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East Molokai and other Kea-trend volcanoes: Magmatic processes and sources as they migrate away from.. Weis, Dominique 2005

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East Molokai and other Kea-trend volcanoes: Magmatic processes and sources as they migrate away from the Hawaiian hot spot Guangping Xu and Frederick A. Frey Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Room 54-1226, Cambridge, Massachusetts 02139, USA (gpxu@mit.edu) David A. Clague Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USA Dominique Weis Pacific Centre for Isotopic and Geochemical Research, Department of Earth Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Melvin H. Beeson U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, California 94025, USA [1] There are geochemical differences between shield lavas from the two parallel trends, Kea and Loa, defined by young Hawaiian volcanoes. The shield of East Molokai volcano, at greater than 1.5 Ma, is the oldest volcano on the Kea trend. Sequences of older tholeiitic to younger alkalic basalt that erupted as this volcano evolved from the shield to postshield stage of volcanism are well exposed. Much younger, 0.34–0.57 Ma, alkalic basalt and basanite erupted during rejuvenated stage volcanism. Like rejuvenated stage lavas erupted at other Hawaiian volcanoes, rejuvenated stage East Molokai lavas have relatively low 87Sr/86Sr and high 143Nd/144Nd. Such ratios reflect a source component with a long-term depletion in abundance of incompatible elements. On the basis of positive correlations of 87Sr/86Sr versus 206Pb/204Pb and negative correlations of these isotopic ratios with Nb/Zr, a smaller proportion of this depleted component also contributed to the late shield/postshield lavas erupted at EastMolokai and the otherKea-trend volcanoes, Haleakala andMaunaKea.At each of theseKea-trend volcanoes, as the volcanomoved away from the hot spot, the extent ofmelting andmagma supply from themantle decreased, the depth ofmelt segregation increased, and there was an increasing role for a component with long-term relative depletion in incompatible elements. This depleted component hasKea-trend Pb isotopic characteristics and relatively low 208Pb/204Pb at a given 206Pb/204Pb, and it is probably not related to oceanic lithosphere or the source of mid-ocean ridge basalt. The overlap in Sr, Nd, and Pb isotope ratios of recent Kilauea shield lavas and 550 kaMaunaKea shield lavas has been used to argue that Kea-trend shield volcanism samples a vertically continuous, geochemically distinct stripe which persisted in the hot spot source for 550 kyr (Eisele et al., 2003; Abouchami et al., 2005). As Kea-trend volcanoes migrate away from the hot spot and evolve from the shield to postshield stage, there are systematic changes in Sr, Nd, and Pb isotope ratios. However, the overlap of Sr, Nd, and Pb isotope ratios in late shield/postshield lavas from Mauna Kea (<350 ka) and East Molokai (1.5 Ma) show that the periphery of the hot spot sampled by Kea-trend postshield lavas also had long-term geochemical homogeneity. Components: 13,340 words, 16 figures, 4 tables. Keywords: East Molokai; Hawaii plume; postshield stage; rejuvenated stage; Sr, Nd, Pb isotope; trace elements. Index Terms: 1037 Geochemistry: Magma genesis and partial melting (3619); 1038 Geochemistry: Mantle processes (3621); 1025 Geochemistry: Composition of the mantle; 1065 Geochemistry: Major and trace element geochemistry; 1040 Geochemistry: Radiogenic isotope geochemistry. G3GeochemistryGeophysicsGeosystems Published by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Article Volume 6, Number 5 28 May 2005 Q05008, doi:10.1029/2004GC000830 ISSN: 1525-2027 Copyright 2005 by the American Geophysical Union 1 of 28Received 12 August 2004; Revised 24 February 2005; Accepted 31 March 2005; Published 28 May 2005. Xu, G., F. A. Frey, D. A. Clague, D. Weis, and M. H. Beeson (2005), East Molokai and other Kea-trend volcanoes: Magmatic processes and sources as they migrate away from the Hawaiian hot spot, Geochem. Geophys. Geosyst., 6, Q05008, doi:10.1029/2004GC000830. 1. Introduction [2] The geochemical characteristics of lavas forming Hawaiian volcanoes reflect the source components associated with the Hawaiian plume, the melting processes and the postmelting pro- cesses that occur during magma ascent. Two major observations are as follows: (1) The geo- chemistry of erupted lavas systematically changes as individual volcanoes evolve through a series of growth stages (i.e., preshield, shield, post- shield and rejuvenated) that are associated with volcano growth as the lithosphere approaches, overrides and recedes from the hot spot [e.g., Chen and Frey, 1985]; (2) recent Hawaiian volcanoes define two subparallel echelon trends, the Kea and Loa trends (Figure 1) [Jackson et al., 1972; Clague and Dalrymple, 1987], whose lavas are generally geochemically distinct [e.g., Tatsumoto, 1978; Lassiter et al., 1996; Abouchami et al., 2005], thereby indicating a large-scale spatial arrangement of different source components. [3] There are good exposures of the shield to postshield transition at the recent (<1.5 Ma) Kea volcanoes, Mauna Kea, Kohala, Haleakala and West Maui. In this paper we characterize the shield to postshield transition on the oldest Kea- trend volcano, East Molokai (>1.5 Ma [McDougall, 1964; Naughton et al., 1980]) (Figure 1). We use major and trace element abundances and Sr, Nd and Pb isotopic ratios for lavas erupted at the end of shield building, that is, during the transition from tholeiitic to alkalic volcanism, and for younger rejuvenated stage basalt, including submarine lavas, to under- stand the final growth stages of East Molokai volcano. We compare the temporal changes in isotopic ratios and correlated trace element abun- dance ratios in these East Molokai lavas with analogous lava sequences at Mauna Kea, Kohala, Haleakala and West Maui. Our overall objective is to understand the processes, such as partial melting and fractionation crystallization, and magma sources that are important as a Kea volcano moves away from the hot spot. 2. Geological Setting [4] The island of Molokai consists of two coa- lesced volcanoes. West Molokai Volcano rises only 421 m above sea level and has not been deeply eroded [Stearns and Macdonald, 1947]. East Molokai Volcano, which rises 1515 m above sea level, forms the eastern two-thirds of the island (Figure 1). The north coast of East Molokai is a series of spectacular sea cliffs, locally more than 915 m high. The sea cliffs, and the large deep valleys of the north coast, provide excellent expo- sure of subaerially erupted lava flows. Beeson [1976] and Clague and Beeson [1980] studied the Kalaupapa section; this thick sequence of interbed- ded tholeiitic, transitional and alkalic lavas is dominated by alkalic lavas near the top of the section which range in age from 1.50 Ma to 1.75 Ma [McDougall, 1964; Naughton et al., 1980]. Rejuvenated stage lavas at East Molokai are exposed at Kalaupapa Peninsula on the north coast where alkalic olivine basalt erupted from a small lava shield surmounted by a deep summit crater (Figure 1). These lavas with an eruption age of about 0.340.57 Ma [Clague et al., 1982] and a lack of significant marine erosion, indicate that the origin of the East Molokai sea cliffs was by landsliding rather than erosion [Holcomb, 1985]. The Wailau landslide deposits are a result (Figure 1). Submarine erupted rejuvenated stage lavas have also been recovered by submersible dives from a terrace on the north slope (Figure 1) [Clague and Moore, 2002]. 3. Samples Studied [5] In addition to 28 samples from the Kalaupapa section (including 26 samples from Beeson, 1976), samples from the Halawa Valley (22), Pelekunu Valley (7), Waikolu Valley (7), a water tunnel section (16) and two alkalic lavas (one from east coast near Mokuhooniki and the other from near a Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 2 of 28gaging station) were studied (Table 1, Figure 1). In the more intensely sampled sections of Kalau- papa and Halawa Valley the lava compositions range from tholeiitic to transitional basalt for the oldest samples to alkalic basalt, and hawaiite/ mugearite for the youngest samples (Figures 2a and 2b). Transitional lavas are defined by their proximity to the tholeiitic basalt-alkalic basalt boundary in a SiO2 versus total alkalis classifi- cation plot (Figure 3). These East Molokai lavas erupted during the evolution from late shield growth to the postshield stage. Most of the samples from the other sections (Pelekuna Valley, Waikolu Valley and water tunnel) are tholeiitic to transitional basalt; therefore they may be slightly older sections than those from Kalaupapa section and Halawa Valley. Although we cannot rigor- ously assign our samples to the shield or post- shield stages, the important point is that intercalated tholeiitic and alkalic basalt is char- acteristic of late shield and early postshield growth of Hawaiian volcanoes [e.g., Clague and Dalrymple, 1987]. In contrast, there is no ambiguity in assigning eleven submarine and subaerial samples to the rejuvenated stage [Clague et al., 1982; Clague and Moore, 2002] (Table 1). 4. Petrography [6] Most of the lavas collected from the Kalau- papa section are porphyritic except for a few in the upper part of the section. The porphyritic lavas commonly contain 20 to 30 vol% pheno- crysts of olivine, clinopyroxene and plagioclase. Olivine phenocrysts are the most abundant in the lower part of the Kalaupapa section. Augite phenocrysts are usually only about a third as abundant as olivine phenocrysts. The plagioclase phenocryst content is less than 10 vol% except for two lavas (69KLPA-28 and 69KLPA-29) Figure 1. Map of Molokai Island showing locations of studied sections [after Beeson, 1976] and location of East Molokai Volcano on the Kea trend of recent Hawaiian volcanoes. The locations for submarine rejuvenated stage lavas, 15 km northeast of Kalaupapa Peninsula [Clague and Moore, 2002], and Wailau landslide lavas, 50 km north of Kalaupapa Peninsula [Tanaka et al., 2002], are not to scale. