Serpentine, the most abundant hydrous mineral in the oceanic lithospheric mantle, plays a key role in the volatile cycle between the Earth's interior and surface. Serpentinite carbonation through the hydrothermal alteration sequestrates CO₂, contributing to the carbon storage in the incoming plate and thus the deep carbon cycle. However, the effect of NaCl on serpentinite carbonation and carbon storage in bending fault systems of slab mantles remains unknown. Here, we precisely constrain the serpentinite carbonation in H₂O–CO₂–NaCl fluids under elevated conditions corresponding to bending fault systems. We for the first time find that NaCl can effectively decrease the serpentinite carbonation efficiency. With an increase in salinity, the serpentinite carbonation rate rapidly drops at low salinities < 5 wt.%, and it slightly decreases at higher salinities. The reduction in efficiency of serpentinite carbonation with the presence of salt is attributed to the reduction in both H₂O and CO₂ activity (a_H2O and a_CO2) and further changes in the solubility of both serpentinite and magnesite. The high porosity in reaction zones ensures the supply of CO₂ to the reaction front, favoring interface-coupled replacement processes. Based on the results, we quantitatively constrained that 5.8–35.9 Mt/yr carbon could be added into slab mantles at the outer rise. Our results shed new light on the carbon uptake in the slab mantle through serpentinite carbonation and on the capacity of serpentinite terranes to sequestrate the anthropogenic CO₂.
 

carbon cycle; slab mantle; bending fault; serpentinite carbonation; salinity; carbonation rate