Vadim S. Kamenetsky a,b,), Anthony J. Crawford a, Stephen Eggins c, Richard Muhe d. Abstract

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1 Ž. Earth and Planetary Science Letters Phenocryst and melt inclusion chemistry of near-axis seamounts, Valu Fa Ridge, Lau Basin: insight into mantle wedge melting and the addition of subduction components Vadim S. Kamenetsky a,b,), Anthony J. Crawford a, Stephen Eggins c, Richard Muhe d a SRC for Ore Deposit Research, UniÕersity of Tasmania, GPO Box 5-79, Hobart, Tasmania 7001, Australia b Research School of Earth Sciences, Australian National UniÕersity, Canberra, ACT 000, Australia c ARC Key Centre for the Geochemistry and Metallogeny of the Continents, Department of Geology, Australian National UniÕersity, Canberra, ACT 000, Australia d Geologisch Palaeontologisches Institut und Museum der UniÕersitat Kiel, Olshausenstr , D-4118 Kiel, Germany Received 1 August 1996; accepted 1 July 1997 Abstract Phenocryst assemblages, and mineral and melt inclusion compositions of magmas erupted at near-axis seamounts on either side of Valu Fa Ridge provide a hitherto unprecedented insight into the complexity of magma generation in this back-arc basin tectonic setting. Two fundamentally different primitive primary melt compositions are identified based on melt inclusion compositions, olivine phenocryst chemistry, and the early co-crystallisation of either magnesian clinopyroxene Ž Mga to 93. or magnesian orthopyroxene Ž Mga to with magnesian olivine Ž to Fo. 94 and Cr-rich spinel Ž Cras One magma type is a H O-rich Ž ;.5 wt%., high-cao Ž ;14 wt%., low-al O Ž ;8 wt%. 3 magnesian basalt, variants of which occur in both the eastern and western seamounts. The other is a low-ca boninite-like magma that only occurs as a component of the western seamount magmas. Large and systematic variations in incompatible trace-element compositions of melt inclusions trapped in primitive olivine phenocrysts, reflect an integration of diverse but geochemically related melt fractions to produce the magmas at each seamount. Trace-element systematics require the variable addition of a LILE-, Pb-, and Cl-rich component to the mantle wedge source with increased influence toward the Tofua arc. This component, as invoked in most models of arc magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting ocean crust beneath the volcanic arc front. We propose that southward propagation of the back-arc basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting of refractory hydrated sub-arc lithosphere, probably veined by clinopyroxene-rich dykes in the case of the high-cao magma series. These near-ridge seamount lavas are very similar to those drilled at ODP Site 839 in the Lau Basin, and we suggest that the Site 839 basalts, as well as other Lau Basin ) Corresponding author. Present address: SRC for Ore Deposit Research, University of Tasmania, GPO Box 5-79, Hobart, Tasmania 7001, Australia. Fax: Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. Ž. PII S001-81X

2 06 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters seamount arc-like magmas, were produced from sub-arc lithosphere during southward propagation of the Eastern Lau Spreading Center ; 3 Ma. q 1997 Elsevier Science B.V. Keywords: Lau Basin; island arcs; magmas; melts; inclusions; boninite; trace elements 1. Introduction The back-arc basin basalts Ž BABB. compositional spectrum from normal mid-ocean ridge basalts ŽN- MORB. toward island arc tholeiites Ž IAT. is believed to reflect progressive addition of a H O-, LILE- Ž large-ion lithophile element., and LREE-rich Ž light rare-earth element. component derived from subducting ocean crust Že.g., wx. 1 to the back-arc mantle source. In several active back-arc basin systems, including the Lau Basin, the amount of added subduction component appears to increase with proxwx. The mechanism imity to the active volcanic arc responsible for adding the subduction component is poorly understood and could reflect: Ž. 1 advection of subduction-modified mantle from beneath the volcanic arc, Ž. direct injection of subduction-derived fluidrmelt into the back-arc basin source region, or Ž. 3 lateral migration of IAT melts from beneath the volcanic arc toward the region of back-arc magma generation. This study has sought to shed light on the nature of melt generation and the nature of the added subduction-derived components in back-arc basins via a detailed investigation of the phenocryst and melt inclusion chemistry of primitive basalts. Several appropriately primitive rocks were obtained from existing collections dredged from near-axis Ž - 8km. seamounts adjacent the Valu Fa spreading center in the southernmost Lau Basin wx 3. The results of this study reveal a striking level of complexity in processes of magma generation in this setting, including the nature of the mantle wedge source composition, the chemistry of added subduction-derived components, and the mechanism of partial melting.. Tectonic setting, sample location and petrography The Lau Basin is a southward propagating spreading system comprising several active spreading segments that are offset and approach the active Tofua arc toward the south ŽFig. 1; w4,5 x.. The southernmost spreading segment, the Valu Fa Ridge, is only 40 km west of the arc volcano, Ata, and is spreading at mmryr wx 6. The samples investigated in this study were obwx 7 and by tained by submersible NAUTILAU dredging during cruises 35 and 67 of the RrV Sonne w3,8,9 x ; 50 km north of the propagating tip of the Valu Fa Ridge. Samples NL17-3 Žherein E1. and SO35-18 Ž E. were recovered in m water depth on a seamount ;8 km east of the Valu Fa Ridge, at X S, X W. Samples SO35-98 Ž herein W1. and SO Ž W. were dredged from another seamount ;8 km to the west of the Valu Fa Ridge at X S, X W Ž Fig. 1. in m water depth. All samples lack obvious signs of alteration and ferromanganese crusts, and are believed to be very young. This is consistent with other near-axis seamounts in this area that erupt lavas of similar geochemistry, and which have marked UrTh disequilibria reflecting magma generation and eruption well within the last 350,000 years w10 x. The Valu Fa Ridge is erupting predominantly basaltic andesite compositions that have strong geow11 13 x. The off-axis seamount lavas also have well-de- chemical affinities to subduction zone magmas veloped subduction zone geochemical characteristics but are geochemically and mineralogy distinct from those erupted at Valu Fa Ridge Žw10 x, this study.. Fig. 1. Map of Lau Basin and Tonga Trench Tonga Ridge system and Lau Ridge showing the areas of the back-arc spreading after Sinha et al. wx 5. The inset shows details of the sample location sites adjacent to the Valu Fa Ridge Ž Eseastern seamount; Wswestern seamount.. NVFR, CVFR, SVFRsnorthern, central and southern Valu Fa Ridge, respectively. Water depths are in meters.