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 3 of 28from the upper section that have abundant pla- gioclase phenocrysts (20 vol%) [Beeson, 1976]. Lavas from the Halawa Valley section have a lower olivine/plagioclase ratio and are like the upper part of Kalaupapa section in that respect. Tholeiitic basalt from Waikolu Valley contains almost no olivine, either as phenocrysts or in the groundmass. Lavas from Pelekunu Valley and water tunnel section are aphyric except that 71PELE-37 and M4.6+150F have olivine as the dominant phenocryst and clinopyroxene is more abundant than plagioclase. [7] Five rejuvenated stage lavas from the Kalau- papa Peninsula contain abundant olivine pheno- crysts (up to 16 vol%) with lesser amounts of plagioclase and clinopyroxene phenocrysts and microphenocrysts [Clague et al., 1982]. Subma- rine rejuvenated stage lavas recovered by Pisces V are olivine phyric with up to 20 vol% olivine phenocrysts [Clague and Moore, 2002]. 5. Analytical Techniques [8] Major element contents were analyzed over several years by different methods in four labora- tories (see Table 2). Trace element abundances (Table 3) were determined at MIT by inductively coupled plasma mass spectrometry (ICP-MS) using a Fisons VG Plasmaquad 2+S with both internal and external drift monitors. Trace element results are reported as the mean of duplicate analyses (usually within ±5%). The chemical procedures and estimates of accuracy and precision were discussed by Huang and Frey [2003]. The subma- rine rejuvenated stage lavas were previously ana- lyzed for trace elements [Clague and Moore, Figure 2. Alkalinity versus stratigraphic position for (a) Kalaupapa section and (b) 71HALW section showing the temporal variation of composition ranging from tholeiitic for the oldest samples to alkalic lavas for the youngest samples. Alkalinity is defined as the vertical deviation from the alkalic-tholeiitic dividing line of Macdonald and Katsura [1964]; see Figure 3. Open symbols are for hawaiite and mugearite using the classification scheme of Figure 3. (c) 87Sr/86Sr and (d) 143Nd/144Nd versus stratigraphic position for Kalaupapa section. From the bottom to the top, 87Sr/86Sr decreases and 143Nd/144Nd increases. The error bars are the 2s calculated from themean of 42 analyses of NBS 987 Sr standard and 18 analyses of La Jolla Nd standard. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 4 of 282002], but were reanalyzed at MIT for internal consistency. A subset of samples had abundances of Sc, Co and Cr determined by instrumental neutron activation analysis (INAA) following the procedures of Ila and Frey [1984, 2000]. Because only a subset of samples was analyzed by INAA, we use the ICP-MS data for Sc in the figures. Sc values determined by the two techniques generally agree within ±5%. [9] Samples for Sr, Nd and Pb isotopic analyses were selected to encompass the entire range of compositions (Table 4). The chemical procedures used are similar to those described by Weis and Frey [1996]. Powder, 150–200 mg, was weighed into a 15 mL Telfon beaker and leached repeatedly with 6.0 N HCl in an ultrasonic bath for approx- imately 15-minute intervals. The leachate and any suspended solids were pipetted off and discarded, and this process was repeated until the resultant leachate was clear (typically four to five washes). The leached powder was rinsed twice with Mill-Q H2O before drying on a hot plate. Samples were then digested for a minimum of 48 hours using concentrated HF and a few drops of 6.0 N HNO3. After verification of complete dissolution, samples were evaporated and the residual cake was dissolved for 12 hours in 6.0 N HCl. Sr and Nd were run on thermal ionization mass spectrometer (Triton) on single Ta filament and triple Re-Ta filaments, re- spectively, and Pb was run on an MC-ICP-MS (Nu021) at University of British Columbia. See Table 4 footnotes for normalization procedures, precision estimates and data for standards. 6. Results 6.1. Major Elements [10] Whole rock compositions are given in Table 2. Volatile free compositions were determined by recalculation of total iron as Fe2O3, subtracting H2O and CO2, and then normalizing to 100 per- cent. These compositions are used in all diagrams involving major elements. Figure 3. Na2O + K2O versus SiO2 classification plot showing that the East Molokai late shield/postshield lavas range from tholeiitic (open symbols) to alkalic basalt (solid symbols) and hawaiite, mugearite, and benmoreite (labeled). Classification fields are from LeMaitre [1991]; the alkalic-tholeiitic dividing line is from Macdonald and Katsura [1964]. Major element data were adjusted to a Fe3+/(Fe2+ + Fe3+) molar ratio of 0.10. Due to K2O mobility, the K2O/P2O5 ratio ranges widely (0.36 to 2.0, except for a benmoreite dike with 2.8) in these late shield/postshield lavas. Therefore the K2O contents of late shield/postshield lavas are corrected to a K2O/P2O5 ratio of 1.71, a typical ratio for fresh Hawaiian lavas [Rhodes, 1995; Garcia et al., 2000]. Benmoreite dike (M4.9+300D) has a high K2O/ P2O5 ratio (2.8) and was not adjusted for potassium loss. Rejuvenated stage lavas did not experience significant potassium loss (K2O/P2O5 > 1.3) and were not adjusted for possible K2O loss. Wailau landslide samples (Figure 1) are tholeiitic basalts derived from the East Molokai shield that were analyzed for Sr, Nd, and Pb isotope ratios by Tanaka et al. [2002]. These samples are not corrected for potassium loss (K2O/P2O5 > 1.1). Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 5 of 28Ta bl e 1. Lo ca tio ns o fE as tM ol ok ai La te Sh ie ld /P os tsh ie ld an d R eju ve na te d St ag e La va s Sa m pl e N um be ro f Sa m pl es Lo ca tio n A lk al in ity Er up tio n En vi ro nm en t R ef er en ce La te Sh ie ld to Po sts hi el d La va s 71 PE LE - 7 Pe le ku nu Va lle y th ol ei iti c to al ka lic su ba er ial th is st ud y 70 W A IK -a n d 71 W A IK - 7 W ai ko lu Va lle y th ol ei iti c to al ka lic su ba er ial th is st ud y M -a n d N E Po rta la 16 W at er Tu n n el th ol ei iti c to al ka lic su ba er ial th is st ud y 71 K PA - 2 K al au pa pa th ol ei iti c to al ka lic su ba er ial th is st ud y 69 K LP A - 26 K al au pa pa th ol ei iti c to al ka lic su ba er ial Be es on [19 76 ] 70 H A LW - an d 71 H A LW - 22 H al aw a Va lle y th ol ei iti c to al ka lic su ba er ial th is st ud y 70 M O L- 1C 1 ea st co as t n ea r M ok uh oo ni ki al ka lic su ba er ial th is st ud y 70 K AW E- 1C 1 n ea r ga gi ng st at io n al ka lic su ba er ial th is st ud y Re juv en ate dS ta ge La va s 74 K A L- 1 1 K al au pa pa Pe ni ns ul a al ka lic su ba er ial Cl ag ue et a l. [19 82 ] 80 K A L- 1 1 K au ha ko Cr at er in K al au pa pa Pe ni ns ul a al ka lic su ba er ial Cl ag ue et a l. [19 82 ] 71 K A U H -1 1 50 0 m to th e n o rth o fK au ha ko Cr at er al ka lic su ba er ial Cl ag ue et a l. [19 82 ] 71 K A U H -2 1 50 0 m to th e n o rth o fK au ha ko Cr at er al ka lic su ba er ial Cl ag ue et a l. [19 82 ] M O E2 1 K au ha ko Cr at er in K al au pa pa Pe ni ns ul a al ka lic su ba er ial Na ug ht on et a l. [19 80 ] P2 52 -2 1 n o rth fla nk o fK al au pa pa Pe ni ns ul a al ka lic su ba er ial Cl ag ue a n d M oo re [20 02 ] P2 53 - 4 n o rth su bm ar in e slo pe o fM ol ok ai Is la nd al ka lic su bm ar in e Cl ag ue a n d M oo re [20 02 ] a W at er Tu n n el sa m pl es w er e co lle ct ed fro m SW Po rta lt o N E Po rta l. D ist an ce s fro m SW Po rta la re M 1. 3F - 1. 96 km ,M 1. 5F - 2. 26 km ,M 2. 5F - 3. 76 km ,M 3. 3F - 4. 96 km ,M 3. 9F - 5. 87 km ,M 4. 1F - 6. 17 km ,M 4. 3F an d M 4. 3+ 1F - 6. 47 km ,M 4. 6F - 6. 92 km ,M 4. 6+ 15 0F - 6. 97 km ,M 4. 9+ 30 0D (di ke )- 7. 46 km ,M 5. 1+ 10 F - 7. 68 km ,M 5. 3+ 40 F - 7. 99 km ,M 5. 3+ 37 5F - 8. 09 km ,M 5. 4+ 50 F - 8. 14 km , N E Po rta l- 8. 17 km . Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 6 of 28Ta bl e 2 (R ep res en tat ive Sa m pl e). M ajo rE le m en tC on te nt s fo rE as tM ol ok ai La va sa [T he fu ll Ta bl e 2 is av ai la bl e in th e H TM L v er sio n o ft hi s ar tic le at ht tp :// w w w . g- cu be d. or g. ] Pe le ku nu Va lle y (L ate Sh ie ld /P os tsh iel d) W ai ko lu Va lle y (L at e Sh ie ld /P os tsh iel d) 71 PE LE -7 7 71 PE LE -7 8 71 PE LE -3 7 71 PE LE -4 2F 71 PE LE -1 9 71 PE LE -2 0 71 PE LE -2 1 71 W A IK -1 4F 71 W A IK -1 0F 71 W A IK -1 70 W A IK -7 70 W A IK -6 70 W A IK -5 PB B B B B B B B B B B B B Si O 2 45 .6 0 47 .1 0 45 .6 0 50 .5 0 49 .7 0 49 .6 0 49 .6 0 48 .9 0 49 .9 0 47 .2 0 48 .3 0 49 .5 0 46 .9 0 Ti O 2 2. 68 4. 09 1. 88 3. 10 3. 00 2. 83 3. 09 2. 93 2. 92 3. 25 3. 48 2. 89 2. 97 A l 2O 3 14 .8 0 15 .1 0 9. 26 13 .2 0 13 .7 0 13 .9 0 13 .8 0 14 .2 0 13 .5 0 13 .5 0 16 .2 0 14 .0 0 12 .3 0 Fe 2O 3 7. 41 5. 46 2. 96 3. 08 3. 70 3. 59 4. 82 3. 09 4. 35 4. 74 3. 50 2. 96 2. 49 Fe O 5. 80 7. 77 10 .2 0 8. 66 8. 91 8. 74 8. 26 9. 37 8. 41 8. 51 8. 19 9. 21 10 .0 0 M nO 0. 16 0. 19 0. 18 0. 21 0. 18 0. 20 0. 20 0. 18 0. 19 0. 18 0. 16 0. 18 0. 18 M gO 8. 81 4. 96 17 .3 0 5. 80 6. 43 6. 49 6. 18 6. 72 6. 01 7. 21 4. 57 6. 69 10 .3 0 Ca O 10 .7 0 9. 83 9. 97 10 .6 0 10 .8 0 10 .8 0 10 .5 0 11 . 30 10 .4 0 10 .4 0 10 .4 0 11 . 10 10 .8 0 N a 2 O 2. 55 3. 53 1. 65 2. 30 2. 38 2. 20 2. 43 2. 27 2. 54 2. 61 3. 30 2. 31 2. 53 K 2O 0. 60 1. 17 0. 36 0. 55 0. 25 0. 24 0. 17 0. 27 0. 37 0. 34 0. 93 0. 37 0. 69 P 2 O 5 0. 36 0. 64 0. 23 0. 33 0. 31 0. 30 0. 34 0. 33 0. 30 0. 44 0. 54 0. 32 0. 42 H 2O + 0. 52 0. 23 0. 31 0. 77 0. 40 0. 66 0. 46 0. 48 0. 42 0. 68 0. 29 0. 47 0. 