Constraints of barium isotopes on recycling of ancient oceanic crust in the mantle of the South China Sea

Abstract

As surface reservoirs (e.g. sediments and altered oceanic crust) have extremely high barium (Ba) concentrations and distinctive Ba isotope compositions compared to the mantle, Ba isotopes have the potential to trace crustal recycling in the mantle. The sediment components and fluids derived from altered oceanic crust in arc lavas and continental basalts have been identified using Ba isotopes, while the geochemical behavior of Ba in subducted oceanic crust remains enigmatic. A series of volcanic breccia that temporally evolved from carbonated silicate clasts to alkali basalts in the South China Sea (SCS) drilled by the Integrated Ocean Discovery Program (IODP) Site U1431 was suggested to originate from the mantle source contaminated by recycled oceanic crust. To understand the potential of using Ba stable isotopes to trace recycled oceanic crust in the mantle, we report high-precision Ba isotope compositions of these samples, together with alkali basalts from adjacent Hainan Island to constrain the Ba isotope characteristics of the Hainan mantle plume. The carbonated silicate clasts and alkali basalts from the SCS display extremely high Ba concentrations and are thus insensitive to magma evolution during ascent through the thin lithospheric mantle and proper to trace crustal materials in their mantle source using Ba isotopes. These samples have variable Ba isotope compositions ranging from δ138/134Ba = −0.10 to +0.05‰, implying local mantle isotopic heterogeneity underlying the SCS. The Ba isotope characteristics of some samples, which are much lighter than those of the mantle-derived carbonatites and global mid-ocean ridge basalts (MORBs), in conjunction with trace element ratios and radiogenic isotope compositions, reflect significant incorporation of recycled carbonated oceanic crust after subduction-related dehydration. The rising of the Hainan mantle plume offers the most reasonable interpretation of the entrainment of the recycled oceanic crust in the carbonated silicate magmatism beneath the SCS. Alkali basalts from Hainan Island display Ba isotope characteristics slightly heavier than the SCS carbonated silicate clasts and alkali basalts. It implies that the Hainan mantle plume underlying Hainan Island contains less recycled-oceanic-crust components than that underneath the SCS. This study highlights Ba isotopes as effective tracers of recycled oceanic crust in the mantle.