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4 08 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters Table 1 Major-element compositions of studied rocks Sample E1 W1 W SiO TiO Al O FeO FeO MnO MgO CaO Na O K O PO H O Total CaOrAl O Samples E1, W1 and W are vesicular, porphyritic pillow basalts Ž Table 1., W1 and W having glassy pillow rims Ž Table 3, NN1 3.. Sample W is olivineq clinopyroxene-phyric, whereas E1 and W1 are olivineq clinopyroxeneq plagioclase-phyric. In all three samples olivine is the dominant phenocryst phase, occurring as euhedral crystals up to 5 mm long. Olivine and clinopyroxene phenocrysts host chromite inclusions, as well as melt and low-density fluid inclusions. The groundmass of these samples consists of devitrified glass, olivine, clinopyroxene, plagioclase and Ti-magnetite. Sample E is a cognate wehrlite nodule composed of large Ž 3 5 mm. clinopyroxene and olivine crystals, minor plagioclase, and interstitial glass. All crystals in the wehrlite contain glassy melt inclusions..1. Mineral chemistry Olivine phenocrysts in samples E1 and E span a large range of primitive compositions Ž Fo Note that all olivines including those with Fo)88 have low NiO contents Ž ; 0.1 wt%. that are unusual for such primitive olivine compositions crystallising from mantle-derived magmas. Their CaO contents are high, similar to olivines in some primitive island arc tholeiite suites Ž Fig. 3b., and decrease with fractionation Ž0.36 to 0.3 wt% CaO; Fig. 3a and b.. Olivine phenocrysts in samples W1 and W are also primitive compositions, in the range Fo but they have high NiO contents Ž wt%. and very variable CaO contents Ž wt%.. Olivine with unusually low CaO contents Ž - 0. wt%. dominate the phenocryst population with Fo)91, and a dispersed trend toward increasing CaO contents occurs with decreasing Fo Ž Fig. 3b.. The low-cao olivine phenocrysts are not xenocrysts, as they contain both tiny crystalline inclusions of low-ca pyroxene and Cr-spinel, and melt inclusions. Similar low- CaO Fo-rich olivine has only been reported from primitive low-ca boninite magmas Ž Fig. 3b.. The wide dispersion of CaO contents in the most primitive olivine in the western seamount samples might reflect mixing between a magma crystallising high- CaO olivine, with similarities to the E1 and E lavas, and a boninitic magma crystallising low-cao olivine. The high NiO contents and compositional trend of the western seamount olivines are consistent with differentiation of melts derived from mantle peridotite, although the observed dispersion along the trend is beyond analytical uncertainty and probably also reflects magma mixing. Clinopyroxene phenocrysts and inclusions in olivine Ž Table, NN1 5; Table 3, NN11, 1. have high Mga Ž85 93, and in samples E1, E and W1rW, respectively., high CaO Ž wt%. and Cr O contents Ž 3 up to 0.6 wt% in E1 and E, and up to 1.% in W1 and W., and low Al O Ž wt%., TiO Ž wt%. 3 and Na O contents Ž wt%.. These compositions are characteristic of primitive clinopyroxene crystallising in arc lavas at low pressure w16,17 x. Orthopyroxene Ž Mga occurs only as small inclusions in low-cao Ž %. olivine Fo in samples W1 and W Ž Fig. b d; Table, NN Orthopyroxene compositions ŽMga, CaO contents. correlate well with those of host olivines Ž Fo and CaO contents.. The orthopyroxene inclusions are often surrounded by glass containing a fluid bubble, and themselves also often contain tiny melt inclusions, attesting to their magmatic origin Ž Fig. b d.. Euhedral inclusions of chromite are common in both olivine Ž Table, NN1 15. and clinopyroxene phenocrysts in all samples. They have high Cra Ž and low TiO contents Ž %., and are typical of spinels in primitive arc basalts and boninites w15,18 x. Due to their small size, no reliable

5 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters analyses of spinels in the low-cao olivines were obtained. Highly calcic plagioclase phenocrysts Ž An are associated with relatively evolved olivine Ž Fo ; 80. and clinopyroxene Ž Mga ;77 83., and are host to broadly andesitic melt inclusions. These plagioclases can be equated with those found in hydrous Ca-rich arc magmas elsewhere w19,0 x... Melt inclusions Melt inclusions up to 300 m in diameter occur in phenocrysts as either glass patches with one or more bubbles Ž Fig. e and f., or as complex aggregates of residual glass and quench crystals. Occasional spherical inclusions of Fe Ni-sulfide have also been observed. Complex melt inclusions comprising glass q orthopyroxeneq bubble occur in low-ca olivine in samples W1 and W Ž Fig. b d. whereas glass q clinopyroxeneq bubble inclusions occur in high-ca olivine Ž Fig. e. in all samples. Partially crystalline inclusions have been homogenised and quenched to glass using a high-temperature heating stage w1x prior to analysis along with the naturally glassy inclusions. For some inclusions the melting of daughter crystals was accompanied by the disappear- Table Representative compositions of olivine and clinopyroxene phenocrysts, and associated minerals trapped as inclusions in phenocrysts Sample W W W W1 W1 W1 W1 W W W W E1 E W1 W Olivine h h h i i h h h h h h h h h h SiO FeO MnO n.d MgO CaO NiO n.d CrO n.d Total Fo ) i-cpx i-cpx i-cpx h-cpx h-cpx i-opx i-opx i-opx i-opx i-opx i-opx i-sp i-sp i-sp i-sp SiO TiO Al O $ Fe O 3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d FeO MnO MgO CaO n.d. n.d. n.d. n.d. Na O n.d. n.d. n.d. n.d. CrO Total Mga Cra Minerals and glasses Ž see also Table 3. were analysed by a Cameca SX50 electron microprobe in the University of Tasmania ŽHobart, Australia. under routine operating conditions. 1 3, 6 11 and 1 15 olivine with clinopyroxene, orthopyroxene and spinel inclusions, respectively; 4, 5 clinopyroxene with olivine inclusions; ph, h, isphenocryst, host and inclusion, respectively. Fo, Mga and Ž q Cra forsterite, Mgr MgqFe. and CrrŽ CrqAl., respectively Ž in mole%.. n.d.snot determined. $ Calculated on the basis of stoichiometry. ) Orthopyroxene inclusion associated with boninite-like glass Ž Table 3, N15..