28 H 2O  0. 31 0. 06 0. 19 0. 69 0. 38 0. 64 0. 42 0. 27 0. 67 0. 65 0. 10 0. 29 0. 04 CO 2 0. 12 0. 06 0. 05 0. 81 0. 14 0. 17 0. 07 0. 11 0. 06 0. 34 0. 07 0. 06 0. 12 LO I 0. 54 < 0. 01 < 0. 01 1. 15 0. 56 0. 97 0. 46 0. 24 0. 90 0. 80 0. 01 0. 25 < 0. 01 To ta l 10 0. 5 10 0. 2 10 0. 1 10 0. 6 10 0. 3 10 0. 4 10 0. 3 10 0. 4 10 0. 0 10 0. 1 10 0. 0 10 0. 4 10 0. 0 a M ajo re le m en tc o n te nt s ar e in w t% .B as ,b as an ite ;P B ,p ic ro -b as al t; B ,b as alt ;H ,h aw ai ite ;M ,M ug ea rit e; B en ,B en m o re ite .T he sa m pl es fro m th et hr ee v al le ys w er e co lle ct ed by M .H . B ee so n in 19 70 an d 19 71 , an d th os ef ro m in sid et he w at er tu nn el w er e co lle ct ed by J. G .M oo re in 19 66 .S am pl es ar e o rd er ed fro m th eo ld es tt o yo un ge st, ac co rd in g to th ei rs tra tig ra ph ic po sit io ns ,w ith in Pe le ku nu Va lle y, W ai ko lu Va lle y, W at er Tu n n el ,K al au pa pa se ct io n, 71 H A LW se ct io n, an d su bm ar in er eju v en at ed st ag el av as .M ajo re le m en tc o n te nt s fo rl av as ar e fro m K al au pa pa se ct io n, ex ce pt 71 K PA -1 an d 71 K PA - 2 w er e ta ke n fro m Be es o n [19 76 ].M ajo r el em en tc o n te nt s fo rs u ba er ia la n d su bm ar in e re juv en at ed st ag e la va sw er e ta ke n fro m Cl ag ue et a l. [19 82 ]a n d Cl ag ue a n d M oo re [20 02 ].A ll th e o th er m ajo r el em en td ata ar e fro m th is st ud y an d w er e an al yz ed by X R F w ith Fe O , H 2O + , H 2O  , an d CO 2 an al yz ed by cl as sic al w et ch em ic al te ch ni qu es at th e U . S. G eo lo gi ca lS ur v ey la bo ra to rie si n D en v er , Co lo ra do ,a n d M en lo Pa rk ,C al ifo rn ia , re sp ec tiv el y. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 7 of 28[11] The East Molokai lavas range widely in major element compositions (e.g., SiO2: 43–55 wt% and MgO: 2–18 wt%); they are dominantly alkalic and tholeiitic basalt, but some of the youngest lavas in the sections include hawaiite, mugearite and ben- moreite (Figures 2 and 3). Lavas from the Kalau- papa section and Halawa Valley are dominantly alkalic and lavas from Pelekuna Valley, Waikolu Valley and Water Tunnel are interbedded tholeiitic and alkalic basalt (Figure 3). Rejuvenated stage lavas range from alkalic basalt to basanite and picro-basalt (Figure 3). [12] With decreasing MgO content, the SiO2, Al2O3, Na2O and P2O5 contents of the late shield/postshield lavas systematically increase with steeper slopes for MgO less than 6.5 wt% (Figure 4). In contrast, the CaO and TiO2 versus MgO trends show marked inflections with their abundance decreasing significantly as MgO content decreases to less than 6 wt% (CaO) or 4 wt% (TiO2) (Figure 4). Qualitatively, such trends are consistent with established liquid lines of descent for Hawaiian shield lavas, that is, dominantly olivine fractionation at 7 wt% MgO with clinopyroxene becoming a fractionating phase at 7 wt% MgO and Fe-Ti oxides at 4 wt% MgO [e.g., Wright and Fiske, 1971; Clague et al., 1995]. [13] Relative to the late shield/postshield stage lavas, the rejuvenated stage lavas at a given MgO content have distinctly high CaO and low TiO2 and P2O5, except for two samples (71KAUH-1 and Table 3 (Representative Sample). Trace Element Contents for East Molokai Lavasa [The full Table 3 is available in the HTML version of this article at http://www.g-cubed.org.] Pelekunu Valley (Late Shield/Postshield) Waikolu Valley (Late Shield/ Postshield) Water Tunnel (Late Shield/Postshield) 71PELE -37 71PELE -19 71PELE -21 71WAIK -14F 71WAIK -10F 71WAIK-1 NE PORTAL M5.3+375 F M5.3+40F M5.1+10F M4.9+300D PB B B B B B B B B B dike/Ben Sc 30.0 32.1 31.5 30.3 33.0 28.6 28.7 30.7 24.7 26.1 4.18 Rb 7.45 3.20 2.10 3.47 5.58 3.88 9.79 5.83 6.95 13.3 48.7 Sr 319 358 349 396 311 498 475 419 353 469 1251 Y 29.1 33.5 35.3 32.6 36.2 34.6 37.3 33.3 27.8 37.1 46.4 Zr 118 187 191 190 170 227 244 187 162 231 546 Nb 12.4 14.8 14.4 15.1 12.7 21.2 19.5 14.5 12.9 20.0 85.2 Ba 112 80.8 89.1 85.9 72.4 181 150 110 102 165 784 La 15.0 12.8 12.5 13.1 11.3 18.2 17.9 13.2 11.8 18.2 67.1 Ce 27.0 31.8 32.0 34.0 29.0 45.2 45.4 34.7 29.5 45.4 146 Pr 4.82 5.01 5.13 5.21 4.54 6.72 6.67 5.31 4.47 6.71 18.2 Nd 22.2 23.8 24.2 24.4 21.8 30.5 31.5 25.0 21.2 30.2 72.5 Sm 5.38 6.41 6.71 6.47 6.05 7.59 8.11 6.84 5.58 7.90 14.2 Eu 1.91 2.20 2.31 2.21 2.07 2.50 2.63 2.27 1.85 2.57 4.31 Tb 0.860 1.10 1.17 1.08 1.13 1.17 1.28 1.12 0.92 1.23 1.66 Gd 5.78 6.82 7.10 6.79 6.70 7.61 8.06 7.05 5.74 7.92 11.55 Dy 4.66 6.11 6.44 5.83 6.33 6.34 6.97 6.11 5.04 6.73 8.56 Ho 0.872 1.16 1.23 1.11 1.23 1.17 1.32 1.16 0.96 1.26 1.54 Er 2.12 2.97 3.09 2.76 3.18 2.92 3.29 2.95 2.40 3.23 3.84 Tm 0.292 0.428 0.449 0.408 0.482 0.408 0.469 0.417 0.343 0.443 0.537 Yb 1.58 2.40 2.56 2.29 2.76 2.33 2.59 2.31 1.91 2.55 3.12 Lu 0.223 0.334 0.351 0.317 0.387 0.319 0.365 0.328 0.262 0.347 0.430 Hf 2.89 4.55 4.66 4.57 4.22 5.39 5.74 4.60 3.89 5.51 11.44 Ta 0.739 0.910 0.895 0.934 0.774 1.29 1.21 0.927 0.773 1.24 4.88 Pb 0.786 1.13 0.975 0.932 1.15 1.44 1.26 1.06 0.844 1.34 4.66 Th 0.805 0.880 0.913 0.946 0.770 1.35 1.23 0.931 0.770 1.30 5.92 U 0.295 0.264 0.283 0.251 0.242 0.273 0.399 0.269 0.957 0.990 0.982 Scb 30.3 31.3 31.6 31.6 27.8 28.6 30.2 25.4 26.5 4.1 Crb 1168 125 80 134 248 114 103 621 145 8.0 Cob 83.8 43.8 44.1 45.2 46.7 38.3 42.8 63.7 39.7 6.1 aTrace element contents are in ppm. bAnalyzed by INAA at MIT, and all the other trace element data were obtained by ICP-MS at MIT following the procedure of Huang and Frey [2003]. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 8 of 28MOE2) from Kauhako crater. These two crater samples have higher TiO2 and P2O5 and lower SiO2 than the other rejuvenated stage lavas (Figure 4). Also rejuvenated stage lavas from East Molokai have relatively higher Na2O and most also have higher Na2O/K2O ratios at a given MgO than late shield/postshield lavas when the latter are adjusted for K loss (Figures 4e and 4g). The wide range of Na2O/K2O ratios shown by the rejuvenated stage lavas is likely a magmatic feature since unal- tered rejuvenated stage glasses, occurring as silt-size grains in a turbidite collected as PistonCore 4, 40 km north of Molokai [Sherman and Garcia, 2002], range widely in Na2O/K2O at 6.3 wt% MgO (Figure 4g). As with Na2O/K2O, the TiO2 and P2O5 of rejuvenated stage glass grains in turbidite cores show a large range in TiO2 and P2O5 (Figures 4d and 4f). 6.2. Trace Elements [14] Like P2O5, the abundance of highly incom- patible trace elements, such as Nb, define a broad inverse trend with MgO content (Figure 5a). In order to define the behavior of incompatible trace elements during the petrogenesis of the shield/ Table 4. Sr, Nd, and Pb Isotopic Compositions for East Molokai Lavasa Sample 87Sr/86Sr 2s 143Nd/144Nd 2s 206Pb/204Pb 2s 207Pb/204Pb 2s 208Pb/204Pb 2s Late Shield/Postshield Lavas 71PELE-19 0.703548 7 0.513017 5 18.5320 63 15.4782 43 38.0378 131 71PELE-19b 18.5566 14 15.4834 13 38.0475 32 71PELE-21 0.703524 7 0.513029 10 18.5325 9 15.4815 8 38.0418 20 71PELE-21b 18.5300 15 15.4792 12 38.0300 37 71WAIK-14F 0.703527 9 0.513015 11 18.5412 43 15.4965 39 38.0761 107 71WAIK-10F 0.703542 8 0.513039 5 18.5316 10 15.4816 9 38.0559 22 NE Portal 0.703564 8 18.5106 11 15.5027 9 38.0541 22 MT 5.3+375F 0.703572 7 0.513021 6 18.4945 11 15.4942 9 38.0259 24 MT 4.6F 0.703521 8 0.513006 6 18.4897 9 15.4929 8 38.0386 28 69KLPA-1 0.703558 7 0.512997 5 69KLPA-2 0.703498 7 0.513019 5 18.4232 11 15.4822 9 37.9557 26 69KLPA-5A 0.703582 7 0.512974 8 18.4339 9 15.4948 8 37.9680 21 69KLPA-5Ab 0.703576 6 0.512991 6 69KLPA-5A UL 0.703626 8 0.512992 5 69KLPA-8B 0.703496 7 0.513008 5 18.4196 18 15.4860 15 37.9567 41 69KLPA-8Bb 0.703506 7 0.513022 5 69KLPA-9A 0.703499 8 0.513016 5 18.4262 15 15.4874 13 37.9662 32 69KLPA-14A 0.703501 8 0.513023 6 18.4357 14 15.4868 11 37.9651 29 69KLPA-16A 0.703346 6 0.513021 7 18.3867 10 15.4812 7 37.9377 19 69KLPA-27 0.703456 7 0.513019 5 69KLPA-30 0.703437 8 0.513022 5 69KLPA-32 0.703472 8 0.513003 10 69KLPA-33 0.703393 7 0.513018 6 71HALW-4 0.703498 7 0.513012 6 18.4568 13 15.4840 12 37.9827 30 71HALW-13 0.703528 7 18.4631 12 15.4880 11 38.0045 28 Rejuvenated Stage Lavas P252-2 0.703146 7 0.513063 7 18.1974 15 15.4502 14 37.7777 42 P252-2b 18.1984 15 15.4452 14 37.7595 34 P252-2 UL 18.2380 13 15.4604 10 37.8132 30 74KAL-1 0.703154 8 0.513068 7 18.1742 130 15.4411 110 37.7357 274 P253-12 0.703172 7 0.513072 6 18.1737 19 15.4496 16 37.7601 39 P253-11B 0.703164 7 0.513064 5 18.1572 31 15.4501 25 37.7486 69 P253-9 0.703181 7 0.513057 6 18.1955 57 15.4601 58 37.7846 134 aAll samples were acid-leached (see text) except for two analyses with UL suffix. For these samples acid-leaching lowered Sr and Pb isotopic ratios but did not change the Nd ratios. Figures only show data from acid-leached analyses. 2s applies to last decimal place(s). Sr data were normalized to 86Sr/88Sr = 0.1194 and Nd were normalized to 146Nd/144Nd = 0.7219. Mean measured 87Sr/86Sr for NBS 987 standard during the course of study was 0.710260 ± 13 (2s, n = 42) and 143Nd/144Nd for La Jolla standard was 0.511858 ± 7 (2s, n = 18). The external reproducibility for 87Sr/86Sr and 143Nd/144Nd based on two duplicates are better than 10  106 and 17  106, respectively, that is, within or slightly larger than the machine in-run uncertainties. Pb isotopic ratios were corrected for instrumental mass fractionation by adding a Tl spike and using a 205Tl/203Tl of 2.3885. Mean measured 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb for NBS 981 Pb standard were 16.9418 ± 21 (2s, n = 105), 15.4979 ± 25 (2s, n = 105), and 36.7184 ± 61 (2s, n = 105), respectively. These numbers are in agreement, within errors, with TIMS triple-spike values [Galer and Abouchami, 1998]. The external reproducibility for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb is better than 136 ppm, 323 ppm, and 482 ppm, respectively. bDuplicate analysis of separate aliquot. Figures show data with the lowest uncertainty. