[S-IT15] Mass and energy transport properties and processes in the crust and the mantle

[SIT15-P01] Sequestration of carbon in the forearc mantle wedge induced from serpentinite carbonation experiment

*Yongsheng HUANG^1,2, Guoji Wu^1,2,3, Tatsumi Tsujimori^4,5, Yuan Li^1,2 (1.Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China, 2.CAS Center for Excellence in Deep Earth Science, Guangzhou, China, 3.College of Earth and Planetary Sciences, University of the Chinese Academy of Sciences, Beijing, China, 4.Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Japan , 5.Center for North Asian Studies, Tohoku University, Sendai, Japan)

Keywords:subduction zone, mantle wedge, carbon cycle, serpentinite carbonation, reaction extent, salinity

Subduction zones control the global carbon (C) cycle. Recently, an increasing number of geochemical and geophysical observations identified extensive serpentinite carbonation in forearc mantle wedges, indicating potentially significant C inventory in the serpentinized mantle wedge. Here, we experimentally investigated the serpentinite carbonation in H₂O–CO₂–NaCl fluids at elevated pressure-temperature (P-T) conditions corresponding to forearc mantle wedges to constrain the reaction extent (RE) of serpentinite carbonation, and thus estimate C storage in the forearc mantle wedge. We found that serpentinite reacted with CO₂ to produce magnesite and talc. Time-series experiments show that the reaction equilibrium was attained within 48 hours. Our results show that RE increased with increasing P-T and X_CO2; whereas RE dramatically decreased with increasing salinity, especially at low salinities (< 10 wt%), which could be attributed to the reduction in CO₂ and H₂O activities (a_CO2 and a_H2O), the change in pH of fluids, and the increase in magnesite solubility in the salt-bearing fluids. Based on our experimental data we fitted an empirical equation for the RE of serpentinite carbonation in serpentinized mantle wedges and then predicted plausible RE according to the P-T conditions and fluid composition in the mantle wedge. We found that even with a small extent of serpentinization (< 0.1 vol%) in each mantle wedge, 42–82% of C in the 15–100 km slab-derived fluids could be sequestrated by the serpentinite carbonation as Magnesium-rich carbonate, resulting in sequestration of 5.0–9.8 Mt C/yr in the mantle wedges globally. A portion of C in the carbonated serpentinite might be stored in the cold and stagnant wedge corner; whereas owing to down-dragging and subduction erosion of the mantle wedge a portion of C in the bottom carbonated layer might transport to the deep depth and contribute to arc volcanic emission, suggesting the possible multi-stages of C migration in the subduction zone. Our results shed new light on the C cycle and seismic response in the subduction zone.