Sequestration of carbon in the forearc mantle wedge

Subduction zones are crucial in governing the global carbon (C) cycle. Recent geochemical and geophysical observations have revealed extensive serpentinite carbonation in the forearc mantle wedges, hinting at a potentially vast C reservoir within these serpentinized areas. In this study, we conducted experimental investigations on serpentinite carbonation in multicomponent (H2O–CO2–NaCl) fluids under various pressure-temperature (P-T) conditions that mimic the mantle wedge. Our aim was to precisely determine the reaction extent (RE, RE = Vmgs / V0mgs, where Vmgs represents the actual magnesite volume in the run product and V0mgs denotes the theoretical magnesite volume through the complete reaction of CO2.) of serpentinite carbonation in the multicomponent fluids and, subsequently, estimate C storage in the forearc mantle wedge. Our findings indicate that serpentinite reacts with CO2 to form magnesite and talc. Time-series experiments demonstrated that reaction equilibrium is attained within 48 hours. The results revealed that RE increases with rising P-T and CO2 concentration in fluids. However, a notable decrease in RE was observed with increasing salinity, particularly at low salinities (< 10 wt%). This decrease can be attributed to the reduced fluid pH, decreased CO2 and H2O activities (aCO2 and aH2O), and increased magnesite solubility in salt-bearing fluids. Based on previous and our experimental data, we derived an empirical equation to describe the RE of serpentinite carbonation in H2O–CO2–NaCl fluids. We extrapolated the potential RE regarding P-T conditions and fluid compositions within the mantle wedge to estimate C inventory in forearc mantle wedges. Our calculations indicated that even a minimal degree of serpentinization (< 10 vol%) in the mantle wedge could result in the sequestration of 49–76% of C from slab-derived fluids originating from depths of 15–80 km through serpentinite carbonation, leading to the formation of magnesium-rich carbonates. On a global scale, this carbonation process has the capacity to sequester 0.02–4.17 million tons of C per year (Mt C/yr) within forearc mantle wedges. Consequently, a portion of the C stored in carbonated serpentinite may persist in the cold, stagnant regions of the wedge, potentially contributing to long-term C storage and seismic activity. Furthermore, due to processes such as down-dragging and subduction erosion, some of the C fixed in the lower carbonated layer may be transported to partial melting regions, ultimately contributing to volcanic emissions in arc regions. This highlights the multi-stage nature of C migration within the mantle wedge. Our study offers significant insights into the C cycle and seismic responses within subduction zones.