Importance of mantle serpentinite carbonation in bending faults for the deep carbon cycle

Serpentinite carbonation contributes to the deep carbon (C) cycle. Recently, geophysical and numerical studies have identified considerable hydrothermal alterations in deep bending faults beneath outer-rise regions, implying potentially significant C storage in the slab mantle. However, quantitative determination of C uptake in outer-rise regions is lacking. Here, we experimentally constrained the serpentinite carbonation in H2O–CO2–NaCl fluids under bending fault conditions to estimate C uptake in the slab mantle. We found that serpentinite carbonation produced talc and magnesite along the serpentinite surface. The porous reaction zones (49.2% porosity) promoted the progress of the carbonation reaction through a continuous supply of CO2-bearing fluids to the reaction front. Strikingly, NaCl effectively decreased the serpentinite carbonation efficiency, particularly at low salinities (< 5.0 wt%), which is likely attributed to the reduction in H2O and CO2 activities (aH2O and aCO2) and transport rate of reactants, the change in pH of fluids, and the enhancement of magnesite solubility. We fitted an empirical equation for the reaction rate of serpentinite carbonation in bending faults and found that this reaction could contribute to a flux of 25–100 Mt C/yr in subduction zones. Our results shed new light on the deep C cycle and the serpentinite carbonation in environments with high salinities.