Significant calcium isotope fractionation driven by partial dissolution of subducting carbonate in slab-derived fluids

Abstract

The burial of carbon from Earth’s surface to its deep interior is vital for sustaining the planet’s habitability, however, uncertainties in the dissolution behavior of subducting carbonates make the efficiency of such a process debatable. In this study, we investigate the dissolution behavior of carbonates in subduction zones by analyzing Ca isotope compositions of ultrahigh-pressure (UHP) marbles from the Sulu orogen, China. Our results reveal a δ^44/42Ca range that extends from protolith-like values (∼0.46 ‰) to significantly lighter ones (0.19 ‰). This variation, correlating positively with ⁸⁷Sr/⁸⁶Sr(i) (0.7038–0.7075), indicates a role of infiltration of external fluids, likely derived from dehydration during the amphibolite to eclogite transition of mafic lithologies with variably low ⁸⁷Sr/⁸⁶Sr(i). Simulations on fluid-carbonate interaction processes at the constrained conditions illustrate that Ca isotope variations of marbles were dominantly governed by dissolution. With heavy Ca isotopes being preferentially enriched in the fluids in equilibrium, δ^44/42Ca of the residual carbonates could have decreased progressively with increasing degrees of dissolution. Utilizing a Rayleigh fractionation model, we estimate that negligible dolomitic marbles versus a substantial proportion—approximately 76 ± 15 %—of Ca-rich marbles have undergone dissolution. Our study illustrates that Ca-rich carbonates can be efficiently dissolved if slab-derived fluids are present, even at continental subduction zones where the slabs are commonly considered to be “dry”. This dissolution process has transferred an isotopically mantle-like Ca flux upward, and left behind residual calcium carbonate with notably lighter δ^44/42Ca. Our findings provide evidence for the contrast dissolution behaviors of calcium versus magnesium carbonates in the subduction channel, and help to explain why light Ca isotope anomalies are commonly identified among mantle-derived magmas far away from magmatic arcs.