[S-IT17] Mass and energy transport in the crust and mantle: from properties to processes

[SIT17-04] Experimental constraints on carbon sequestration in the mantle wedge

*guoji wu^1,2,3, Yongsheng HUANG^2,1, Michihiko Nakamura⁴, Tatsuki Tsujimori⁴, Yuan Li⁵ (1.Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences, 2.CAS Center for Excellence in Deep Earth Science, 3.College of Earth and Planetary Sciences, University of the Chinese Academy of Sciences, 4.Department of Earth Science, Graduate School of Science, Tohoku University, 5.Bayerisches Geoinstitut, University of Bayreuth)

Keywords:subduction zone, mantle wedge, carbon cycle, carbonation, multicomponent fluid

Subduction zones play a crucial role in regulating the global carbon (C) cycle. Recent geochemical and geophysical findings have revealed widespread carbonation of mantle rocks, including serpentinite and olivine, within mantle wedges, indicating the existence of a potentially vast C reservoir. Nevertheless, the quantitative assessment of C inventory resulting from the carbonation of mantle rocks in multicomponent subduction fluids remains poorly understood. In this study, we investigated serpentinite and olivinite carbonations in H₂O–CO₂–NaCl fluids under various pressure-temperature conditions mimicking those in the mantle wedge. Our results indicate that the carbonation reaction extent increases with rising pressure-temperature and CO₂ concentration in fluids, but decreases with increasing salinity, especially at low levels (< 10 wt%). This decrease is due to reduced fluid pH, decreased CO₂ and H₂O activities, and increased magnesite solubility in salt-bearing fluids. Based on previous and our data, we derived an empirical equation to describe the carbonation reaction extent in these fluids. By extrapolating these findings to mantle wedge conditions, we estimate that this process can sequester 33.7–42.4 million tons of C per year globally within mantle wedges. Some of this stored C may remain in cold, stagnant regions of the wedge, potentially contributing to long-term C storage and seismic activity. Additionally, processes like down-dragging and subduction erosion can transport C to partial melting regions, leading to volcanic emissions in arc regions. This study provides valuable insights into the C cycle and seismic responses within subduction zones.