The fate of subducting carbon tracked by Mg and Zn isotopes: A review and new perspectives

Abstract

Tracking the final fate of subducting carbon is crucial to understanding global carbon cycles and climate changes in the history of the Earth. Available geochemical tracers such as carbon isotopes are apt to identify recycled organic carbon but usually insufficient to discriminate between primordial carbon in the mantle and carbon derived from recycled carbonate sediments. In the past decade, magnesium and zinc isotope systematics have been proposed as novel proxies for subducting carbon owing to the noticeable isotopic offsets between carbonate sediments and the mantle (i.e., δ26Mgcarbonate < δ26Mgmantle; δ66Zncarbonate > δ66Znmantle). Nonetheless, isotopic effects induced by subduction-zone processes and crystal-melt differentiation may obscure the information of Mg and Zn isotopic compositions of mantle-derived magmas. In this paper we firstly discuss how these processes modify the Mg and Zn isotopic systematics of mantle-derived magmas. Based on the fact that different carbonate species (calcite, dolomite, and magnesite) possess distinct Mg and Zn contents and their stabilities in subduction zones vary with pressure, we then develop the two isotope systematics as tools to track the final storage depth of subducting carbon. We test this application by collating available Mg and Zn isotopic compositions of ultramafic xenoliths and basaltic lavas sourced from various mantle depths. The lack of light Mg and heavy Zn isotopic anomalies of global arc lavas supports experimental and theoretical prediction that the dissolved carbonate species in the sub-arc mantle−if any−is dominated by calcium-rich carbonate. The findings of pervasive low-δ26Mg and high-δ66Zn ultramafic xenoliths and basaltic lavas sourced from the sub-continental lithospheric mantle (SCLM) suggest that the SCLM is an important storage of subducting carbon via metasomatism by dolomite that can be substantially dissolved by supercritical fluids at depths of >160 km. Intraplate alkali basalts with low δ26Mg and high δ66Zn are commonly restricted to the regions with stagnant slabs at depths of ~410–660 km, suggesting that the mantle transition zone is another global storage of subducting carbon composed mainly of Mg-rich carbonates. Overall, observations on mantle-derived rocks, with Mg and Zn isotopes as the tracers, indicate that a significant flux of Earth's surface carbon has survived the arc regime and been recycled into the deeper mantle. Future studies that explore a quantitative relationship between Mg-Zn isotopic ratios and the flux of subducting carbon will further promote the application of the paired isotopic proxies.