09:15 〜 09:30
[SIT17-02] Important role of mantle serpentinite carbonation of bending faults in the deep carbon cycle
キーワード:carbon cycle, slab mantle, bending fault, serpentinite carbonation, salinity, carbonation efficiency
The deep carbon cycle governs the Earth’s surface CO2 concentration and temperature and, thus, the Earth’s habitability. Serpentinite carbonation contributes to the deep carbon cycle. Recently, geophysical and numerical studies found considerable hydrothermal alterations in deep bending faults, implying a potentially larger carbon storage in slab mantles. However, a quantitative determination of carbon uptake at outer rise regions is lacking. Here, we experimentally constrain the carbonation reaction in serpentinite–H2O–CO2–NaCl systems under mantle bending fault conditions (180 MPa and 300–500 °C) to estimate carbon uptake in slab mantles. In all systems, run products are composed of residual serpentinite and a reaction zone. The reaction zone primarily consists of magnesite and talc. The thickness of the reaction zone (L) increases with increasing temperature and running time but decreases with increase in salinity. Importantly, we find that NaCl can effectively decrease the serpentinite carbonation efficiency, especially at low salinities < 5 wt.%. The reduction of carbonation efficiency induced by NaCl is attributed to the reduction of H2O and CO2 activity (aH2O and aCO2) in systems under the presence of salt. Based on the reaction rate under 3 wt.% NaCl conditions, we estimate that 5.8–35.9 × 106 Mt carbon can be added into slab mantles via the serpentinite carbonation at outer rise regions, contributing a subducted carbon flux of 5.8–35.9 Mt/yr. Our study provides a quantitative estimation for carbon uptake in slab mantles and a potential scenario for the deep carbon cycle.