Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones

Abstract

Subduction zones modulate the chemical evolution of the Earth's mantle. Water and volatile elements in the slab are released as fluids into the mantle wedge and this process is widely considered to result in the oxidation of the sub-arc mantle. However, the chemical composition and speciation of these fluids, which is critical for the mobility of economically important elements, remain poorly constrained. Sulfur has the potential to act both as oxidizing agent and transport medium. Here we use zinc stable isotopes (δ ⁶⁶Zn) in subducted Alpine serpentinites to decipher the chemical properties of slab-derived fluids. We show that the progressive decrease in δ ⁶⁶Zn with metamorphic grade is correlated with a decrease in sulfur content. As existing theoretical work predicts that Zn-SO ₄ ²⁻ complexes preferentially incorporate heavy δ ⁶⁶Zn, our results provide strong evidence for the release of oxidized, sulfate-rich, slab serpentinite-derived fluids to the mantle wedge.

Abstract

During subduction, fluids are released into the mantle wedge, but the exact compositions of these fluids are unclear. Pons et al. by analysing zinc isotopes from serpentinite provide evidence that oxidized, sulphate rich fluids are released to the mantle wedge during subduction.