Petrological constraints on the origin of arc picrites, New Georgia Group, Solomon Islands

Subduction related picrites from the New Georgia archipelago (Solomon Islands) range in bulk MgO from 13 to 30 wt%. Two generations of olivine are identified based on CaO contents: High-CaO olivine phenocrysts from the picritic parental melt and low-CaO olivine xenocrysts incorporated from either lithospheric or asthenospheric upper mantle. There is also evidence that some of the low-CaO olivines are boninitic in origin. The bulk MgO range in the picrites is largely controlled by assimilation of low-CaO olivine xenocrysts. Oxidation states of the melt (ΔFMQ+2.2), calculated from magnetite activities in liquidus chromites, constrain the MgO content of the parental melt to 13 wt%, assuming Fe Mg exchange equilibrium between melt and liquidus olivine composition. The dry liquidus temperature of the parent melt based on this MgO content is 1340°C, about 80°C above the temperature obtained with the olivine clinopyroxene Ca-exchange thermobarometer. The residence time of the low-CaO olivine xenocrysts in the magma, estimated from Ca- and Fe Mg interdiffusion profiles, did not exceed 1 year.