6 10 Table 3 Compositions of matrix and pillow-rim glasses and representative melt inclusions in olivine and clinopyroxene SampleW1 W E E1 E1 E1 E E E E E E W1 W1 W1 W W W Grain r r r r6 4r3 cpx cpx SiO FeO MgO CaO NiO CrO Total Ž. Fo Mga T h T q Glasses Melt inclusions SiO TiO Al O FeO MnO MgO CaO Na O K O PO Total V. S. Kamenetsky et al.rearth and Planetary Science Letters 151 ( 1997 ) 05 3 S n.d n.d. n.d n.d n.d. n.d. n.d. n.d Cl n.d n.d. n.d n.d n.d. n.d. n.d. n.d H O n.d..4 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. CaOr Al O 3

7 B n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Sc n.d Ti Rb b.d.l Sr Y Zr Nb Ba La Ce Pr n.d. n.d. n.d Nd Sm Eu Gd Dy Er Yb Lu n.d n.d n.d n.d. n.d Hf Ta b.d.l Pb n.d Th b.d.l U , and 3 pillow-rim and matrix glasses, respectively; 4 10, melt inclusions trapped in olivine; 11, 1 melt inclusions trapped in clinopyroxene Žin wt%, TiO 0.09 and 0.05, Al O 0.67 and 0.86, Na O 0.06 and 0.10 in host clinopyroxene phenocrysts NN11 and 1, respectively; 15 melt associated with orthopyroxene Ž Table, N6. 3. Ž q T stemperature of homogenisation; T stemperature of quenching if homogenisation was not achieved. Fo, Mga forsterite content of olivine and Mgr MgqFe. h q of clinopyroxene, in mole%. n.d.snot determined; b.d.l.sbelow detection limit. H O in glasses was determined by infrared spectroscopy in the University of Tasmania Ž Hobart, Australia.; see Ref. w14x for the details of technique. Trace elements in ppm. Trace elements were analysed by Ar F excimer laser ablation ICPMS at RSES, ANU Ž Canberra. using a range of spot diameter from 5 to 75 mm, depending on the melt inclusion size. Instrument was calibrated using NIST61 and 43 Ca as the internal standard. Typical analytical precision Ž s. is -% for Ti, Sr and Ba; 5% for Sc, V, Y, Zr, Nb, La and Ce; 5 10% for Ga, Rb, Nd, Eu, Gd, Er, Yb, Hf, Ta and Th; and 10 15% for Sm, Lu, Pb and U. V. S. Kamenetsky et al.rearth and Planetary Science Letters 151 ( 1997 )

8 1 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters ance of bubbles Ž i.e., complete homogenisation. at temperatures between 1108 and 1308C. No correlation was found between the homogenisation temperatures and host olivine composition. This may indicate the overheating of some inclusions, resulting in homogenisation temperature above true liquidus temperatures. Consequently, the subset of lowest homogenisation temperatures for a given olivine composition Že.g., T s 1308C for melt inclusion in olivine Fo ; Table 3, N gives our preferred estimates of liquidus temperatures. Representative analyses of both glassy and homogenised melt inclusions are listed in Table 3. All melt inclusions are basaltic with the homogenised melt inclusions being the most magnesian. Their low analysis totals Ž av. 97.1"1. wt%. are due to high dissolved H O contents, as confirmed by infrared spectroscopy measurements of matrix glasses and a

9 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters small number of large melt inclusions, for which H O contents range between ; 1.4 and.4 wt% Ž Table 3.. The melt inclusions in samples E1 and E display a systematic decrease in CaOrAlO3 values Ž 1.85 to 0.7 and 1.5 to 0.9, respectively. as host olivines evolve toward more Fe-rich compositions Ž Fig. 3c.. This trend requires the early fractionation of a CaO-rich phase and is consistent with evidence for the co-crystallisation of highly magnesian clinopyroxene and olivine in these samples. Melt inclusions in primitive olivine Ž Fo. ) 91 from the western seamount samples span a similarly large but lower range in CaOrAl O values Ž , Fig. 3c.. This is also consistent with the mixing of a primitive boninitic low-cao magma, represented by melt inclusions trapped together with orthopyroxene in low-ca olivine Ž Table 3, N15., and an equally primitive high-cao Ž ; wt%., low-al O Ž 3 ; 8 10 wt%. basaltic magma Že.g., Table 3, NN4 6, 14.. Sulfur and chlorine concentrations in matrix glasses and melt inclusions have been measured by electron-probe Ž Table 3.. Chlorine concentrations are high, and similar to other subduction-related magmas Že.g., w x, our unpublished data.. Both sulfur and chlorine contents are generally higher in the western seamount ŽSs0.08"0.03 wt%; Cls0.0"0.04 wt%. than the eastern seamount samples ŽS s 0.05 "0.01 wt%; Cls0.1"0.0 wt%.. Melt inclusion S abundances significantly exceed those of the pillowrmatrix glasses Ž ; 0.0 wt%., and tend to correlate positively with the composition of the host olivine Ž not shown.. This could reflect sulfur loss during differentiation due to sulfide fractionation andror volatile degassing. Only one melt inclusion occurring in low-ca olivine Ž Fig. d. was sufficiently large for analysis by electron microprobe and by laser ablation ICP MS. This melt inclusion Ž Table 3, N15; Fig. 4. occurs in sample W1 and is associated with an orthopyroxene inclusion Ž Table, N6. in olivine Fo 9.. Its composition is broadly boninitic, with high SiO Ž 57.9 wt%., low CaOrAl O Ž and extremely low K O and PO5 contents Ž 0.01 and 0.0 wt%, respectively...3. Trace-element geochemistry of matrix glasses and melt inclusions Concentrations of incompatible trace elements in matrix Ž pillow rim. glasses and melt inclusions in high-ca olivine and clinopyroxene, have been measured by laser ablation ICP MS. These data are listed in Table 3, along with relevant analysis details, and are summarised as primitive mantle-normalised diagrams in Fig. 4. The melt inclusions and matrix glasses from either seamount are notable for their virtually identical element abundance patterns, with the obvious exception of the low-ca melt inclusion from the western seamount. This strong geochemical similarity is significant because it indicates: Ž. 1 the melt inclusions and phenocrysts are genetically related to their host magmas; and Ž. that the melt inclusions have accurately sampled the bulk melt geochemistry from which the phenocrysts were crystallising. The trace-element abundance patterns of the matrix glasses and melt inclusions from both the eastern and western seamounts have well-developed highfield-strength element Ž HFSE Nb, Ta, Zr, Hf, Ti. Fig.. Examples of magmatic inclusions trapped in olivine. Ž. a A fragment of low-ca olivine in sample W with a planar zone of tiny inclusions of melt, fluid, pyroxene and spinel. Ž. b A zone of complex Ž orthopyroxeneqglassqbubble. inclusions in low-ca olivine in sample W. Note the occurrence of small melt inclusions in the cores of orthopyroxene inclusions. Ž. c Euhedral inclusion of orthopyroxene combined with glass trapped in low-ca olivine in sample W. The orthopyroxene itself contains melt inclusions Ž glassqbubbleqsilicate daughter crystals., which provides evidence for a magmatic origin of the host pyroxene and host olivine. Ž. d Large complex Ž euhedral orthopyroxeneqglassqbubble. inclusion within a zone of tiny magmatic inclusions in low-ca olivine in sample W1. The orthopyroxene itself contains very small melt inclusions, which are distributed evenly within the crystals interior, and outline a relict crystal melt interface. Glass from an identical inclusion has been analysed for major and trace elements Ž Table 3, N15.. Ž. e Glass melt inclusion with a bubble Ž hole. and trapped crystal of clinopyroxene in high-ca olivine in sample E1. Ž. f Typical glassy melt inclusion with numerous small bubbles and larger bubble next to a Cr-spinel crystal within high-ca olivine in sample E.

10 14 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters

11 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters Fig. 4. Primitive mantle normalised incompatible trace-element abundances in western and eastern seamount glasses. Circles joined by thick lines are matrix Ž pillow rim. glasses and thin lines are melt inclusions. The boninitic melt inclusion in the western seamount is denoted by crosses joined by thin lines. depletions and Th, U, and large-ion lithophile element Ž LILE Rb, Ba, K, Sr. enrichment, as well as Pb enrichment in the eastern seamount, relative to rare-earth elements Ž REE. of similar incompatibility Ž Fig. 4.. These specific enrichments and depletions are typical of subduction zone magmas, including those of the nearby Tofua arc. They are widely attributed to the selective addition of LILE) LREE ) HFSE by melt andror fluid derived from subducting oceanic crust Že.g., w4,5 x.. The melt inclusion occurring in low-ca olivine from the western seamount is distinct from all the Fig. 3. Compositional trends in olivine phenocrysts Ž NiO, CaO, in wt%. and between melt inclusions Ž CaOrAl O. and host olivines. In Ž b. 3 fields for olivine from low-ca boninites ŽCape Vogel, Papua New Guinea; Setouchi volcanic belt, Japan; Howqua, Victoria, Australia; New Caledonia. and island arc tholeiites Ž IAT: Hunter Ridge Hunter Fracture Zone. are based on our unpublished data and data from Sigurdsson et al. w15 x. In Ž. c the field for melt inclusions from Cape Vogel low-ca boninites is based on our unpublished data.

12 16 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters other seamount melts compositions. It is set apart by its strong LREE depletion wž LarYb. s 0.7 x N, and lower incompatible-element abundance levels, and low Sc Ž16.5 ppm cf. ) 30 ppm in other melt inclusions. Ž Fig. 4; Table 3, N15.. The very low HFSE, HREE and Sc levels in this melt inclusion Fig. 5. Bivariate, element and LarYb vs. La concentration plots for eastern and western seamount glasses Ž matrix and melt inclusions. compared to lava compositions reported in the literature for the Valu Fa Ridge, Tofua Arc, and the Central and Eastern Lau Spreading Centres Ždata from w,10 13,5,7 x..

13 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters Fig. 6. Selected element ratio vs. LarYb plots, comparing eastern and western seamount glasses Ž matrix and melt inclusions. and lava compositions reported in the literature for the Valu Fa Ridge, Tofua Arc, and the Central and Eastern Lau Spreading Centres Ždata from w,10 13,5,7 x.. The illustrated element ratios Ž except LarYb. are unlikely to be fractionated from the mantle source value by partial melting. All show considerable deviations from typical mantle values, including that of the regional convecting upper mantle Žas indicated by the mantle wedge field which has been based on Central and Eastern Lau Spreading Centre compositions clustering near the average N-MORB composition w3 x.. Fields labelled A and B denote approximate end-member compositions on the seamount glass data arrays. A is believed to be an LILE-, Pb- and Cl-rich aqueous fluid.

14 18 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters indicate a source peridotite at least as refractory as the source of low-ca boninites. However unlike the low-ca boninites, this depleted melt inclusion shows no LILE or Zr enrichment. A puzzling feature is its marked positive Ti anomaly, for which we have no explanation. Similar melt inclusion compositions, also associated with low-ca pyroxene inclusions, have been observed in primitive low-ca olivines from ultramafic high-k lavas in Kamchatka w6 x. Though their geochemical patterns are broadly similar, key differences occur in the trace-element geochemistry of melt compositions from the two seamounts. Bivariate element abundance plots in Fig. 5 elucidate these differences further. The western seamount melts have distinctly higher abundances of certain highly incompatible elements, including the LREE and Sr, and generally higher abundances of many others, with the exception of Pb, K, and Ba. They also have more LREE-enriched REE patterns Ž not shown.. A comparison with lavas of the Tofua arc, Valu Fa Ridge and the Central and Eastern Lau Spreading Centres is also made in Fig. 5 and reveals the seamount melt compositions to be the most LILE-rich Ž particularly Ba. and HFSE- ŽNb, Zr, Hf, Ti., HREE- and Y-depleted in the region. The Tofua arc lavas display the closest similarities to the seamount compositions, and particularly to those of the eastern seamount. Large Ž up to ten-fold. variations occur in the incompatible-element abundances of melt inclusions from the individual seamounts. These variations are highly correlated, particularly between the more incompatible trace elements ŽHFSE, LREE, Ba, Th, U, K, Cl, Pb, Sr., forming extended linear arrays ŽFig. 5.. The eastern and western seamount compositions typically form separate sub-parallel or intersecting arrays that do not always regress linearly through the origin. Non-zero intercepts are observed for certain element combinations that include, but are not restricted to those shown for Ba, Pb, K, and the less incompatible trace elements Ž e.g., Y and Ti. vs. La. The failure of arrays to pass through the origin indicates that the element ratios must change systematically with varying abundance levels, as shown for LarYb in Fig. 6. It also precludes the arrays from being an artefact of dilutionrenrichment trends resulting from crystallisationrdissolution of the melt inclusionrhost olivine. In fact, this possibility is also ruled out by the comparatively small amounts Ž wt%. of differentiation undergone by the melt inclusions. In short, the trace-element variability cannot be attributed to shallow-level magmatic differentiation or to post-entrapment modification of the melt inclusions. The most plausible explanation is that the arrays reflect primary compositional variability produced during partial melting of the mantle sources of the two seamounts. Fig. 6 shows a series of plots of incompatible-element ratios vs. LarYb. The latter ratio varies systematically as a function of partial melting, and for a given source composition, plots of similarly incompatible elements such as CerPb, KrLa, BarLa should define essentially horizontal arrays against LarYb. However, Fig. 6 shows that most such ratios vary significantly and systematically with changing LarYb. Note that neither end-member A nor B, defined by the spectrum of melt inclusion compositions, plots close to the MORB field, as denoted for the Lau Basin by data from the Central and Eastern Lau Spreading Centres with minimal or no contribution from the subducted slab. Although two separate compositional arrays appear to be present in some cases Ž e.g., KrLa and BarLa plots; Fig. 6., the western seamount melts always plot near the extension of the more prominent eastern seamount array. In all cases, the principal compositional array formed by the melt inclusions bypasses and does not trend toward the mantle wedge field. However, the melt inclusion array is linked to the mantle wedge field by a more or less orthogonal array of compositions comprising the Tofua arc, Valu Fa Ridge and some Central and Eastern Lau Spreading Center lavas. The Tofua arc samples are notable for clustering on or toward the LREE-depleted end of the seamount array Ž Field A., whereas the Valu Fa samples tend, in some cases, to plot toward an intermediate position along the A B array. 3. Discussion 3.1. Nature of the mantle wedge source The temperature of the mantle wedge source region can be constrained from liquidus temperatures

15 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters of the primitive seamount magmas. Our best estimates of liquidus temperature based on melt inclusion homogenisation temperatures are of the order of ;1308C for a melt inclusion in Fo Likewise, the dry crystallisation temperature of the most magnesian olivine can be calculated using the model of Ford et al. w8x and allowing for a reduction of between 408 and 808C for wt% dissolved H O w9 x, and is 1300" 08C. These values are consistent with the crystallisation temperatures of primitive MORB w30,31x and support the notion that the mantle wedge temperature is similar to that of the N-MORB source and within an acceptable range of estimates for the upper-mantle potential temperature. The incompatible trace-element geochemistry and phenocryst assemblages and chemistry of the seamount magmas are characteristic of subduction zone magmas sensu stricto. The magnesian olivine Ž to Fo. and Cr-rich spinel compositions Ž Cra) of the Valu Fa seamounts are notably more extreme than those found in MORB, and also BABB with N-MORB geochemistry, where olivines are always less magnesian than Fo9 and usually Fo - 90, and chromites seldom have higher Cra values than 60 wdick, 1984 a16 x. These particular characteristics implicate the melting of refractory mantle wedge peridotite compositions. This is also consistent with the low HFSE abundances of the matrix glasses and melt inclusions compared to the composition of N- MORB and to BABB from the Central and Eastern 3.. Origin of high-ca melt compositions? Lau Spreading Centres Ž Fig. 5. and with certain incompatible HFSE ratios Že.g., NbrZr, ZrrTi; Fig. 6. that indicate the melting of a mantle source more depleted than the N-MORB source. Similar refractory and incompatible element-depleted mantle compositions have been widely implied from arc mag- mas based on similar evidence w3,33 x. The origin of these depleted mantle sources compositions is widely attributed to prior melting of the mantle wedge. The high-cao melt inclusion compositions ŽTable 3; Fig. 3c. contrast with those of the bulk-rock and matrix glass compositions Ž Tables 1 and 3.. Similar primitive high-cao magmas with CaOrAlO3 values )1 are unusual but do occur in some western Pacific island arcs, including Vanuatu w16,34 x, Kamw6 x., and Indonesia w35 x. However, some of chatka these rocks are often strongly clinopyroxene-phyric Ž ankaramites., and some uncertainty surrounds whether or not their compositions are the product of clinopyroxene phenocryst accumulation. The existence of melts with high CaOrAlO3 compositions is unequivocal in the case Valu Fa near-axis seamounts, but this then presents a dilemma as to their origin. Melts with CaOrAlO 3) 1 are not typical mantle-derived compositions, nor are they products of peridotite melting experiments, where it is well established that CaOrAlO3 values increase with degree of partial melting but only up to the peridotite bulk composition value, which is ; 0.8 for typical mantle compositions Že.g., w36 x.. The CaOrAlO3 values of the melt inclusions do not correlate with trace-element parameters that could be anticipated to vary with degree of melting Žnot shown., nor with other major-element variables that are known to vary with extent of peridotite melting, such as SiO and FeO contents Že.g., w37 x.. However, this is not surprising as such systematics are likely to be masked by the effects of early clinopyroxene crystallisation, as indicated by the differentiation controlled correlation between olivine Fo content and melt inclusion CaOrAl O values Ž Fig. 3c. 3, and also post-entrapment modification of melt inclusion FeO and SiO compositions by olivine growth on the inclusion walls and subsequent re-equilibration. A clue as to the origin of the high CaOrAlO3 melt compositions may exist in the remarkably low NiO contents of the most primitive olivine phenocrysts from the eastern seamount Ž Fig. 3a., which equate to a primitive equilibrium melt composition with only ppm Ni Žusing the K from w38 x. d. We propose that a possible solution to these enigmatic chemical compositions may exist in the melting of a clinopyroxene-rich and Ni-Ž olivine. -poor source material. This might be a wehrlitic mantle source Že.g., w34 x., or clinopyroxene-rich veins or layers, similar to those observed in supra-subduction zone ophiow39 x. Because peridotite veiningrlayering lites by wehrlites is common in arcrfore-arc lithosphere, the source rocks of the eastern seamount high-ca magmas might be remnants of the Lau Ridge fore-arc and sub-arc lithosphere currently being invaded and wx split by the propagating Valu Fa Ridge 4.

