Cadmium isotopes as a tracer for deep carbon recycling

Abstract

Global marine carbonates have cadmium (Cd) concentrations much higher than those of mantle peridotites, and especially, Phanerozoic carbonates are uniquely characterized by heavier Cd isotopic composition (expressed as δ^114/110Cd against NIST 3108) relative to the mantle. These characteristics make the Cd systematics potentially useful for tracing recycled carbonates in the mantle. We test the feasibility of this application through analyses of seventeen well-characterized Cenozoic intraplate basalts from Eastern China, and for comparison, six mid-ocean ridge basalts (MORB) are also investigated. The MORB have a mean δ^114/110Cd value of 0.13 ± 0.04 ‰ (2sd), in accord with previous analyses for other MORB, illustrating a relatively homogeneous Cd isotopic composition of global MORB and the depleted upper mantle. The Eastern China basalts have δ^114/110Cd values varying between 0.17 ± 0.05 ‰ (2sd) and 0.65 ± 0.05 ‰ (2sd), which are slightly to notably higher than those of the mantle and MORB. The maximum δ^114/110Cd offset (∼0.5–0.6 ‰) between these basalts and the mantle fairly exceeds the magnitude of Cd isotope fractionation even at low degrees of partial melting. The high δ^114/110Cd values cannot also be explained by volatilization-induced Cd loss, given the positive correlation between δ^114/110Cd and Cd concentrations. Instead, this correlation points to a key role of recycled carbonates, supported by the “carbonated” geochemical signals of these basalts (e.g., low Ti/Eu ratios). Binary mixing model indicates that recycling of approximately 2–10 % carbonates into sources can explain the high δ^114/110Cd observed in these basalts. Given that Cd is a trace element in the mantle (∼38 ng/g), the Cd isotopic composition of mantle rocks is sensitive to even a tiny amount of recycled carbonates. Therefore, we propose for the first time that Cd isotopes are a new and powerful tool of tracing Earth's deep carbon recycling over geologic time.