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

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 wt% were derived from their primary magmas, which contain ~18–20 wt% 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. 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: (1) crystallization of clinopyroxene and garnet in a chamber at the base of the lithosphere (~90–100 km), and (2) re-equilibration with harzburgite by assimilating orthopyroxene while precipitating olivine during ascent through the lithosphere (~60–10 km). Unlike isotopic evidence, the major and rare earth element compositions of Hawaiian post-shield alkali basalts and shield tholeiites suggest a common source with variable orthopyroxene assimilation.

Junlong Yang, Chao Wang, Junfeng Zhang, Zhenmin Jin