Zinc isotopic systematics of the South China Sea basalts and implications for its behavior during plate subduction

Abstract

Zinc isotopes is a new tracer for deep materials recycling. However, its behavior during plate subduction remains obscure. To address this issue, here we present high precision Zn isotopic compositions (δ66Zn) of basalts recovered from International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea (SCS), which have been influenced by various amounts of subducted materials. Based on (La/Sm)N values, these basalts are classified into N-MORB ((La/Sm)N < 1) and E-MORB ((La/Sm)N ≥ 1) types. The results show that both of the N-MORB and E-MORB type basalts have similar and homogeneous Zn isotopic compositions with δ66Zn ranging from 0.22 to 0.28‰ and 0.24 to 0.31‰, respectively. These values are similar to that of typical MORB reported in the literature (0.28 ± 0.03‰), but are systematically higher than the estimated average value of depleted MORB-type mantle (DMM, 0.18‰).The lack of correlations between δ66Zn and geochemical indicators of magma differentiation (e.g., MgO, SiO2, CaO/Al2O3) suggests that crystal fractionation has a limited effect on Zn isotope fractionation. Even though the mantle sources of the SCS basalts are clearly affected by various amounts of subducted materials, there are no correlations between δ66Zn and indicators of subducted components (e.g., Ce/Pb, Nb/U, Ba/La, 87Sr/86Sr). This implies that insignificant amount of Zn was transported from the subducting slab to the mantle sources of the SCS basalts, so that subducted materials have a limited effect on Zn isotopic compositions of the SCS basalts studied here. Therefore, the observed offset of δ66Zn (~0.10‰) between the mantle and the SCS basalts studied here should be primarily controlled by Zn isotope fractionation during mantle partial melting with no need for contributions from subduction materials.