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 9 of 28Figure 4. MgO versus SiO2, Al2O3, CaO, TiO2, Na2O, P2O5, and Na2O/K2O. Open symbols indicate tholeiitic lavas, and solid symbols indicate alkalic lavas. In the SiO2 panel the most evolved postshield lavas and two rejuvenated stage lavas with lower SiO2 contents are labeled. Two lavas with abundant clinopyroxene phenocrysts (M4.6+150F and 71PELE-37) are labeled in Al2O3 and CaO panels. In the Na2O/K2O panel, late shield/postshield lavas were adjusted to a constant K2O/P2O5 ratio for potassium loss as in Figure 3. The benmoreite dike and rejuvenated stage lavas were not adjusted. The thick line is a liquid line of descent calculation for fractional crystallization using MELTS [Ghiorso and Sack, 1995] at a pressure of 1 kbar, fO2 = FMQ-2, and 1.0 wt% H2O and rejuvenated lava P253-11B as a starting composition. The choices of fO2 and water content are based on data for North Arch lavas [Dixon et al., 1997]. This choice of pressure yields only olivine as a fractionating phase in the interval from 16 to 6.5 wt% MgO. The dashed rectangles in Figures 3d, 3f, and 3g show the range for East Molokai rejuvenated stage glasses from Clague and Moore [2002] (square) and Sherman and Garcia [2002] (triangle). The green fields in the insets in Figures 4d, 4f, and 4g are enlargements of the dashed rectangles. These glass data show that rejuvenated stage magmas are compositionally more diverse than the whole rock data. Mauna Kea data from West et al. [1988], Frey et al. [1990, 1991], Rhodes [1996], and Rhodes and Vollinger [2004]; Kohala data from Feigenson et al. [1983], Lanphere and Frey [1987], and Spengler and Garcia [1988]. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 10 of 28postshield and rejuvenated stage East Molokai lavas, we plot abundance of various incompatible elements versus Th which is highly incompatible in the phenocryst phases, insensitive to minor alter- ation, and has a large abundance variation, factor of 8. The best correlations are for the relatively immobile, highly incompatible elements Ba, Nb and Ta (not shown) (Figure 6). Correlation coef- ficients of Rb (0.92) and U (0.91, not shown) with Th content are not as strong. In particular, the wide range in Rb abundance (0.5 to 47.3 ppm), a factor of 100, compared to less than 10 for other incompatible trace elements shows the well estab- lished result that Rb, like K (factor of 17 in abundance range, Table 2), was mobile during postmagmatic alteration processes [e.g., Feigenson et al., 1983; Fodor et al., 1987; Frey et al., 1990, 1994; Jackson et al., 1999]. [15] As expected, evolved alkalic lavas have the highest abundance of Rb, Ba, Nb and Pb and the tholeiitic lavas generally have the lowest abun- dance (Figure 6). For these elements the rejuve- nated stage lavas largely overlap the field defined by the late shield/postshield basalt (Figure 6). As elements plotted on the vertical axis in Figure 6 become less incompatible (e.g., Zr, Sr, Y and Yb), their trends become increasingly convex upward. In fact, the late shield/postshield alkalic lavas with relatively high Th contents have relatively uniform Y and Yb contents (abundance range of 2.7 and 2.5, respectively); sample 69KLPA-33, a mugear- ite, is an exception. In addition, for these elements rejuvenated stage lavas are clearly offset from the late shield/postshield lavas to low X/Th ratios (Figures 6e–6h). [16] A relative depletion of Th for all East Molokai lavas is apparent in primitive mantle (PM) normal- ized plots (Figure 7). For example, all East Molo- kai lavas have (Ba/Th)PM significantly greater than unity and this is a characteristic of all Hawaiian lavas [Hofmann and Jochum, 1996; Huang and Frey, 2003; Yang et al., 2003]. The most extreme enriched patterns (e.g., high La/Yb) are for mugearite (69KLPA-33) that is relatively depleted in Sr, Hf and Ti and the benmoreite (M4.9+300D) that has a relative depletion in Ti (Figure 7). An important feature in the primitive mantle normal- ized plots is the negative slope from Ta to Yb (Figure 7). Most late shield/postshield lavas have subparallel patterns for highly and moderately incompatible elements (from Rb to Ti), but from tholeiitic to alkalic basalt there is a progressive increase in abundance of highly incompatible ele- Figure 5. Abundance of Nb and Cr and abundance of olivine phenocryst versus MgO content. Open symbols indicate tholeiitic lavas, and solid symbols indicate alkalic lavas as defined in Figure 3. Abundance of the highly incompatible trace element, Nb, is negatively correlated with MgO content, whereas the compatible element Cr is positively correlated. The strong correla- tion between modal olivine abundance and MgO content for lavas from the Kalaupapa section reflects the important role of olivine fractionation and accumu- lation during the evolution of lavas with MgO contents greater than 6 wt%. Modes of olivine phenocryst for Kalaupapa section are from Beeson [1976]. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 11 of 28ments. The lowest abundances are in tholeiitic basalt, presumably main shield lavas, from the Wailau landslide. From Rb to Sr the alkalic reju- venated stage lavas overlap the field for basaltic late shield/postshield lavas, but they range to lower abundances for the more compatible elements from Nd to Yb (Figure 7). 6.3. Isotopes (Sr, Nd, Pb) 6.3.1. Late Shield/Postshield Lavas [17] As Hawaiian volcanoes evolve from the shield building to the postshield stage there is a temporal trend to lower 87Sr/86Sr and higher 143Nd/144Nd. This trend was first recognized for lavas from Haleakala (East Maui) [Chen and Frey, 1985] and Kohala [Hofmann et al., 1987; Lanphere and Frey, 1987] and more recent studies of the Hawaii Scientific Drilling Project (HSDP) cores from Mauna Kea show a similar trend [Lassiter et al., 1996; Abouchami et al., 2000; J. G. Bryce et al., Sr, Nd and Os isotopes in a 2.84 km section of Mauna Kea volcano: Implications for the geochemical structure of the Hawaii plume, submitted to Geo- chemistry, Geophysics, Geosystems, 2005 (herein- after referred to as Bryce et al., submitted manuscript, 2005)]. Late shield/postshield lavas from Kalaupapa section of East Molokai show a similar temporal trend (Figures 2c and 2d). The range of Sr and Nd isotopic variation is relatively large for Haleakala, that is, the range from Hono- manu (late shield) to Kula and Hana (postshield) in Figure 8a. Note that the Hana Volcanics were reclassified as postshield lavas by Sherrod et al. [2003]. In contrast, late shield/postshield lavas from the other four Kea-trend volcanoes (Mauna Kea, Kohala, West Maui and East Molokai) are less Figure 6. Th versus Rb, Ba, Nb, Pb, Zr, Sr, Y, and Yb. Open symbols indicate tholeiitic lavas, and solid symbols indicate alkalic lavas as defined in Figure 3. Although Th abundance in East Molokai late shield/postshield lavas is strongly correlated with these elements, the correlation coefficient decreases as the element on the vertical axis increases in compatibility. In Figure 6a the highly evolved lavas are labeled. The trend for rejuvenated stage lavas overlaps those for late shield/postshield lavas in Figures 6a to 6d, but in Figures 6e to 6h the rejuvenated stage lavas are offset to high Th/X ratios (X = Zr, Sr, Y or Yb) which exceed those of primitive mantle. The dashed line has a slope equal to the primitive mantle ratio [Sun and McDonough, 1989]. The fields in Figures 6c, 6e, 6f, 6g, and 6h for Honolulu Volcanics and North Arch lavas designated by blue lines [Yang et al., 2003] show that offsets to high Th/X are characteristic of rejuvenated stage lavas. Rejuvenated stage lava 74KAL-1 is labeled in Figure 6a because it has lost Rb during alteration (see text). Among East Molokai rejuvenated stage lavas labeled, sample 71KAUH-1 in Figure 6a has the highest abundances of incompatible elements. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 12 of 28variable in 87Sr/86Sr and 143Nd/144Nd and most importantly their fields in Figure 8a overlap. [18] When lavas from several Hawaiian volcanoes are considered there is a well-defined inverse correlation between 143Nd/144Nd and 87Sr/86Sr, but this correlation is not well defined by East Molokai and West Maui late shield/postshield lavas (Figure 8a). For example, East Molokai alkalic and tholeiitic lavas span a similar limited range in 143Nd/144Nd ratios, but the tholeiitic lavas are offset to higher 87Sr/86Sr ratios (Figure 8b). Nine acid-leached East Molokai lavas analyzed by Basu and Faggart [1996] also range more widely in 87Sr/86Sr than 143Nd/144Nd (Figure 8b). [19] In 206Pb/204Pb versus 207Pb/204Pb and 208Pb/204Pb plots the late shield/postshield lavas from the five Kea-trend volcanoes define two groups: Mauna Kea, West Maui, East Molokai, and Kohala (Hawi Volcanics) compared to Kohala (Pololu Volcanics) and Haleakala (Figures 9a and 9c). Both groups have relatively low 208Pb/204Pb ratio at a given 206Pb/204Pb, a characteristic of Kea- type lavas [Abouchami et al., 2005]; however, lavas from the first group range to higher 206Pb/204Pb than lavas from the second group (Figure 9a). [20] Shield stage lavas define a negative 87Sr/86Sr versus 206Pb/204Pb correlation, but as first clearly demonstrated by postshield lavas from Haleakala, the Kula and Hana Volcanics, a positive correla- tion is characteristic of postshield lavas [West and Leeman, 1987] (Figure 9d). This reversal of slope reflects a marked temporal change in source components contributing to growth of Hawaiian volcanoes. Figure 9d shows that four Kea volcanoes define a positive 87Sr/86Sr versus 206Pb/204Pb trend during late-shield and post- shield growth. A trend for postshield lavas from Kohala (Hawi Volcanics) is not shown because there are insufficient data. 6.3.2. Rejuvenated Stage Lavas [21] The East Molokai rejuvenated stage lavas are relatively homogenous in Sr and Nd isotopes with higher 143Nd/144Nd and lower 87Sr/86Sr ratios than late shield/postshield lavas (Figure 8a). Relative to the fields for other rejuvenated stage lavas they overlap the Koloa Volcanics from Kauai and Lahaina Volcanics from West Maui, and are offset to lower 87Sr/86Sr than the Honolulu Volcanics from Oahu (Figure 8c). East Molokai rejuvenated stage lavas define the same positive trend as East Molokai late shield/postshield lavas on 206Pb/204Pb Figure 6. (continued) Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 13 of 28versus 208Pb/204Pb, but they have lower 206Pb/204Pb ratios (Figure 9a). The trend defined by East Molokai rejuvenated stage lavas overlaps with North Arch lavas in 206Pb/204Pb-208Pb/204Pb space but is distinct from Honolulu Volcanics and Koloa Volcanics which are offset to higher 208Pb/204Pb at given 206Pb/204Pb (Figure 9b). In the 87Sr/86Sr versus 206Pb/204Pb plot, rejuvenated stage lavas from East Molokai overlap the fields for North Arch lavas and Lahaina Volcanics (Figure 9d). In general the positive correlations defined by late-shield/postshield lavas trend toward the fields of rejuvenated stage/North Arch lavas (Figure 9d). 