An unmetasomatized source for the Malaitan alnoite (Solomon Islands): Petrogenesis involving zone refining, megacryst fractionation, and assimilation of oceanic lithosphere

Abstract

The Malaitan alnoite contains a rich and varied megacryst suite of unprecedented compositional range. We have undertaken trace element and isotope modeling in order to formulate a petrogenetic scheme which links the host alnoite to its entrained megacrysts. This requires that a proto-alnoite magma is the product of zone refining initiated by diapiric upwelling (where the initial melt passes through 200 times its volume of mantle). Isotopic evidence indicates the source of the proto-alnoite contains a time-integrated LREE-depleted signature (143Nd/144Nd = 0.51274). Impingement upon the rigid lithosphere halts or dramatically slows the upward progress of the mantle diapir. At this point, the magma cools and megacryst fractionation begins with augites crystallizing first, followed by subcalcic diopsides and finally phlogopites. Garnet probably crystallizes over the entire range of clinopyroxene fractionation. Estimated proportions of fractionating phases are 30% augite, 24.5% subcalcic diopside, 27% garnet, 12.9% phlogopite, 5% bronzite, 0.5% ilmenite, and 0.1% zircon. As this proto-alnoite magma crystallizes, it assimilates a subducted component of seawater-altered basalt which underplates the Ontong Java Plateau. This is witnessed in the isotopic composition of the megacrysts and alnoite.