Chromium Isotope Behavior During Serpentinite Dehydration in Oceanic Subduction Zones

Abstract

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 δ 53Cr, while samples with Cr contents >∼1,800 ppm typically preserve mantle-like δ 53Cr. Antigorite serpentinites have an average δ 53Cr 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.