6.3.3. East Molokai Lavas Recovered From the Wailau Landslide [22] The submarine landslide deposits north of Molokai Island are inferred to be derived from East Molokai volcano (see Figure 1 and also Moore and Clague [2002, Figures 13 and 15]). Since it is likely that basalt exposed in these landslide deposits is older than our subaerially collected samples, the landslide samples provide information about the temporal variation of geo- chemical characteristics during growth of the East Molokai shield. Glass-rich volcaniclastic rocks are abundant in the landslide deposits. The glasses are tholeiitic basalt and document the diversity of lava compositions erupted during shield growth [Clague et al., 2002; Shinozaki et al., 2002]. Five whole rocks from landslide blocks have been analyzed for major and trace element abundance and isotopic ratios of Sr, Nd and Pb [Tanaka et al., 2002]. Compared to the late shield/postshield lavas, these samples are not as enriched in highly incompatible elements (Figure 7). They define two distinct groups in isotopic ratios (Figures 8a, 9a, 9c, and 9d). In subsequent discussion of trace element and isotopic characteristics we compare these submarine landslide samples with our data for subaerial late shield/postshield lavas. 7. Discussion [23] Our broad objectives are to define the changes in magma sources, melting process and crustal evolution of magmas as Kea-trend volcanoes mi- grate away from the hot spot. We compare the transition from late shield to postshield volcanism at five Kea-trend volcanoes and also the subse- quent transition to rejuvenated stage volcanism at Figure 7. Incompatible trace element abundances normalized to the primitive mantle (PM) estimates of Sun and McDonough [1989]. Important features of the fields are the negative slopes from Ta to Yb, the high Ba/Th ratios, and the relatively low abundances in rejuvenated stage lavas. Individual patterns are shown for three highly evolved lavas and 71HALW-9, which has a pattern different from other last shield/postshield lavas. Three evolved lavas have prominent depletions in Ti. Main shield samples occurring in the Wailau landslide deposits are tholeiitic basalt with relatively low abundances [Tanaka et al., 2002]. The gray shaded field is for all the other late shield/postshield lavas except the four marked with sample names. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 14 of 28East Molokai and West Maui volcanoes. We first focus on lavas from East Molokai by considering the effects of crustal processes on their geochem- ical characteristics. 7.1. Crustal Processes: Compositional Effects of Postmagmatic Alteration 7.1.1. Late Shield/Postshield Lavas [24] Clague and Beeson [1980] noted that lavas from the Kalaupapa section of East Molokai can be divided into two groups on the basis of K/Ba, that is, groups with average K/Ba of 16 and 26 (see Figure 10b); they inferred that K/Ba differences reflected magmatic characteristics and a role for residual phlogopite during melting. Since publica- tion of this paper there has been recognition that K abundance in Hawaiian lavas is commonly affected by postmagmatic alteration [e.g., Feigenson et al., 1983; Fodor et al., 1987; Frey et al., 1990, 1994; Jackson et al., 1999]. Typically loss of K results in bulk rock K2O/P2O5 ratios less than unity which contrast with ratios of 1.5–2 in unaltered Hawaiian lavas (e.g., historical Mauna Loa lavas have K2O/ P2O5 of about 1.6 [Rhodes, 1995]; Puu Oo Kilauea lavas, 1.84 [Garcia et al., 2000]). Also Rb is Figure 8 Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 15 of 28more severely affected than K so that lavas with low K2O/P2O5 typically have anomalously high K/ Rb [e.g., Frey et al., 1994]. Five samples from the Kalaupapa section (69KLPA-2, 69KLPA-5A, 69KLPA-8B, 69KLPA-9A and 69KLPA-14A) have low K2O/P2O5 (1.0), extremely high K/Rb (>1600) and low K/Ba (<20) (Figure 10). Conse- quently we conclude that the K/Ba differences among East Molokai lavas emphasized by Clague and Beeson [1980] reflect postmagmatic alteration rather than magmatic processes. 7.1.2. Rejuvenated Stage Lavas [25] Except for one sample, 74KAL-1, East Molo- kai rejuvenated stage lavas have high K2O/P2O5 (>1.5) and low K/Rb (385–565) (Figure 10a). In contrast, sample 74KAL-1 has the lowest K2O/ P2O5 (1.35) and K/Ba (12.8) and the highest K/Rb (840) (Figure 10); among the rejuvenated stage lavas it was the most significantly affected by postmagmatic alteration. 7.2. Crustal Processes: Role of Crystal Fractionation 7.2.1. Late Shield/Postshield Lavas [26] The importance of crystal fractionation, depends in part on magma supply rates from the mantle. For example, the transition from shield to postshield volcanism occurs as a Hawaiian volcano moves away from the hot spot. Initially this tran- sition is a change from tholeiitic to alkalic volca- nism, commonly interpreted as reflecting a decrease in extent of melting [e.g., Chen and Frey, 1985]. The resulting decrease in magma supply leads to magma stagnation, cooling and crystal fractionation. Consequently, evolved alkalic lavas, for example, hawaiites, are important in postshield stage volcanism. Also there is evidence that the depth of fractionation increases with decreasing age [e.g., Clague, 1987; Frey et al., 1990]. In this section, we discuss the role of crystal fractionation during the late evolution of East Molokai volcano and then compare the late stage evolution of several Kea-trend volcanoes. [27] For East Molokai lavas with greater than 8 wt% MgO the major control on compositions was fractionation and accumulation of olivine phenocrysts and chromite which occurs as inclu- sions in olivine phenocrysts and as micropheno- crysts in the groundmass. This inference is based on (1) the strong positive correlation between modal olivine abundance and MgO content in the Kalaupapa section (Figure 5c); (2) the positive MgO-Cr correlation (Figure 5b); (3) the uniform CaO/Al2O3 ratios (0.74–0.88) with varying MgO content, except for two lavas with abundant augite phenocrysts (71PELE-37 and M4.6+150F) and higher CaO/Al2O3 ratios (0.98 and 1.08, respec- tively) (Figure 11a); and (4) the slight increase in Sc content with decreasing MgO from 18 to 8 wt% (Figure 11b). [28] For lavas with less than 6.5 wt% MgO, an important role for clinopyroxene fractionation is inferred from the positive trends for CaO and Sc abundance, and CaO/Al2O3 ratios versus MgO con- tent (Figures 4c and 11). Three evolved (hawaiite/ mugearite/benmoreite) postshield lavas (69KLPA- Figure 8. (a) The 87Sr/86Sr-eNd fields for late shield/postshield lavas from the five Kea-trend volcanoes and East Molokai rejuvenated stage lavas. The Sr-Nd isotopic ratios of late shield/postshield lavas from East Molokai broadly overlap the fields for West Maui, Haleakala, Kohala, and Mauna Kea late shield/postshield lavas. Three of the five East Molokai samples from the Wailau landslide overlap the field for East Molokai late shield/postshield samples. Mauna Kea late shield/postshield lavas include on-land sections and subaerially erupted lavas cored by the Hawaiian Scientific Drilling Project (HSDP). eNd is calculated from 104  [(143Nd/144Nd)sample/(143Nd/144Nd)CHUR  1], where (143Nd/144Nd)CHUR = 0.512638. (b) Note that tholeiitic lavas (open circles) from East Molokai have a higher 87Sr/86Sr ratio than most alkalic lavas (solid circles). Plus symbol denotes acid-leached samples analyzed by Basu and Faggart [1996] and collected from the same Kalaupapa section which is predominantly alkalic lavas. The error bars are the 2s calculated from the mean of 42 analyses of NBS 987 Sr standard and 18 analyses of La Jolla Nd standard. (c) 87Sr/86Sr-eNd for Hawaiian rejuvenated stage/North Arch lavas. Rejuvenated stage lavas from East Molokai overlap fields for Lahaina Volcanics and Koloa Volcanics and are offset to lower 87Sr/86Sr relative to the Honolulu Volcanics. The error bars are the 2s calculated for the mean of 42 analyses of NBS 987 Sr standard and 18 analyses of La Jolla Nd standard. Data sources: Mauna Kea, Kennedy et al. [1991], Lassiter et al. [1996], Bryce et al. (submitted manuscript, 2005); Kohala, Stille et al. [1986], Hofmann et al. [1987]; Haleakala, West and Leeman [1987], Chen et al. [1990, 1991]; West Maui, Hegner et al. [1986], Tatsumoto et al. [1987], Gaffney et al. [2004]; East Molokai, Basu and Faggart [1996], Tanaka et al. [2002] for Wailau landslide and this study; Lahaina Volcanics, Hegner et al. [1986], Tatsumoto et al. [1987], Gaffney et al. [2004]; Honolulu Volcanics, Stille et al. [1983], Lassiter et al. [2000]; Koloa Volcanics, Reiners and Nelson [1998]; North Arch lavas, Frey et al. [2000]; EPR MORB, Niu et al. [1999], Regelous et al. [1999], Castillo et al. [2000]. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 16 of 28Figure 9. (a) 206Pb/204Pb-208Pb/204Pb for late shield/postshield lavas from the five Kea-trend volcanoes. The Loa- Kea boundary is defined by Abouchami et al. [2005]. Most lavas from Kea-trend volcanoes lie to the right side of this trend. Data points shown for East Molokai have maximum 2s less than the size of the symbol. East Molokai late shield/postshield lavas have relatively higher 206Pb/204Pb ratios than lavas from Haleakala volcano but overlap the field of Mauna Kea and West Maui late shield/postshield lavas. Pb isotopic data for West Maui shield/postshield lavas from Hegner et al. [1986] and Tatsumoto et al. [1987] are not plotted due to their large uncertainty. Postshield stage lavas from Hualalai, a Loa-trend volcano, are shown for comparison. (b) 206Pb/204Pb-208Pb/204Pb for four suites of Hawaiian rejuvenated stage and North Arch lavas compared with fields for late shield/postshield lavas from five Kea-trend volcanoes and EPR MORB triple-spike data from Galer et al. [1999]. The black line is the regression line for East Molokai late shield/postshield lavas (solid circles) and rejuvenated stage lavas (open circles). (c) 206Pb/204Pb-207Pb/204Pb for East Molokai lavas and fields for late shield/postshield lavas from four Kea-trend volcanoes. Also shown are rejuvenated stage and North Arch lavas. (d) 87Sr/86Sr-206Pb/204Pb isotope fields for five Kea-trend volcanoes and rejuvenated stage/North Arch lavas. Hawaiian shield lavas define an inverse trend, whereas postshield (e.g., Haleakala) define a positive trend. East Molokai late shield/postshield stage lavas define a broadly positive trend overlapping the field of Mauna Kea late shield/postshield lavas and trend toward the fields of Hawaiian rejuvenated stage/North Arch lavas. Data sources: Kilauea, Abouchami et al. [2005]; Mauna Kea, Abouchami et al. [2000, 2005], Eisele et al. [2003]; Mauna Loa, Abouchami et al. [2000, 2005]; Hualalai, Cousens et al. [2003]; Kohala, Holcomb et al. [2000], Abouchami et al. [2005]; Haleakala, West and Leeman [1987], Chen et al. [1990, 1991], Ren et al. [2005]; West Maui, Gaffney et al. [2004]; East Molokai, A. Basu (unpublished data), Tanaka et al. [2002], and this study; Koolau, Roden et al. [1994]; Lahaina Volcanics, Hegner et al. [1986], Tatsumoto et al. [1987], Gaffney et al. [2004]; Honolulu Volcanics, Lassiter et al. [2000], Fekiacova and Abouchami [2003]; Koloa Volcanics, Lassiter et al. [2000]; North Arch lavas, Frey et al. [2000]; EPR MORB, Galer et al. [1999], Niu et al. [1999], Regelous et al. [1999], Castillo et al. [2000]. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 17 of 2833, 70KAWE-1C and dike M4.9+300D) with low abundances of MgO and Sc, low CaO/Al2O3 ratios (Figure 11) and negative Ti anomalies (Figure 7) are offset to higher Nb/Zr ratios than other late shield/ postshield lavas (Figure 12a). These evolved lavas reflect fractionation of clinopyroxene and Fe-Ti oxides (e.g., titanomagnetite and ilmenite). Since fractionation of Fe-Ti oxides cannot increase theNb/ Zr ratios [Nielsen et al., 1994; Nielsen and Beard, 2000; Jang and Naslund, 2003], the high Nb/Zr ratios require extensive fractionation of clinopyrox- ene (Figure 12a). [29] Although not obvious from the negative Al2O3 versus MgO trend (Figure 4b) the broad positive correlation of Sr/Ce versus Eu/Eu* (Figure 13), and decreasing Sr/Ce with increasing abundance of a highly incompatible element, such as Nb (Figure 12b), indicate that plagioclase fractionation also occurred. Figure 10. K/Rb and K/Ba versus K2O/P2O5 for East Molokai lavas. Five lavas (69KLPA-2, 69KLPA-5A, 69KLPA-8B, 69KLPA-9A, and 69KLPA-14A) from the Kalaupapa section with relatively low K2O/P2O5 ratios have high K/Rb ratios, indicating loss of K and Rb during postmagmatic alteration. Five other labeled late shield/postshield lavas have K2O/P2O5 < 1 and ‘‘normal’’ K/Rb. The only rejuvenated stage lava showing evidence for alteration is subaerial sample 74KAL-1, which has lower K2O/P2O5 (1.35) and higher K/Rb (840) ratios than other rejuvenated stage East Molokai lavas. On the basis of K/Ba ratios, lavas from the Kalaupapa section of East Molokai can be divided into the two circled groups [Clague and Beeson, 1980]; lavas with low K/Ba ratios have low K2O/P2O5 and high K/Rb ratios, thereby indicating loss of K and Rb. PM denotes value for primitive mantle from Sun and McDonough [1989]. Figure 11. (a) MgO-CaO/Al2O3 and (b) MgO-Sc for East Molokai lavas. For the late shield/postshield lavas the positive trend between MgO versus CaO/Al2O3 at low MgO contents reflects the control of clinopyroxene fractionation. Sc defines a complex trend reflecting a change from olivine (negative slope) to clinopyroxene dominated fractionation (positive slope). The rejuve- nated stage lavas show no evidence for clinopyroxene fractionation. Evolved late shield/postshield lavas and two rejuvenated stage lavas from Kauhako crater are labeled in Figure 11a. Two lavas (M4.6+150F and 71PELE-37) with abundant clinopyroxene phenocrysts and high CaO/Al2O3 ratios are also labeled. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 18 of 28[30] Within the postshield stages at Mauna Kea, Kohala and West Maui there is an abrupt transition both in eruption age and composition from older, dominantly basalt to younger, solely hawaiite to trachyte lavas with an obvious gap in major ele- ment composition [Stearns and Macdonald, 1942; Spengler and Garcia, 1988; Frey et al., 1990]. This gap is also apparent in Figure 12b; specifically the relatively young evolved lavas, hawaiites and mugearites, forming the Laupahoehoe and Hawi Volcanics at Mauna Kea and Kohala, respectively, define a distinct trend offset to high Nb at a given Sr/Ce (Figure 12b). Also at East Molokai, three samples are offset to high Nb/Zr at a given Tb/Yb and high Nb at a given Sr/Ce (Figure 12). These may be relatively young lavas; sample M4.9+300D Figure 12 Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 19 of 28is a dike, 69KLPA-33 is the youngest sample in the Kalaupapa section (Figure 2a), and 70KAWE-1C is a surface flow. Frey et al. [1990] inferred that offset to high Nb at a given Sr/Ce reflects domi- nantly clinopyroxene fractionation and derivation of hawaiite from basalt at moderate pressure, perhaps at a depth of 20 km. Therefore a general characteristic of the postshield evolution of Kea- trend volcanoes is that as the volcano enters the postshield stage, fractionation of a plagioclase- bearing assemblage occurs at low pressure but as the volcano migrates further away from the hot spot there is a diminishing supply of basaltic magma from the plume and basaltic magma stagnates deep within the crust or at the crust/mantle boundary where the fractionating mineral assemblage is ini- tially dominated by clinopyroxene [Feigenson et al., 1983; see also Clague, 1987, Figure 4; Frey et al., 1990, Figure 15]. Plagioclase fractionation does not occur at these depths until high Al2O3 contents of 17–18% are attained (Figure 4b). 7.2.2. Rejuvenated Stage Lavas [31] Rejuvenated stage lavas have MgO ranging from 6.3 wt% to 16 wt% (Figure 4). Consistent with the abundance of olivine phenocrysts, liquid line of descent calculations using MELTS [Ghiorso and Sack, 1995] show that olivine fractionation is the dominant process in the evolution of rejuve- nated stage lavas. The negative trends for MgO- CaO and MgO-Sc and uniform CaO/Al2O3 ratios (0.79–0.84) show that clinopyroxene fractionation was not an important process for these lavas (Figures 4c and 11). Also the negative trend on MgO-Al2O3 imply that fractionation of plagioclase was not important during evolution of rejuvenated stage East Molokai lavas (Figure 4b). In summary, like other rejuvenated stage lavas (Honolulu Vol- canics and Koloa Volcanics) the olivine-dominated crustal evolution of East Molokai rejuvenated stage lavas differs substantially from that of the alkalic postshield lavas. 7.3. Constraints on the Melting Process: Mineralogy of Residual Phases and Extents of Melting 7.3.1. Late Shield/Postshield Stage Lavas [32] East Molokai lavas define three subparallel trends for Nb/Zr versus Tb/Yb (Figure 12a). Nb is more incompatible than Zr in clinopyroxene and garnet and both phases can control Nb/Zr; in Figure 12. (a) Tb/Yb versus Nb/Zr for the East Molokai lavas. East Molokai late shield/postshield stage lavas display a positive trend overlapping Mauna Kea subaerial (late shield/postshield) lavas. Three evolved lavas with relative Ti depletion and low Sc abundances are offset to higher Nb/Zr. Clinopyroxene fractionation is the only likely process to cause such an increase in Nb/Zr. The clinopyroxene fractional crystallization trend, with 10% increments shown as green plus symbols, used the partition coefficients for clinopyroxene/alkalic basalt from Hart and Dunn [1993]. These intervals are maximum estimates because the partition coefficients will increase as the melt evolves from basalt to hawaiite. Such extensive amounts of clinopyroxene fractionation are qualitatively consistent with the low CaO/Al2O3 and Sc of these samples (Figure 11). The rejuvenated stage lavas define a positive trend offset from late shield/postshield lavas to higher Nb/Zr ratios, presumably because of a source with high Nb/Zr. PM denotes value for primitive mantle from Sun and McDonough [1989]. Symbols as in Figure 3. (b) Nb (ppm) versus Sr/Ce for East Molokai late shield/postshield lavas. Three highly evolved East Molokai lavas are labeled. For comparison, fields are shown for Mauna Kea and Kohala. The inset shows the fractionation trends of the mineral assemblages listed in parentheses. Cpx, plag, and oliv are abbreviations of clinopyroxene, plagioclase, and olivine, respectively. The basalt to hawaiite transitions at Mauna Kea and Kohala reflect fractionation of a plagioclase-poor and clinopyroxene-rich assemblage at moderate pressure. Data sources: East Molokai, this study; Wailau landslide, Tanaka et al. [2002]; Kohala, Spengler and Garcia [1988]; Mauna Kea, West et al. [1988], Frey et al. [1990, 1991], Huang and Frey [2003]. Figure 13. Sr/Ce versus Eu/Eu* for East Molokai lavas. All the East Molokai lavas (except Wailau landslide samples) define a positive trend, indicating plagioclase control. Symbols as in Figure 3. PM denotes value for primitive mantle from Sun and McDonough [1989]. