Transfer of Strontium and Carbon From Subducting Sediment Into the Leading Edge of the Mantle Wedge

Carbon mobilization at subduction zones can be constrained by focusing on Sr as a proxy, as Sr readily substitutes for Ca—a major cation in the crystal structures of carbonates. Unmelted high/ultrahigh pressure (HP/UHP) pelitic metasediments exhumed from less than 100 km depth along the slab-mantle interface show loss of Sr and retention of most other incompatible trace elements, when compared with their potential protoliths, for example, average shales/average upper continental crust/average subducting sediments. Sr and CaO/SiO₂ in subducting sediments and unmelted HP/UHP metasediments are correlated, suggesting that Ca-carbonate is one of the major hosts for Sr in subducting sediments, and that Sr and carbonates are lost during subsolidus devolatilization of subducting HP/UHP metasediments beneath forearcs. In turn, the elevated Sr in arc magmas suggests that carbonated Sr-rich wedge peridotites may be dragged down into the source of arc magmas via wedge convection, where they contribute to C and Sr during partial melting. Decoupling of Sr from most of the other trace elements in subduction zone metasediments suggests that previous mass balance attempts using Sr, Nd, and Pb concentrations and isotope ratios to constrain the relative contributions of mantle peridotites, subducting sediments and subducting oceanic crust to arc melts need to be reexamined. Metasediments could have contributed more to arc magmas than previously estimated.