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

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-are basin tectonic setting, Two fundamentally different primitive primary melt compositions are identified based on melt inclusion compositions, olivine phenocryst chemistry, and the ea rly co-crystallisation of either magnesian clinopyroxene (Mg# to 93) o r magnesian orthopyroxene (Mg# to 93.5) with magnesian olivine (to Fo( 94)) and Cr-rich spinel (Cr# = 0.78-0.87). One magma type is a H2O-ric h (similar to 2.5 wt%), high-CaO (similar to 14 wt%), low-Al2O3 (simil ar to 8 wt%) magnesian basalt, variants of which occur in both the eas tern 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 o f melt inclusions trapped in primitive olivine phenocrysts, reflect an integration of diverse but geochemically related melt fractions to pr oduce the magmas at each seamount. Trace-element systematics require t he variable addition of a LILE-, Pb-, and Cl-rich component to the man tle wedge source with increased influence toward the Tofua are. This c omponent, as invoked in most models of are magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting o cean crust beneath the volcanic are front. We propose that southward p ropagation of the back-are basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting o f refractory hydrated sub-are lithosphere, probably veined by clinopyr oxene-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 a s other Lau Basin seamount are-like magmas, were produced from sub-are lithosphere during southward propagation of the Eastern Lau Spreading Center similar to 2-3 Ma. (C) 1997 Elsevier Science B.V.