Three-dimensional variations in carbon inputs to a subduction zone based on Mg–Zn isotopes

Abstract

Subduction zones are the main pathways for the transport of surficial carbon into the deep Earth. Heterogeneity of decarbonation was recognized in different subduction zones on a global scale. What is unclear yet is the small-scale along-strike variations in carbon released from slab in a subduction zone. In this paper, we present Mg–Zn isotope data for Miocene basalts distributed parallel to the Japan Trench that can be linked to different slab decarbonation efficiencies. The sampling sites can be divided into three groups based on the Mg–Zn isotope data: northern, central, and southern. The northern samples have high δ66Zn (0.40 ‰ to 0.62 ‰) and low δ26Mg (–0.48 ‰ to –0.37 ‰) values (i.e., there is coupling of the Mg–Zn isotopic compositions). The central samples have δ66Zn values that overlap with those of global MORBs (0.25 ‰ to 0.31 ‰), and they have slightly lower δ26Mg values (–0.35 ‰ to –0.19 ‰) than normal mantle (i.e., there is slight decoupling of the Mg–Zn isotopic compositions). In contrast, the southern samples have low δ26Mg (–0.56 ‰ to –0.02 ‰) and normal δ66Zn values (0.26 ‰ to 0.40 ‰) (i.e., there is decoupling of Mg–Zn isotopic compositions). The northern samples with coupled Mg–Zn isotopes indicate that dissolved carbonate sediment in slab-derived fluids contributes to arc/back-arc volcanism. In contrast, the southern samples have decoupled Mg–Zn isotopes, implying that carbonate sediment is retained in the slab and transported to great depths and long distances beneath the mantle wedge. This is consistent with the widespread occurrence of wehrlite xenoliths within associated intraplate volcanoes. These differences in carbon inputs correspond to the slab thermal structure caused by slab geometry and dynamics of mantle flow, which is hot in the north and cold in the south. Globally, the small-scale thermal structure of different subduction zones therefore affects the spatial variations in carbon input in short-distance along the trench, which are probably responsible for the elusive geophysical anomalies and redox heterogeneities of the deep mantle.