16 0 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters Origin of the boninite-like component? The microscopic evidence for involvement of a boninitic magma component in the western seamount is compelling despite the lack of macroscopic evidence. The relatively high proportion of olivine phenocrysts with low CaO Ž - 0.1%, Fig. 3b. in samples W1 Ž ; 38%. and W Ž ; 9%., and which also occasionally contain magnesian orthopyroxene inclusions Ž Fig. b d; Table., indicates that the boninitic component may constitute a significant portion Ž0 40%. of the bulk magma. We noted above that the occurrence of low-ca olivines is restricted to low-ca boninites, which are widely believed to erupt exclusively in fore-arc settings based on Eocene localities in the Western Pacific w18 x. Boninites are also recorded in several back-arc basin settings, all from seamounts in close proximity to spreading centres w15,40,41 x; however, these back-arc basin-located boninites are all high-ca boninites, and have olivine with generally higher CaO contents Ž wt%.. A general model to explain the petrogenesis of these boninites w18,4,43x involves emplacement of asthenospheric MORB source mantle into hydrated sub-arc lithosphere, leading to partial melting of the latter. We propose that juxtaposition of hot asthenospheric mantle, such as is currently supplying the well-established Central and Eastern Lau Spreading Centres, against hydrated shallow sub-lau Ridge arc lithosphere in the mantle wedge, triggered partial melting, first of the more fusible wehrlitic veins, and subsequently of the extremely refractory residue and host peridotite of the wehrlite veins. The high-ca magmas were derived from wehrlite melting, the highly depleted boninite-like melt component was produced by partial melting of the residue and perhaps adjacent refractory peridotite. Mixing of these magmatic components occurred in conduits of small magma chambers beneath the seamount, and gave rise to the spectrum of compositions observed ŽFig. 3b and c Melting process and the added subduction zone component The variable incompatible-element ratios and large variations in concentration levels of the near-axis Valu Fa Ridge seamount melt compositions are most reasonably explained by primary diversity produced during partial melting of the mantle source. This interpretation is consistent with the relatively undifferentiated nature of the melt inclusions and their primitive host crystals, and the equally implausible possibility of producing such large compositional variations by shallow-level differentiation process. We suggest that the melt inclusions are snapshots of diverse primitiverprimary melt compositions that have been aggregated and mixed to produce the magmarlava composition erupted at the different seamounts. This interpretation is further supported by the observation that the matrix glass compositions closely approximate a mean for the melt inclusion compositions from each seamount Ž Fig. 4.. It also has significant ramifications for the mechanisms of melt generation and segregation within the mantle wedge. This large range in incompatible-element abundances might suggest a near-fractional melting process. However, even this melting process is unlikely to be able to produce the magnitude and systematics of the variations observed for certain equally incompatible trace-element ratios Že.g., BarRb, CerPb, BarLa, KrLa, ClrLa.. The local domains from which these compositionally diverse melt fractions are generated must possess these distinctive but systematically varying trace-element characteristics. We can suggest several relatively simple melting scenarios that could produce the required systematics within a tectonically consistent framework of adding a H O-, LILE-, Pb-, and Cl-rich subduction-derived component to the mantle wedge. Ž. 1 Spatial or temporal variation in the amount of the H O- and LILE-rich subduction component to the mantle wedge fluxes the degree of melting as it progressively enriches the mantle source composition in LILE, Pb and Cl. Alternatively, Ž. uniform addition of the same subduction-derived component to a variably depleted mantle source region would produce identical geochemical systematics, with the subduction component dominating the chemistry of melts derived from the more depleted source regions. It would follow that the change in trace-element chem- istry from the western to the eastern seamount Žfrom B to A in Fig. 6., and the concurrence of the Tofua arc lavas with the eastern seamount compositions,

