Chromium Isotope Behavior During Serpentinite Dehydration in Oceanic Subduction Zones

Abstract

Fluids released through the dehydration of serpentinite are 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. The low‐grade serpentinites affected by significant Cr loss during serpentinization exhibit remarkably higher δ53Cr, while samples with Cr contents > ∼1800 ppm typically preserve mantle‐like δ53Cr. Antigorite serpentinites have an average δ53Cr 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.

Stable Chromium Isotope Fractionation During the Alteration of Abyssal Peridotite: Implications for Marine Chromium Isotope Mass Balance

“…Abyssal peridotites are commonly exposed on the seafloor underlying oxic seawater and have undergone prolonged fluid-rock interactions (e.g., Niu, 2004;Snow & Dick, 1995). Hence, considering the high mobility of Cr(VI) that can be formed during fluid-rock interactions and potential isotope fractionation during oxidation (Frei et al, 2014), the primary Cr isotopic compositions of abyssal peridotites can be affected by high-T alteration related to redox reactions such as serpentinization (Farkaš et al, 2013;Shen et al, 2021;Wang et al, 2016;Xiong et al, 2023). The δ 53 Cr values of abyssal peridotites investigated in this study are slightly higher and more scattered than the altered basaltic oceanic crust ( 0.18‰ on average, Wang et al, 2016; Figure 5).…”