Chromium Isotope Behavior During Serpentinite Dehydration in Oceanic Subduction Zones

Fluids released through the dehydration of serpentinite can be rich in Cl-, which enables the significant mobility of Cr in subduction zones. However, the Cr isotope behavior accompanying the mobility of Cr during serpentinite dehydration is still poorly constrained. Here, we report high-precision Cr isotope data for a unique suite of serpentinites that represent metamorphic products at different depths in oceanic subduction zones. Low-grade serpentinites affected by significant Cr loss during serpentinization exhibit remarkably higher delta Cr-53, while samples with Cr contents >similar to 1,800 ppm typically preserve mantle-like delta Cr-53. Antigorite serpentinites have an average delta Cr-53 value of -0.17% +/- 0.19% (n = 12, 2SD), which is statistically lower than those of low-grade serpentinite (-0.05% +/- 0.30%, n = 80, 2SD) and higher-grade chlorite harzburgite (-0.10% +/- 0.27%, n = 22, 2SD). This suggests that resolvable Cr isotope fractionation occurs during serpentinite dehydration, which is explained by the variability of Cr isotope behavior in the presence of Cl-bearing fluids at different dehydration stages. No obvious Cr isotope fractionation was found during chlorite harzburgite dehydration, probably related to the limited Cr mobility in a Cl-poor fluid. Other processes, such as melt extraction, external fluid influx and retrograde metamorphism, have negligible effects on the Cr isotope systematics of meta-serpentinites. Fluids released by serpentinite dehydration may have a great effect on the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas. Plain Language Summary Serpentinite plays an important role in crust-mantle interaction and arc magmatism in oceanic subduction zones. Serpentinite-derived fluids potentially mobilize Cr in the form of Cl complexes. However, there is limited information available about the Cr isotope compositions of subducted serpentinites, hindering the understanding of Cr isotope behavior during the dehydration of serpentinite. Here, we first report high-precision Cr isotope data for a unique suite of (meta-)serpentinite that encompasses low-pressure oceanic serpentinite to metamorphic products of complete dehydration at great depth. Our results fill the gap in the Cr isotope composition of meta-serpentinites metamorphosed under diverse P-T conditions in oceanic subduction zones. We found that there is variable and significant Cr isotope fractionation during prograde dehydration of serpentinites. Such serpentinite-derived fluids may contribute to the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas.