Isotopic evidence for iron mobility during subduction

Subduction zones are one of the most important sites of chemical interchange between the Earth’s surface and interior. One means of explaining the high Fe3+/ΣFe ratios and oxidized nature of primary arc magmas is the transfer of sulfate (SOX), carbonate (CO3–), and/or iron (Fe3+) bearing fluids from the slab to the overlying mantle. Iron mobility and Fe stable isotope fractionation in fluids are influenced by Fe redox state and the presence of chlorine and/or sulfur anions. Here we use Fe stable isotopes (δ56Fe) as a tracer of iron mobility in serpentinites from Western Alps metaophiolites, which represent remnants of oceanic lithosphere that have undergone subduction-related metamorphism and devolatilization. A negative correlation (R2 = 0.72) is observed between serpentinite bulk δ56Fe and Fe3+/ΣFe that provides the first direct evidence for the release of Fe-bearing fluids during serpentinite devolatilization in subduction zones. The progressive loss of isotopically light Fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate-rich and/or hypersaline fluids, which preferentially complex isotopically light Fe in the form of Fe(II)-SOX or Fe(II)-Cl2 species. Fe isotopes can therefore be used as a tracer of the nature of slab-derived fluids.