[S-IT17] TRANSPORT PROPERTIES AND PROCESSES IN THE EARTH

[SIT17-02] Important role of mantle serpentinite carbonation of bending faults in the deep carbon cycle

*Yongsheng HUANG^1,2, Satoshi Okumura³, Kazuhisa Matsumoto³, Naoko Takahashi³, Hong Tang⁴, Tatsumi Tsujimori^3,5, Michihiko Nakamura³, Yuan Li^1,2 (1.Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 2.CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China, 3.Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan, 4.South China Sea Environmental Monitoring Center, SOA, Guangzhou 510300, China., 5.Center for North Asian Studies, Tohoku University, Sendai, 980-8576, Japan)

Keywords: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 (a_H2O and a_CO2) 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.