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 20 of 28contrast because heavy rare earth elements are compatible in garnet, Tb/Yb is much more sensi- tive to garnet than clinopyroxene [e.g., Frey et al., 2000, Figure 10c; Pertermann et al., 2004]. There- fore the positive Nb/Zr versus Tb/Yb trends for East Molokai lavas (Figure 12a) reflect residual garnet. In detail, the relatively older East Molokai tholeiitic (and transitional) lavas have lower Tb/Yb ratios than alkalic lavas, and the lowest Tb/Yb ratios are in the presumably older tholeiitic basalt from the Wailau landslide (Figure 12a). A similar temporal trend characterizes Mauna Kea volcano where the late shield to postshield, subaerially erupted lavas have relatively higher Tb/Yb and show a clear trend of garnet control, whereas older submarine lavas have lower Tb/Yb and a poor Tb/ Yb-Nb/Zr trend (Figure 12a). We infer that as Kea volcanoes age and pass from the tholeiitic shield to alkalic late shield/postshield magmatism there is an increasing role for residual garnet. An enhanced role for garnet is inconsistent with the change to a more depleted source composition, that is, lower 87Sr/86Sr and higher 143Nd/144Nd (Figure 2) and presumably less garnet because of lower Al2O3 and CaO contents. This contradiction can be alleviated if the alkalic basalt was generated at lower extents of melting and segregated at higher pressure. 7.3.2. Rejuvenated Stage Lavas [33] Hawaiian rejuvenated stage lavas are alkalic and the well-studied Honolulu Volcanics and Koloa Volcanics are highly enriched in incompat- ible elements [Clague and Frey, 1982; Feigenson, 1984]. Such enrichments are attributed to deriva- tion by low extents of melting of an incompatible- element enriched, garnet-bearing source [e.g., Clague and Frey, 1982; Clague and Dalrymple, 1988; Yang et al., 2003]. Samples with the highest abundance of incompatible elements also have anomalous ratios of incompatible elements such as Zr/Sm, Nb/La, Ti/Eu, K/Ce, that are typically uniform in oceanic basalt. These variations are attributed to the effects of minor residual phlogo- pite, amphibole, and Ti-rich phases [e.g., Clague and Frey, 1982; Feigenson, 1984; Class and Goldstein, 1997; Yang et al., 2003]. [34] The abundance ratios Nb/La, Zr/Sm, Ti/Eu and K/Ce are highly correlated in lavas from the Honolulu Volcanics, North Arch and rejuvenated stage East Molokai lavas (Figure 14). Honolulu Volcanics with the lowest SiO2 contents have the most extreme ratios (Figure 15), that is, very different from primitive mantle ratios in Figure 14. These samples are inferred to have formed by the lowest extent of melting, and they were most affected by residual phlogopite and Ti-rich phases [Clague and Frey, 1982; Yang et al., 2003]. In contrast, rejuvenated stage East Molokai lavas have relatively high SiO2 content and particularly the submarine samples have near primitive mantle ratios of Zr/Sm and Ti/Eu (Figure 14). Compared to these submarine samples, the subaerial East Molokai samples range to lower K/Ce, Zr/Sm and Nb/La (Figure 14). Given the isotopic similarity of submarine and subaerial rejuvenated stage lavas we infer a range in extent of melting. Finally, compared to the lavas that we studied, an even larger range in relative extent of melting is indicated by the wide range of TiO2, P2O5 and Na2O/K2O at a given MgO (Figures 4d, 4f, and 4g) of submarine glass grains derived from rejuvenated stage East Molokai lavas [Sherman and Garcia, 2002]; no trace element data are available for these glasses. 7.4. Constraints on Source Components 7.4.1. Comparison of Late Shield/ Postshield East Molokai Lavas With East Molokai Shield Lavas From Wailau Landslide [35] Three of five lavas recovered from the Wailau landslide [Tanaka et al., 2002] have Sr and Nd isotope ratios within the field of late shield/post- shield East Molokai lavas (Figure 8a) and two of these have high 206Pb/204Pb and 208Pb/204Pb that are on an extrapolation of the East Molokai trend; that is, they are on the Kea trend (Figure 9a). Two other samples have Sr, Nd and Pb isotopic ratios unlike those of other East Molokai lavas; they are most similar to the late shield lavas from Haleakala (Honomanu Volcanics in Figure 8a). Their Loa- trend Pb isotopic character (Figure 9a) may be another example of a Kea volcano with a subset of lavas having the Pb isotopic ratios characteristic of Loa-trend volcanoes. Other examples are Mauna Kea [Eisele et al., 2003] and Haleakala [Ren et al., 2005]. 7.4.2. Source Components in Late Shield/ Postshield East Molokai Lavas [36] The isotopic ratios 87Sr/86Sr and 206Pb/204Pb of East Molokai late shield/postshield lavas are correlated with their compositions; for example, these isotopic ratios are positively correlated with SiO2 and negatively correlated with Nb/Zr (Figure 16). Such trends require two geochemically Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 21 of 28distinct source components. One component lies within the field of Hawaiian shield lavas (Figure 16b). In regard to the other component the important result is that the negative Nb/Zr versus 206Pb/204Pb trend defined by late shield/ postshield East Molokai lavas contrasts markedly with the positive trend defined by Hawaiian shield lavas (Figure 16b). A similar contrast was noted in 87Sr/86Sr versus 206Pb/204Pb (Figure 9d). In each case (Figures 9d and 16b), the trend of late shield/ postshield East Molokai lavas extrapolates toward the field for rejuvenated stage lavas from East Molokai. It is apparent that the low 87Sr/86Sr, low 206Pb/204Pb and high Nb/Zr source component that dominates the East Molokai rejuvenated stage lavas also contributed to the late shield/postshield lavas. As an aside, the strong correlation of Nb/Zr with 87Sr/86Sr for East Molokai late shield/post- shield lavas also suggests that the poor correlation of 87Sr/86Sr versus 143Nd/144Nd (Figure 8b) is not a result of alteration effects on 87Sr/86Sr. 7.4.3. Comparison of Late Shield/ Postshield Lavas From Kea-Trend Volcanoes [37] Shield stage lavas from the geographically defined Kea- and Loa-trend volcanoes (Figure 1) have little overlap in Pb isotope space [e.g., Tatsumoto, 1978; Stille et al., 1986; Abouchami et al., 2005]. In detail, Pb isotopic analyses of lavas recovered by HSDP from Mauna Kea, a Kea-trend volcano, show that the lavas define three distinct Pb isotope arrays labeled as ‘‘Kea-lo8’’, ‘‘Kea- mid8’’ and ‘‘Kea-hi8’’ [Eisele et al., 2003]. Most of the shield lavas belong to the Kea-mid8 group. Eisele et al. [2003] and Abouchami et al. [2005] found that modern Kilauea and 350–550 ka Mauna Kea shield lavas, forming the Kea-mid8 array, have the same Pb isotopic signature. Because at these respective times the locations of the Kilauea and Mauna Kea shields were similar relative to plume center, they argued for a long-lived, spatially con- strained source for Kea lavas. On the basis of Sr- Nd-Hf-Pb similarities of lavas from the uppermost West Maui shield with Mauna Kea and Kilauea lavas, Gaffney et al. [2004] also argued that the Kea end-member has maintained its distinctive geochemical character for 1.5 Myr. [38] Assuming that Kea volcanoes evolve from the shield to postshield stage at a uniform distance from the plume, that is, constant relative migration rate of Pacific plate over the plume, another test of the long-term uniformity of the sources and pro- cesses generating Hawaiian magmas is to compare the isotopic characteristics of late shield/postshield stage lavas from Kea-trend volcanoes. Indeed there is a first order similarity in temporal variation of isotopic ratios. Chen and Frey [1985] and Chen et al. [1991] showed that as Haleakala volcano evolved from late shield to postshield volcanism isotopic ratios of 87Sr/86Sr and 206Pb/204Pb de- crease. We find the same result for East Molokai (Figure 2c). Recent studies of late shield and postshield lavas at Mauna Kea [Lassiter et al., Figure 14. Na/La versus (a) Zr/Sm, (b) Ti/Eu, and (c) K/Ce for Hawaiian rejuvenated stage and North Arch lavas. These lavas define strong positive trends. The large solid square is the primitive mantle value from Sun and McDonough [1989]. Error bars (±5%, 2s) for Nb/La, Zr/Sm, Ti/Eu, and K/Ce are shown for East Molokai rejuvenated stage lavas. The pink field designates the submarine rejuvenated stage lavas from East Molokai. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 22 of 281996; Blichert-Toft et al., 2003] show the same temporal trend. [39] In both the 87Sr/86Sr versus 206Pb/204Pb and Nb/Zr versus 206Pb/204Pb plots late shield/post- shield lavas from East Molokai, Mauna Kea, Haleakala and West Maui lavas lie on the same trend (Figures 9d and 16b). Clearly the late shield/ postshield lavas at these four Kea volcanoes show the influence of the low 206Pb/204Pb, low 87Sr/86Sr and high Nb/Zr component that was important in creating rejuvenated stage lavas. 