Carbon and Sulfur Retention in Forearc Serpentinites: Evidence from the Heimulin Serpentinites, Central China

Subduction zones are crucial for regulating volatile exchange between the Earth’s surface and interior. Specifically, volatile migration in the mantle wedge controls arc magma genesis and outfluxes. However, the poorly constrained capacity of the forearc mantle wedge to retain volatiles limits our ability to quantify global volatile cycling. This study focuses on serpentinites from the Heimulin area and investigates volatile behavior during shallow forearc serpentinization and subsequent recrystallization within the forearc mantle wedge. This is achieved through analyses of carbon and sulfur contents and isotopic compositions, combined with thermodynamic modeling. The carbon content and isotopic composition of the two sample types, which represent different degrees of serpentinization, show no significant difference. However, carbon enrichment and magnesite formation were observed in serpentinites containing ribbon-textured lizardite. Sulfur systematics suggest that slab-derived dehydrating fluids can introduce sulfur into the mantle wedge, where it can be effectively retained in serpentinite systems as pyrite under low water–rock ratios. These findings imply that forearc serpentinites may play a role in volatile transport and serve as reservoirs for carbon and sulfur, which may have implications for understanding volatile cycling in subduction zones.