Iron isotope constraints on the lithological heterogeneity of the upper mantle in the South China Sea

Abstract

The South China Sea is located at the junction of the Eurasian, Indian, Australian, and Pacific plates, and thus its evolution is complex and the nature of its sub-ridge mantle remains unclear. Here we present new iron (Fe) isotopic data for mid-ocean ridge and intraplate basalts from the South China Sea region, in order to examine the origins of compositional heterogeneity in the sub-ridge mantle beneath the South China Sea. The investigated mid-ocean ridge basalts are from International Ocean Drilling Program Sites U1431 and U1433 in the eastern and southwestern sub-basins of the South China Sea, respectively. The primary melts of the Site U1431 and U1433 basalts have mean δ56Feprim = +0.06‰ ± 0.04‰ (2SD; n = 10) and +0.02‰ ± 0.02‰ (2SD; n = 3), respectively. The primary melts of the intraplate rocks have highly variable δ56Feprim (+0.04‰ to +0.22‰). Our new and previously published data show that the δ56Feprim values and Fe/Mn ratios of the Site U1431 and U1433 basalts are positively correlated (r2 = 0.42). This result indicates that the heterogeneous Fe isotopic compositions of the studied basalts reflect distinct mantle source lithologies (i.e. pyroxenite and peridotite) that have also been identified from the chemistry of olivine phenocrysts. Co-variations between δ56Feprim values and Mg–Sr–Nd isotopes for the South China Sea basalts suggest that the end-member components in the sub-ridge mantle include an enriched recycling-related pyroxenitic component with high δ56Fe and low δ26Mg, and a ubiquitous depleted subduction-modified peridotitic component with low δ56Fe and high δ26Mg.