7.4.4. East Molokai Compared to Mauna Kea and West Maui [40] Huang and Frey [2003] defined the basaltic postshield group of Mauna Kea lavas as including the oldest (75 to 250 ka) subaerially exposed Hamakua Volcanics and the uppermost subaerially erupted part of the HSDP cores. The shield to postshield transition is gradual and with decreasing age, alkalic basalt is intercalated with tholeiitic basalt, SiO2 content decreases and ratios such as La/Yb and Nb/Zr increase. This postshield group defines the Kea-lo8 Pb-Pb array of Eisele et al. [2003]. Mauna Kea postshield group lavas are the comparable evolutionary stage to East Molokai late shield/postshield lavas. Late shield/postshield lavas from East Molokai, Mauna Kea and West Maui lavas overlap in Sr-Nd-Pb space (Figures 8a, 9a, 9c, and 9d), thereby indicating a long-term simi- larity in the source components and processes contributing to this phase of Kea volcano growth. 7.4.5. East Molokai Compared to Kohala and Haleakala [41] Two other Kea-trend volcanoes, Kohala and Haleakala, have also evolved to the postshield Figure 15. SiO2 contents versus Zr/Sm, Ti/Eu, Zr/Hf, and Nb/Zr for Hawaiian rejuvenated stage and North Arch lavas. SiO2 contents are corrected for olivine fractionation and accumulation by adding or subtracting equilibrium olivine until the whole rock composition is in equilibrium with olivine with Fo = 90. Error bars shown for Zr/Sm, Ti/ Eu Zr/Hf, and Nb/Zr are ±5% (2s) for East Molokai rejuvenated stage lavas. The pink field designates the submarine rejuvenated stage lavas from East Molokai. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 23 of 28stage. Late shield/postshield lavas from Haleakala and Kohala lavas overlap East Molokai and Mauna Kea late shield/postshield lavas in Sr-Nd space (Figure 8a). For Pb isotopic ratios the evolved alkalic lavas of the postshield Hawi Volcanics (Kohala) overlap the field for East Molokai late shield/postshield lavas, but the late shield Pololu Volcanics (Kohala) have less radiogenic ratios (Figure 9a). Also postshield Haleakala lavas (Kula & Hana Volcanics) range to low Pb ratios that overlap East Molokai rejuve- nated stage lavas. 7.4.6. Source Components in East Molokai Rejuvenated Stage Lavas [42] Compared to shield and postshield lavas, the relatively low 87Sr/86Sr and high 143Nd/144Nd of rejuvenated stage lavas (Figures 8a and 8c) require a larger role for a depleted component, that is, with long-term low Rb/Sr and high Sm/Nd in the source of rejuvenated stage lavas. An important result is that the East Molokai rejuvenated stage lavas lie on the same 206Pb/204Pb versus 208Pb/204Pb and Nb/Zr versus 206Pb/204Pb and 87Sr/86Sr trends defined by the late shield/postshield lavas (Figures 9b, 16b, and 16c). These linear trends indicate that East Molokai late shield/postshield lavas and rejuvenated stage lavas sampled a common depleted component with low 206Pb/204Pb and 87Sr/86Sr. The rejuvenated stage East Molokai lavas contain a larger proportion of this depleted component. Hence a relatively unradiogenic Pb component with relatively low 87Sr/86Sr and high 143Nd/144Nd was available for 1 Myr; that is, rejuvenated Figure 16. 206Pb/204Pb versus SiO2 contents and Nb/ Zr versus 206Pb/204Pb and 87Sr/86Sr for East Molokai lavas. (a) 206Pb/204Pb ratios are positively correlated with SiO2 contents for late shield/postshield lavas. Despite the sensitivity of SiO2 content to crystal fractionation, late shield/postshield alkalic lavas have lower SiO2 contents and 206Pb/204Pb ratios than tholeiitic lavas except the highly altered sample 69KLPA-5A. Regression lines for East Molokai late shield/postshield lavas in the SiO2-206Pb/204Pb panel and all East Molokai lavas in the Nb/Zr versus 206Pb/204Pb and 87Sr/86Sr panels are also shown. (b and c) East Molokai lavas, late shield/postshield and rejuve- nated stage, define a negative trend in Nb/Zr versus 206Pb/204Pb and 87Sr/86Sr. Late shield/postshield lavas from West Maui and Mauna Kea overlap late shield/ postshield East Molokai lava. The postshield Kula and Hana Volcanics fromHaleakala span the gap between late shield/postshield and rejuvenated stage East Molokai lavas. Note that the negative trend for Nb/Zr and 206Pb/204Pb defined by late shield/postshield lavas contrasts markedly with the positive trend for Hawaiian shield stage lavas. Data sources for shield fields are the same as Figure 9d plus Chen et al. [1996], Garcia et al. [1993, 1995, 1998], Rhodes [1996], Norman and Garcia [1999], and Pietruszka and Garcia [1999]. Kohala lavas are not shown because there are noNb/Zr data for samples analyzed for isotopic ratios. Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 24 of 28stage lavas are 0.34–0.57 Ma [Clague et al., 1982] and postshield stage lavas are 1.35–1.49 Ma [McDougall, 1964]. This component lies on the upper boundary of the EPR (East Pacific Rise) MORB field in a Pb-Pb isotopic ratio plot, but it is distinct from the MORB field in a 87Sr/86Sr versus 206Pb/204Pb (Figures 9b and 9d). Frey et al. [2005] have argued that this depleted component is intrin- sic to the Hawaiian hot spot and not related to MORB-related lithosphere or asthenosphere. 8. Summary and Implications [43] Rejuvenated stage lavas from East Molokai are alkalic lavas with Sr and Nd isotope ratios that overlap fields for rejuvenated stage lavas from other Hawaiian volcanoes (e.g., Koloa Volcanics from Kauai). These lavas were derived by variable extents of melting of a depleted source, that is, a source with 143Nd/144Nd greater than and 87Sr/86Sr less than primitive mantle, with garnet as a residual phase. Rejuvenated stage lavas from Hawaiian volcanoes define linear trends of Zr/Sm, Ti/Eu and K/Ce versus Nb/La (Figure 14), and these ratios are correlated with SiO2 content (Figure 15). These trends reflect variable extents of melting, relatively low for the Honolulu Volcanics with low SiO2 and (Zr/Sm)PM < 1 and relatively high for submarine rejuvenated stage lavas from East Molokai which have higher SiO2 content and (Zr/Sm)PM  1. [44] Late shield/postshield lavas from East Molo- kai volcano include intercalated tholeiitic and alkalic basalt with a few highly evolved alkalic lavas, hawaiite to benmoreite. Where stratigraphic control is available, with decreasing eruption age alkalinity increases, 87Sr/86Sr decreases and there is an increasing role for residual garnet. East Molokai shares these geochemical features with three other Kea-trend volcanoes (Mauna Kea, Kohala and Haleakala); the temporal trend of 87Sr/86Sr at West Maui is more complex [e.g., Tatsumoto et al., 1987; Gaffney et al., 2004]. The change from tholeiitic to alkalic volcanism reflects a decreasing extent of melting as the volcano overrides the cooler outer parts of the plume. The increase in depth of melt segregation implied by an increasing role for residual garnet is not inferred from dynam- ical model of Ribe and Christensen [1999] for plume melting, but it is consistent with the prism shaped plume proposed by Lassiter et al. [1996, Figure 9], which shows that the depth of melt segregation is restricted to greater depths near the edge of the plume. [45] Late shield/postshield lavas from five Kea- trend volcanoes (Mauna Kea, Hawi Volcanics of Kohala, Kula and Hana Volcanics of Haleakala, West Maui and East Molokai) are variable in isotopic ratios of Sr, Nd and Pb, but they define overlapping fields in 87Sr/86Sr versus 143Nd/144Nd and have the low 208Pb/204Pb ratio at a given 206Pb/204Pb that is typical of Kea-trend volcanoes (Figures 8a and 9a). 87Sr/86Sr and 206Pb/204Pb in these lavas are inversely correlated with Nb/Zr (Figure 16). This trend can be explained by mixing of the Kea shield component (i.e., the relatively low 87Sr/86Sr, high 143Nd/144Nd, high 206Pb/204Pb end of range defined by Hawaiian shield lavas) with a depleted component that dominates rejuve- nated stage lavas. Compared to the Kea shield component, this component has lower 87Sr/86Sr, higher 143Nd/144Nd and lower 206Pb/204Pb, and is not MORB-related lithosphere or asthenosphere (Figure 9d) [Frey et al., 2005]. Rejuvenated stage and late shield/postshield lavas from East Molokai contain variable amounts of the same depleted component as previously inferred on the basis of relative magma volumes [Clague, 1987, p. 246]. [46] The similar temporal variations of radiogenic isotopic ratios in late shield/postshield lavas from the four Kea volcanoes (Mauna Kea, Kohala, Haleakala and East Molokai) are consistent with horizontal heterogeneity in the plume; that is, as the volcano moves off the hot spot a common depleted source component was sampled by each volcano. Moreover, the complete overlap in Sr-Nd- Pb isotopic ratios of the relatively young postshield Mauna Kea lavas (<350 ka) and older (1.5 Ma) shield/postshield East Molokai lavas are consistent with long-term vertical continuity of geochemically distinct stripes at the edge of the plume, as well as in the interior [see Eisele et al., 2003, Figure 13f]. An interesting question is, do Kea- and Loa-trend volcanoes sample the same depleted component as they move off the hot spot? Isotopic data are sparse for postshield lavas from Loa volcanoes, but postshield lavas from Hualalai retain their distinctive Loa Pb isotopic signature (Figure 9a). Apparently, the edge of the plume is not concentri- cally zoned; that is, the distinctive Loa- and Kea- trend signatures extend to the plume edges. Acknowledgments [47] The paper benefited from the review comments of M. Feigenson, Z.-Y. Ren, and editors Y. Tatsumi and W. White. We are indebted to J. G. Moore (U.S. Geological Survey), who, with M. H. Beeson, collected nearly all the samples used in this Geochemistry Geophysics Geosystems G3 xu et al.: kea-trend volcanoes 10.1029/2004GC000830 25 of 28study. Our trace element and isotopic study would not have been possible without their joint effort. 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