Petrogenesis of middle Okinawa Trough volcanic rocks: Constraints from lead isotopes in olivine-hosted melt inclusions

摘要

Olivine-hosted melt inclusions provide crucial information about their parental magma composition and evolution and represent ideal targets for determining the nature of the poorly understood and still-debated magma sources in the Okinawa Trough, a back-arc basin in the western Pacific. We present the first analyses of the lead (Pb) isotope compositions of olivine-hosted melt inclusions to evaluate the mantle properties and petrogenesis of middle Okinawa Trough volcanic rocks. The melt inclusions have more variable major and trace elements and Pb isotope compositions than the host whole-rock samples. We report the discovery of both high-~(207)Pb/~(206)Pb (> 0.865) and low-~(207)Pb/~(206)Pb (< 0.865) isotope compositions in melt inclusions in individual volcanic rocks, even within a single host olivine, indicating a compositionally heterogeneous magma source. The trace element and Pb isotope characteristics of the melt inclusions show that the magma source is affected by enriched components. We modeled the injection of enriched components into a magma source to explain the generation of the magma heterogeneity. The results indicate that the mixing of Pacific Ocean-type mantle (MORB), an EMI-like component from recycled lower continental crust and EMII-like material from subducted sediments can explain the low-~(207)Pb/~(206)Pb isotope values observed in the melt inclusions. The discovery of small proportions of melt inclusions with high ~(207)Pb/~(206)Pb ratios, high K_2O, P_2O_5, Rb and U contents and low Pb and Cu contents in the studied andesites suggests that the andesitic magma may have been formed by the mixing of materials with different elemental and isotopic compositions. Our study results suggest that pervasive magma mixing may have occurred in the magma source prior to eruption in the Okinawa Trough.