Insights from olivine chemistry into crustal magmatic processes and the mantle source lithology of basalts from Hainan Island, China

Abstract

Olivine in ocean island basalts can provide unique insights into lithological heterogeneity (i.e., peridotite versus pyroxenite) in the mantle. Here we present high-precision major and trace element data for olivine crystals in basalts from Hainan Island, China, and use these data to investigate their mantle source. Many of the olivines are in disequilibrium with their whole-rock compositions (i.e., Mg# values), and the basalts contain both normally and reversely zoned olivine crystals, which are indicative of magma mixing. The decoupled Fo–NiO profiles and concave-down trends defined by data in a Fo–NiO diagram suggest that diffusive re-equilibration controlled the olivine chemistry. The olivine compositions were significantly affected by crustal magmatic processes, such as fractional crystallization, magma mixing, and diffusive re-equilibration. However, the lack of clear correlations between Mn/Fe, Mn/Zn, Zn/Fe, and Ga/Sc ratios and olivine Fo contents suggests these trace element ratios were not significantly affected by crustal processes, and thus preserve information about the mantle source. The Mn/Zn, 100 × Mn/Fe and 10,000 × Zn/Fe ratios of the examined olivines indicated that the Hainan basalts had a mixed pyroxenite-peridotite mantle source. This is consistent with the whole-rock geochemical data and previously reported isotope geochemistry data for the Hainan Island basalts. However, crustal magmatic processes also affected the compositions of the primitive olivines that crystallized in the mantle-derived magma. Therefore, careful assessment of post-melting processes using olivine textures and minor element chemistry needs to be undertaken before discriminating between pyroxenitic and peridotitic source components based on olivine chemistry.