The role of serpentinites in cycling of carbon and sulfur: Seafloor serpentinization and subduction metamorphism

We summarize the uptake of carbon and sulfur during serpentinization of seafloor peridotites, and discuss the fate of these volatiles during subduction of serpentinite. We use a simplified classification to divide seafloor serpentinization into high-temperature and low-temperature processes. High-temperature serpentinization typically involves heat and mass transfer from gabbro intrusions, leading to addition of hydrothermal sulfide sulfur (up to >1wt.%) having high δ³⁴S values (+5 to +10‰). Total carbon contents of bulk rocks are elevated (0.008-0.603wt.%) compared to mantle values and δ¹³C_{Total C} values of -3‰ to -17.5‰ result from mixtures of organic carbon and seawater-derived carbonate. Low-temperature serpentinization is generally characterized by microbial reduction of seawater sulfate, which leads to addition of sulfide sulfur (up to 1.4wt.%) having negative δ³⁴S values (down to -45‰), although local closed-system conditions can lead to reservoir effects and positive δ³⁴S values (up to +27‰). Extensive circulation of cold seawater can cause oxidation, loss of sulfide, and addition of seawater sulfate resulting in high δ³⁴S_Total-S values. High total carbon contents (0.006-7.2wt.%) and δ¹³C values of -26 to +2.2‰ result from addition of variable proportions of organic carbon and seawater-derived carbonate to serpentinite. We estimate that serpentinization at mid ocean ridges is a sink for 0.35-0.64×10¹¹molCy^-1 and 0.13-1.46×10¹¹molSy^-1, comparable to the sinks of these elements per unit volume of mafic oceanic crust. Serpentinization in the subducting plate at subduction zones may further affect chemical budgets for serpentinization.During subduction metamorphism, sulfur and carbon contents remain unaffected by recrystallization of seafloor lizardite and chrysotile to antigorite, and formation of minor olivine. Dehydration of antigorite-serpentinites to chlorite-harzburgites at higher pressure and temperature results in loss of 5wt.% water, and an average of 260ppm sulfur is lost as sulfate having δ³⁴S=14.5‰, whereas carbon is unaffected. These volatiles can induce melting and contribute to ³⁴S enrichments and oxidation of the sub-arc mantle wedge. Serpentinized oceanic peridotites carry isotopically fractionated water, carbon and sulfur into subduction zones. Up to 0.49×10¹¹molsulfury^-1 and 1.7×10¹¹molcarbony^-1 are subducted in serpentinites, less than 3% of the total subduction budgets for each of these elements. Isotopically fractionated carbon, sulfur, and water remain in serpentinite dehydration products, however, and can be recycled deeper into the mantle where they may be significant for volatile budgets of the deep Earth.