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 early co-crystallisation of either magnesian clinopyroxene (Mg# to 93) or 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-rich (similar to 2.5 wt%), high-CaO (similar to 14 wt%), low-Al2O3 (similar to 8 wt%) 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 are. This component, as invoked in most models of are magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting ocean crust beneath the volcanic are front. We propose that southward propagation of the back-are basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting of refractory hydrated sub-are 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 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.