MORB-like δ56Fe values unveil the effect of subduction on the South China Sea basalts

Abstract

Clarifying the genesis of backarc basin basalts (BABBs) is of great significance for understanding the cycling of subducted materials. Here we show MORB-like Fe isotopic signatures (δ56Fe) of basalts recovered from the South China Sea (SCS), a well-developed backarc basin, as an indicator to investigate the influence of subduction on the BABBs. Whole-rock chemical compositions and olivine chemistry indicate the presence of pyroxenitic components in the peridotitic mantle. Melting of a pyroxenite-bearing peridotite indicates an addition of ~10–40% of pyroxenitic melts to the peridotitic mantle in the SCS, which should result in higher δ56Fe values than that of global MORBs. It is inconsistent with the MORB-like δ56Fe values we observed in the SCS basalts. Therefore, a component with low δ56Fe signatures may exist in the source of the SCS basalts, which is most likely derived from subducted materials. Such a scenario can be well corroborated by mixing depleted MORB mantle with subducted fluids and pyroxenitic melts. This study emphasizes that the MORB-like Fe isotopic signatures in backarc basin basalts can be caused by multi-factors, especially in the case of heterogeneous mantle source.