Stratified distribution of recycled components in the mantle of the South China Sea unveiled through magnesium isotopes

Abstract

Plate subduction is pivotal in creating mantle heterogeneity and facilitating deep material recycling. However, the precise origins, input processes, and distribution of recycled components in the upper mantle remain poorly constrained by natural observations. Compared to large ocean basins, marginal sea basins are more likely to preserve detailed information about recycled components due to their close spatial relationship with subduction processes, relatively smaller size, and shorter evolutionary timescales. In this study, we present new Mg–Sr–Pb isotope data, in conjunction with previously published major and trace element analyses, as well as Nd–Hf isotope data for mid-ocean ridge basalt (MORB) samples from Holes U1431E, U1433B, and U1434A of the International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea (SCS), a marginal sea basin in the Western Pacific with a complex subduction history. MORBs from the SCS exhibit a wide range of Mg isotope ratios (δ²⁶Mg = −0.32‰ to 0.05‰, with an average of −0.16‰), with some samples displaying lower or higher values compared to the average mantle (δ²⁶Mg = −0.25 ± 0.04‰). These variable Mg isotope compositions likely originate from mantle sources affected by subduction-related metasomatism at varying depths, given the limited impact of post-eruptive alteration, diffusion, and high-temperature magmatic processes. The heavy Mg isotopes in the SCS basalts can be attributed to metasomatism by serpentinite components related to surrounding subduction restricted to the shallow mantle (<200 km). Conversely, MORBs with lighter Mg isotopes also show high Hf/Hf* (>1) and low CaO/Al₂O₃ values, which were derived from recycled carbonated eclogites formed after decarbonization of residual subducted carbonate-bearing sediments in the upper mantle at depths exceeding 200 km or within the mantle transition zone (410–660 km). The southwestern subbasin records more information about serpentinite components due to its low expansion rate, insufficient magma supply, and weak mantle mixing, as well as recycled carbonated eclogites carried by subduction-induced mantle-return flows. In contrast, in the eastern subbasin, a greater proportion of recycled carbonated eclogites from the mantle transition zone was incorporated into the mantle source by an upwelling mantle plume. The data show that MORBs from the SCS preserve a record of derivation from various depths of the convecting mantle having a distinct geochemical and isotopic imprint, including evidence for a carbon-rich reservoir in the deep upper mantle.