Petrogenesis of basalt from the middle Okinawa Trough: New insights from olivine-hosted melt inclusions

Silicate melt inclusions (SMIs) in basalt from the middle Okinawa Trough are investigated in detail to understand the source geochemical characteristics and petrogenesis of the basalt. The middle Okinawa Trough basalts contain abundant SMIs within olivine phenocrysts. The major element compositions of the SMIs are analysed using an electron microprobe. After correcting for olivine postentrapment crystallization, the major element compositions of the SMIs are more variable in terms of SiO2 (49.7-54.8 wt%) and MgO (5.1-7.2 wt%) than those of the host basalt (SiO2, 52.4 wt%; MgO, 5.7 wt%), which could indicate the existence of more primitive melts than erupted bulk rock. The major element compositions of the SMIs indicate that the SMI-recorded melt evolution process was mainly dominated by the fractional crystallization of olivine. According to laser ablation inductively coupled mass spectrometer analyses, the trace element compositions of the SMIs are similar to those of the host bulk rock and feature obvious enrichment in Pb and large-ion lithophile elements relative to high field strength elements, rare earth elements, and highly negative Nb and Ta anomalies. The low Ce/Pb ratios, high Pb contents, and trace element distribution patterns in the SMIs are related to contributions from subducted sedimentary components in the magma source. Based on the electron microprobe analyses, the chemical composition of the olivine suggests that the lithology of the basalt source may be pyroxenite. The basalts may have been generated by the partial melting of pyroxenite during interactions between mantle-wedge peridotite and Si-rich melts that were released from the subducted slab.