17 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters which have the lowest LarYb and largest relative LILE, Pb and Cl enrichment Že.g., highest BarLa, PbrCe, ClrLa., could reflect a spatial increase in the amount of added subduction component toward the volcanic arc andror an increase in the degree of melting and mantle source depletion toward the volcanic arc. The chemical characteristics of the added subduction component in either case would be appropriate for component A in Fig. 6. The geochemical characteristics of A might be ascribed to an aqueous fluid, based on its LILE-, Cl-, and Pb-rich 4 LREE4 HFSE-poor characteristics. The release of this component into the mantle wedge appears to dominate at the Tofua volcanic arc, and in some eastern seamount melt inclusions, but diminishes toward the back-arc. Interestingly, the Valu Fa Ridge tend to plot towards an intermediate position along the A B array, suggesting it is affected to lesser extent than the eastern seamount source but to greater than the western seamount source. The identity and origin of component B is perhaps more obscure. Silicate melts of broadly tonalitic composition, derived from partial melting of the subducted slab, are implicated in the genesis of boninites w44 x. It may be that the boninite-like component identified only in the western seamount is genetically related to the interaction of mantle peridotite with a slab-derived melt, and we note that the high LarYb and SrrY Ž )30, Table 3. values are appropriate for postulated slab melt compositions. We interpret the seamount magmas to be products of melting shallow, hydrated supra-subduction zone lithosphere due to conductive heating and decompression caused by entrainment into upwelling MORB-source mantle of the developing Valu Fa Ridge, that eventually rifts the fore-arcrarc to initiw45x suggested a ate a back-arc basin. Falloon et al. similar model for primitive arc-like lavas forming seamounts in the Lau Basin. As the spreading ridge propagated through the pre-existing hydrated arc lithosphere, the initial invasion of MORB-source peridotite heated and partially melted the pre-existing lithosphere at relatively shallow levels ŽP- 10 kbar.. With continued spreading enabling decompression of MORB-source mantle to sub-crustal depths, magma generation increasingly became more MORB-like at the expense of arc-lithosphere derived components. This model is able to plausibly explain a gradual change in crustal style and composition to true oceanic crust, such as that characterising the mature Lau Basin farther north than the Valu Fa Ridge. Finally, we note that lavas compositionally and mineralogically very close to those reported herein were drilled farther north in the Lau Basin at ODP Site 839 Ž Leg 135., at a location where the crust changes from older horst and graben-style, to younger, true back-arc basin oceanic crust. Ewart et al. w5x suggested that the Site 839 lavas may have been produced in an arc volcanic edifice that was later split by southward propagation of the Eastern Lau Spreading Center. Lines of such seamounts, and their unusual arc-like primitive magmas, may record the progress of the propagating tip of back-arc spreading centers in supra-subduction zone settings. Acknowledgements We thank Maya Kamenetsky and Les Kinsley for technical assistance, and Leonid Danyushevsky for useful comments and making his program Petrolog available to us. J.W. Hawkins, C.H.Langmuir, R.L. Nielsen and an anonymous reviewer are thanked for their reviews and suggestions that have improved the original manuscript. This work and VSK are supported by an ARC Large Grant to AJC and funding from RSES, ANU. AJC and SME acknowledge an ARC Senior Fellowship and Research Fellowships, respectively. RM acknowledges funding by the BMFT, Federal Ministry for Research and Technology and the DFG, German Science Foundation Žgrants BMFT-FK 03R399C and DFG Sta 81r1-1..[ CL] References wx 1 E. Stolper, S. Newman, The role of water in the petrogenesis of Mariana Trough magmas, Earth Planet. Sci. Lett. 11 Ž wx J.A. Pearce, M. Ernewein, S.H. Bloomer, L.M. Parson, B.J. Murton, L.E. Johnson, Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity, in: J.L. Smellie Ž Ed.., Volcanism Associated with Extension at Consumed Plate Margins, Geol. Soc. London, Spec. Publ. 81 Ž wx 3 G. Frenzel, R. Muhe, P. Stoffers, Petrology of the volcanic

