar of the air standard to be 295.5 and 0.1869, respectively [Matsumoto and Kobayashi, 1995]. The initial 40Ar/36Ar is calculated from measured 38Ar/36Ar assuming mass‐dependent isotopic fractionation during rock formation. The air standard was ana- lyzed with every sample and a hot blank was measured every five to ten samples. SORI93 biotite was used for calibration of the air standard. Blank levels were less than 1.7 × 10−8 cm3STP for mass 40. No peak drift was observed during analyses. Errors for 40Ar, 40Ar/36Ar and 38Ar/36Ar were estimated from multiple analyses of the air stan- dard, and were 2.0%, 0.2%–0.4% and 0.4%–0.8%, respectively. For measurement of potassium con- tent, a flame emission spectrometer Asahi Rika FP‐33D was used in a peak integration mode with a lithium internal standard. Analytical error for potassium measurement is ∼2%, estimated from standard deviation of multiple analyses of standard JB‐3 and JA‐2. See Ozawa et al. [2005] for addi- tional information on methods. A3. University of Massachusetts XRF Analytical Methods [45] Fifty‐two samples were selected for XRF analysis. These samples were unaltered or con- tained only minor amounts of iddingsite on olivine phenocrysts, and/or calcite, and zeolites in vesicles. For XRF analysis, rocks were broken into small (1–8 mm) fragments in a Rocklabs hydraulic piston crusher between WC plates. Fragments with signs of alteration were removed before powdering the sample. Rock chips were cleaned in a beaker with deionized water and dried in an oven at 70°C for 24 h to drive off excess water. Fragments were powdered in a tungsten carbide mill using a Rocklabs WC shatterbox for 1–3 min. [46] All XRF analyses were performed at the University of Massachusetts XRF Laboratory where whole‐rock, major element abundances were measured using the procedures of Rhodes and Vollinger [2004]. One sigma accuracy and preci- sion estimates for the XRF data are ∼0.5% for major elements [Rhodes, 1996]. Loss on ignition (LOI) analysis represents a measurement of alter- ation from volatile loss. Five grams of a powdered sample were heated in a muffle furnace at 1020°C for 10 min to limit the amount of ferrous iron formation [Rhodes and Vollinger, 2004]. The weight loss is LOI. Major elements were measured on a fused La‐bearing lithium borate glass disc using a Siemens MRS‐400 spectrometer with a Rh X‐ray tube operating at 2700 W. Trace element concentrations (Rb, Sr, Ba, Ce, Nb, Zr, Y, Pb, Zn, Ga, Ni, Cr, V) were measured on a separate powder pellet using a Philips PW2400 sequential spec- trometer with a Rh X‐ray tube. Precision and accuracy estimates for the trace element data are described by Rhodes [1996] and Rhodes and Vollinger [2004]. Results for each major element analysis are the average of two separate analyses. A4. PCIGR Trace Element and Isotopic Analytical Methods [47] A subset of twenty‐four samples was selected for high‐precision trace element analysis and Sr, Nd, Pb, and Hf isotopic analysis at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia (UBC; Tables 2 and 3). Samples were selected from the 52 samples analyzed by XRF, based on major and trace element chemistry, alteration (low LOI), and sample location. These samples were powdered in a planetary mill using agate jars and balls that were cleaned with quartz sand between samples. [48] Samples were prepared for trace element analysis at the PCIGR by the technique described by Pretorius et al. [2006] on unleached rock powders. Sample powders (∼100 mg) were weighed in 7 mL screw‐top Savillex® beakers and dissolved in 1 mL ∼14N HNO3 and 5 mL 48% HF on a hotplate for 48 h at 130°C with periodic ul- trasonication. Samples were dried and redissolved in 6 mL 6N HCl on a hotplate for 24 h and then dried and redissolved in 1 mL concentrated HNO3 for 24 h before final drying. Trace element abun- dances were measured with a Thermo Finnigan Element2 High Resolution–Inductively Coupled Plasma–Mass Spectrometer (HR‐ICP‐MS) follow- Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 