Water-soluble chlorides in massive seafloor serpentinites: a source of chloride in subduction zones

Dissolved cation and anion species were measured in the water-soluble fraction of massive seafloor serpentinites leached with distilled H2O. Soluble chloride contents ranged from 0.09 to 0.51 wt.% (average=0.26 wt.%), SO42− to 0.09 wt.%, Mg2+ to 0.16 wt.% and Na+ to 0.09 wt.%. The concentration of evolved ions was the same in samples equilibrated with water at 25 and 230 °C, demonstrating that a significant amount of Cl present in serpentinites occurs as water-soluble salts. A nearly equal amount of Cl exists in the leached serpentinite, likely sited in serpentinite itself. The formation of intergranular salts in the serpentinite can be explained by seawater hydrothermal alteration of peridotite. Hydration reactions and H2O-incorporation in the serpentine leave the residual fluid with ever-increasing salinity and ultimately lead to precipitation of water-soluble chlorides. Chlorine isotope ratios in the water-soluble fraction average −1.49‰ (vs. standard mean ocean chlorine, SMOC) from a core at the Iberian Abyssal Plain, and +0.16‰ from a much younger core at Hess Deep. The δ37Cl value of insoluble Cl is ∼0.33‰ higher than the soluble fraction. The nonzero δ37Cl values demonstrate that the Cl in the serpentinites is not an artifact of seawater incorporation during drilling. llowing conclusions are presented: (1) Serpentinite may be the major conduit for surficial chlorine transfer to mantle depths and an important part of the global chlorine cycle. The Cl flux in serpentinites into the mantle is larger than all other previously identified fluxes. (2) Cl is sited in both a water-soluble and -insoluble component. (3) A high-salinity fluid ‘plume’ is evolved during serpentinite dehydration of a subducting slab. The high-salinity fluid affects metamorphic reactions and melting properties above the slab and can cause intense metasomatism. The unusual bulk composition of the whiteschist from Dora Maira can be explained by channeled passage of a high-salinity fluid. (4) High-salinity fluids reduce the stability field of serpentine, causing its dehydration to occur over a broad interval, which may partly explain the broad locus of intermediate-depth earthquakes.