Genesis of Hawaiian lavas by crystallization of picritic magma in the deep mantle

Abstract

Olivine is the dominant phenocryst or xenocryst of Hawaiian tholeiitic basalts, and the general consensus is that lavas with MgO concentrations from 7.5 to about 15 weight percent were derived from their primary magmas, which contain ~18–20 weight percent MgO, by only olivine crystallization. However, the major element composition of estimated primary magmas through olivine crystallization correction is inconsistent with direct partial melting of either mantle peridotite or its hybrid with subducted oceanic crust. Our melting experiments on peridotite-derived melt composition show that this discrepancy can be resolved if the primary magmas experienced two other processes before abundant olivine fractionation. First, the primary magmas experienced crystallization of clinopyroxene and garnet in the chamber at the base of the lithosphere (approximately the depths of 90–100 km). Second, the evolved magmas re-equilibrated with harzburgite when passing through the lithospheric mantle (approximately the depths of 60–10 km). Different from the isotopic evidence, the major and rare earth element compositions of Hawaiian post-shield alkali basalts and shield tholeiites suggest that they form from the same source by assimilating different amounts of orthopyroxene.