18 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters rocks from the Lau Basin, southwest Pacific, Geol. Jahrb. 9 Ž wx 4 J.W. Hawkins, Evolution of the Lau Basin Insights from ODP Leg 135, in: B. Taylor, J.P. Natland Ž Eds.., Active Margins and Marginal Basins of the Western Pacific, Am. Geophys. Union, Washington, DC, Geophys. Monogr. 88 Ž wx 5 M.C. Sinha, Segmentation and rift propagation at the Valu Fa ridge, Lau Basin: Evidence from gravity data, J. Geophys. Res. 100 Ž wx 6 M. Bevis, F.M. Taylor, B.E. Schutz, J. Recy, B.L. Isacks, S. Helu, R. Singh, E. Kendrick, J. Stowell, B. Taylor, S. Calmant, Geodetic observations of very rapid convergence and back-arc extension at the Tonga arc, Nature Ž London. 374 Ž wx 7 Y. Fouquet, U. von Stackelberg, J.L. Charlou, J.P. Donval, J. Erzinger, J.P. Foucher, P. Herzig, R. Muhe, S. Soakai, M. Wiedicke, H. Whitechurch, Hydrothermal activity and metallogenesis in the Lau back-arc basin, Nature Ž London. 349 Ž wx 8 G. Sunkel, Origin of petrological and geochemical variations of Lau Basin lavas Ž SW Pacific., Mar. Min. 9 Ž wx 9 U. von Stackelberg, U. von Rad, V. Riech, SONNE cruise SO-35 in the Lau and north Fiji basins, southwest Pacific Ocean, Geol. Jahrb. 9 Ž w10x G. Loock, Character and distribution of the Indian Ocean domain: a study of the mantle source compositions of Lau Basin volcanics Ž SW Pacific. and Indian Ocean mid-ocean ridge basalts, Thesis, Univ. Koln, 199. w11x G.A. Jenner, P.A. Cawood, M. Rautenschlein, W.M. White, Composition of back-arc basin volcanics, Valu Fa ridge, Lau Basin: evidence for a slab-derived component in their mantle source, J. Volcanol. Geotherm. Res. 3 Ž w1x T.L. Vallier, G.A. Jenner, F.A. Frey, J.B. Gill, A.S. Davis, A.M. Volpe, J.M. Hawkins, J.D. Morris, P.A. Cawood, J.L. Morton, D.W. Scholl, M. Rautenschlein, W.M. White, R.W. Williams, A.J. Stevenson, L.D. White, Subalkaline andesite from Valu Fa Ridge, a back-arc spreading center in southern Lau Basin: petrogenesis, comparative chemistry, and tectonic implications, Chem. Geol. 91 Ž w13x X. Boespflug, L. Dosso, H. Bougault, J.-L. Joron, Trace element and isotopic Ž Sr and Nd. geochemistry of volcanic rocks from the Lau Basin, Geol. Jahrb., Reihe D 9 Ž w14x L.V. Danyushevsky, T.J. Falloon, A.V. Sobolev, A.J. Crawford, M. Carroll, R.C. Price, The H O content of basalt glasses from Southwest Pacific back-arc basins, Earth Planet. Sci. Lett. 117 Ž w15x I.A. Sigurdsson, V.S. Kamenetsky, A.J. Crawford, S.M. Eggins, S.K. Zlobin, Primitive island arc and oceanic lavas from the Hunter ridge-hunter fracture zone Evidence from glass, olivine and spinel compositions, Mineral. Petrol. 47 Ž w16x M. Barsdell, Petrology and petrogenesis of clinopyroxene-rich tholeiitic lavas, Merelava volcano, Vanuatu, J. Petrol. 9 Ž w17x S.M. Eggins, Origin and differentiation of picritic arc magmas, Ambae Ž Aoba., Vanuatu, Contrib. Mineral. Petrol. 114 Ž w18x A.J. Crawford, T.J. Falloon, D.H. Green, Classification, petrogenesis and tectonic setting of boninites, in: A.J. Crawford Ž Ed.. Boninites, Unwin Hyman, London, 1989, pp w19x T.W. Sisson, T.L. Grove, Experimental investigations of the role of H O in calc-alkaline differentiation and subduction zone magmatism, Contrib. Mineral. Petrol. 113 Ž w0x Y. Panjasawatwong, L.V. Danyushevsky, A.J. Crawford, K.L. Harris, An experimental study of the effects of melt composition on plagioclase melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-an plagioclase, Contrib. Mineral. Petrol. 118 Ž w1x A.V. Sobolev, L.V. Dmitriev, V.L. Barsukov, V.N. Nevsorov, A.B. Slutsky, The formation conditions of high magnesium olivines from the monomineral fraction of Luna-4 regolith, 11th Lunar Planet. Sci. Conf., 1980, pp wx M.O. Garcia, N.W.K. Liu, D.W. 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19 ( ) V. S. Kamenetsky et al.rearth and Planetary Science Letters floor and Geochemical Leg 8, in: H. Bougault, S.C. Cande, et al. Ž Eds.., Init. Rep. DSDP, 8 Ž w31x R.J. Kinzler, T.L. Grove, Primary magmas of mid-ocean ridge basalts,. Applications, J. Geophys. Res. 97 Ž w3x A. Ewart, W.B. Bryan, B.W. Chappell, R.L. Rudnick, Regional geochemistry of the Lau Tonga arc and backarc systems, in: J.W. Hawkins, L.M. Parson, J.F. Allan, et al. Ž Eds.., Proc. ODP Sci. Results, 135 Ž w33x J. Woodhead, S. Eggins, J. Gamble, High field strength and transition element systematics in island arc and back-arc basin basalts: evidence for multi-phase melt extraction and a depleted mantle wedge, Earth Planet. Sci. Lett. 114 Ž w34x M. Barsdell, R.F. Berry, Origin and evolution of primitive island arc ankaramites from Western Epi, Vanuatu, J. Petrol. 31 Ž w35x J. Foden, The petrology of calcalkaline lavas of Rindjani Volcano, east Sunda Arc; a model for island arc petrogenesis, J. Petrol. 4 Ž w36x T.J. Falloon, D.H. Green, Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: Implication for the origin of primitive MORB glasses, Mineral. Petrol. 37 Ž w37x E.M. Klein, C.H. Langmuir, Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness, J. Geophys. Res. 9 Ž w38x S.R. Hart, K.E. Davis, Nickel partitioning between olivine and silicate melt, Earth Planet. Sci. Lett. 40 Ž w39x K. Benn, A. Nicolas, I. Reuber, Mantle crust transition zone and origin of wehrlitic magmas: evidence from the Oman ophiolite, Tectonophysics 151 Ž w40x T.J. Falloon, D.H. Green, A.J. Crawford, Dredged igneous rocks from the northern termination of the Tofua magmatic arc, Tonga and adjacent Lau Basin, Aust. J. Earth Sci. 34 Ž w41x T.J. Falloon, D.H. Green, M.T. McCulloch, Petrogenesis of high-mg and associated lavas from the north Tonga Trench, in: A.J. Crawford Ž Ed.., Boninites, Unwin Hyman, London, 1989, pp w4x A.J. Crawford, L. Beccaluva, G. Serri, Tectono-magmatic evolution of the West Philippine Mariana region and the origin of boninites, Earth Planet. Sci. Lett. 54 Ž w43x S. Meffre, J.C. Aitchinson, A.J. Crawford, Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia, Tectonics 15 Ž w44x J.A. Pearce, M.F. Thirlwall, G. Ingram, B.J. Murton, R.J. Arculus, S.R. van der Laan, Isotopic evidence for the origin of boninites and related rocks drilled in the Izu-Bonin Ž Ogasawara. forearc, Leg 15, in: P. Fryer, J.A. Pearce, L. Stokking, et al. Ž Eds.., Proc. ODP Sci. Results, 15 Ž w45x T.J. Falloon, A. Malahoff, L.P. Zonenshain, Y. Bogdanov, Petrology and geochemistry of back-arc basin basalts from Lau Basin spreading ridges at 158, 188and 198S, Mineral. Petrol. 47 Ž

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