Cadmium isotope compositions of basalts from the East Pacific Rise and Southwest Indian Ridge: Implications for magma differentiation and mantle heterogeneity

Abstract

Cadmium (Cd) isotopes have significant potential for tracing planetary differentiation, yet their behavior during high-temperature magmatic processes remains unclear. We report Cd isotope data for mid-ocean ridge basalts (MORBs) from the East Pacific Rise (EPR, ∼3°S) and the Southwest Indian Ridge (SWIR, 46°–56°E) to assess the effects of magmatic processes and mantle source heterogeneity. The EPR MORBs yield δ^114/110Cd (relative to NIST-3108) values of −0.08 ‰ to 0.21 ‰, consistent with existing MORB data. Correlations of these δ^114/110Cd values with indices of magmatic differentiation (e.g., MgO, CaO, and TiO₂ contents) suggest that crystal fractionation can cause at least 0.3 ‰ of Cd isotope fractionation. The estimated Cd isotope composition of the depleted MORB mantle is −0.05 ‰, within the range of the bulk silicate Earth (BSE; δ^114/110Cd_BSE = −0.06 ‰ to 0.11 ‰). In contrast, the SWIR MORBs show a much larger δ^114/110Cd range (−0.42 ‰ to 0.82 ‰) that cannot be explained by magmatic degassing and differentiation. Their δ^114/110Cd values correlates with Sr isotopes and trace-element ratios (e.g., Ba/La and Th/Yb), indicating that mantle source heterogeneity dominates their Cd isotope variability. We propose that the observed heterogeneity stems from mantle metasomatism by slab-derived fluids and sediment with heavy Cd isotopes during ancient subduction. This fluid-modified mantle wedge was subsequently recycled into the mantle source beneath the central SWIR. Thus, variable melting of this recycled subarc mantle material ultimately accounts for the observed Cd isotope range at SWIR. Our study highlights the role of ancient subduction in the Cd geochemical cycle and that Cd isotopes can be used to trace MORB mantle heterogeneity and subduction recycling.