24 of 30 ing the procedures described by Pretorius et al. [2006] within 24 h of redissolution. High field strength elements (HFSE) and large ion lithophile elements (LILE) were measured in medium reso- lution mode at 2000x dilution using a PFA Teflon spray chamber washed with Aqua Regia for 3 min between samples. Rare earth elements (REE) were measured in high‐resolution mode, and U and Pb in low‐resolution mode, at 2000x dilution using a glass spray chamber washed with 2% HNO3 between samples. Total procedural blanks and reference material (Kil93) was analyzed with the batch of samples. Indium was used as an internal standard in all samples and standard solutions. Background and standard solutions were analyzed after every 5 samples to detect memory effects and mass drift. [49] Sample digestion for purification of Sr, Nd, Hf, and Pb for column chemistry involved weigh- ing each sample powder. All samples were initially leached with 6N HCl and placed in an ultrasonic bath for 15 min. Samples were rinsed two times with 18 mega W cm H2O between each leaching step (15 total) until the supernatant was clear (fol- lowing the technique described in detail by Nobre Silva et al. [2009]. Samples were then dried on a hotplate for 24 h and weighed again. Sample solutions were then prepared by dissolving ∼100– 250 mg of the leached powder dissolved in 1 mL ∼14N HNO3 and 10 mL 48% HF on a hotplate for 48 h at 130°C with periodic ultrasonication. Sam- ples were dried and redissolved in 6 mL 6N HCl on a hotplate for 24 h and then dried. Pb was separated using anion exchange columns and the discard was used for Sr, REE, and Hf separation. Nd was separated from the REE and Hf required two additional purification steps. Detailed procedures for column chemistry for separating Sr, Nd, and Pb at the PCIGR are described by Weis et al. [2006] and Hf purification is described by Weis et al. [2007]. Sr and Nd isotope ratios were measured on a Thermo Finnigan Triton Thermal Ionization Mass Spectrometer (TIMS) in static mode with relay matrix rotation on a single Ta and double Re‐Ta filament, respectively. Four to 5 filaments per barrel of 21 were occupied by standards (NIST SRM 987 for Sr and Rennes for Nd) for each barrel where samples were run. Sample Sr and Nd isotopic compositions were corrected for mass fractiona- tion using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Each sample was then normalized using the barrel average of the reference material rela- tive to the values of 143Nd/144Nd = 0.511858 and 87Sr/86Sr = 0.710248 [Weis et al., 2006]. During the period when the West Ka‘ena samples were analyzed, the Rennes standard gave an average value of 0.511957 ± 26 (n = 18) and NIST SRM 987 standard gave an average of 0.710260 ± 5 (n = 6; 2s error is reported as times 106). 147Sm/144Nd ratio errors are approximately ∼1.5%, or ∼0.006. PCIGR internal reference material from the Pu‘u ‘Ō‘ō eruption of Kīlauea Volcano on 8 July 1993 (Kil93) was processed with the samples and yiel- ded Sr and Nd isotopic ratios of 0.703594 ± 7 and 0.512974 ± 4, respectively. PCIGR internal refer- ence material Ko‘olau (base of the Makapu’u sec- tion) was also processed with the samples and yielded Sr and Nd isotopic ratios of 0.704086 ± 7 and 0.512760 ± 6, respectively. In both case, the results are within analytical uncertainty of the accepted values (see I. G. Nobre Silva et al., Constraining the origin of the Ninetyeast Ridge, Indian Ocean and its relation with the Kerguelen, Amsterdam and St. Paul hotspots—New insights from new high‐precision Pb‐Sr‐Nd‐Hf isotopes, manuscript in preparation, 2010). [50] Pb and Hf isotopic compositions were ana- lyzed by static multicollection on a Nu Plasma (Nu Instruments) Multiple Collector–Inductively Cou- pled Plasma–Mass Spectrometer (MC‐ICP‐MS). The detailed analytical procedure for Pb isotopic analyses on the Nu at the PCIGR is described by Weis et al. [2005]. The configuration for Pb anal- yses allows for collection of Pb, Tl, and Hg together. Tl and Hg are used to monitor