ORIGIN AND DIFFERENTIATION OF PICRITIC ARC MAGMAS, AMBAE (AOBA), VANUATU

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg approximately 80 to 93.4) and clinopyroxene (mg approximately 80 to 92), and include accessory Cr-rich spinel (cr approximately 50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to approximately 20 wt% MgO) to high-alumina basalts (< 5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene ne + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with approximately 10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (f(o2) approximately FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has approximately 15 wt % MgO and an mg(Fe2) value of approximately 81. It requires deep (approximately 3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T(p) approximately 1300-degrees-C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respectively. A third component is attributed to either different degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.