instru- mental mass discrimination and isobaric overlap, respectively. All sample solutions were analyzed with approximately the same Pb/Tl ratio (∼4) as the reference material NIST SRM 981. To accomplish this, a small aliquot of each sample solution from the Pb columns was analyzed on the Element2 to determine the precise amount of Pb available for analysis on the Nu Plasma. The SRM 981 standard was run after every two samples on the Nu Plasma. During the time samples were run, analyses of the SRM 981 Pb reference material gave values of 206Pb/204Pb = 16.9422 ± 22, 207Pb/204Pb = 15.4985 ± 19, and 208Pb/204Pb = 36.7194 ± 44 (n = 38; 2s error is reported as times 104); these values are in excellent agreement with reported TIMS triple‐ spike values of Galer and Abouchami [1998]. Results were further corrected by the sample‐ standard bracketing method or the ln‐ln correction method described by White et al. [2000]. Refer- ence material Kil93 yielded Pb isotopic ratios of 206Pb/204Pb= 18.4143 ± 8, 207Pb/204Pb = 15.4775 ± 7, and 208Pb/204Pb = 38.0697 ± 18. Ko‘olau reference material yielded Pb isotopic ratios of 206Pb/204Pb = Geochemistry Geophysics Geosystems G3 GREENE ET AL.: VOLCANISM BETWEEN KAUA‘I AND O‘AHU 10.1029/2010GC003233 25 of 30 17.8375 ± 7, 207Pb/204Pb = 15.4339 ± 7, and 208Pb/204Pb = 37.7358 ± 16. Both sets of values are in agreement with PCIGR averages [see Nobre Silva et al., 2009, also manuscript in preparation, 2010]. [51] Hf isotopic compositions were analyzed fol- lowing the procedures detailed by Weis et al. [2007]. The configuration for Hf analyses moni- tored Lu mass 175 and Yb mass 172 to allow for interference correction to masses 174 and 176. Hf isotopic ratios were normalized internally for mass fractionation to a 179Hf/177Hf ratio of 0.7325 using an exponential correction. Standards were run after every two samples and sample results were nor- malized to the ratio of the in‐run daily average and a 176Hf/177Hf ratio for JMC‐475 of 0.282160. During the course of analyses, the Hf standard JMC‐475 gave an average value 0.282161 ± 12 (n = 31). Reference materials Kil93 and Ko‘olau were processed with the samples and yielded Hf isotopic ratios of 0.283098 ± 5 and 0.282953 ± 5, respectively, and in agreement with PCIGR avera- ges (see Nobre‐Silva et al., manuscript in prepara- tion, 2010). A5. Electron Microprobe Analyses of Olivine [52] Olivine phenocrysts were measured for the oxides of Si, Fe, Mg, Ca, Ni, and Mn with a JEOL Hyperprobe JXA‐8500F at the University of Hawai‘i at Mānoa. A minimum of 15 olivine grains were measured in each of 5 selected samples, with 4 analyses on each olivine grain (3 core, 1 rim to check for compositional zoning). The analytical conditions were 20 kV accelerating voltage with long counting times (60 s) for each element and high current of (200 nA) to obtain low detection limits and higher precision (0.01 wt %) following the procedures of Sobolev et al. [2007]. San Carlos olivine standard was analyzed as an unknown to monitor accuracy of olivine measurements. Oxide concentrations of all the olivine measurements and the San Carlos olivine standard are presented in the auxiliary material. Acknowledgments [53] We would like to thank the science team participants of the Northern Hawaiian Islands Jason2 Expedition in 2007 for contributing to this research. We are also grateful to the Jason2 team from Woods Hole Oceanographic Institute and the crew of the University of Hawai‘i research vessel Kilo Moana. Insights and advice from James Scoates are greatly appreciated. We are grateful to Brian Jicha and Brad Singer for help with Ar‐Ar ages. Thanks to Shichun Huang for pro- viding compiled data for Hawaiian shields, Jane Barling and Bruno Kieffer for assistance with isotope analyses, Claude Maerschalk for sample preparation, Michael Vollinger for XRF analyses, and Eric Hellebrand for help with electron microprobe analyses. We appreciate reviews by Ryoji Tanaka and an anonymous reviewer. 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