Coupled Serpentinization and Carbonation in the Outer-Rise Mantle: Implications for Slab Volatile Budgets

Volatile cycling in subduction zones plays a pivotal role in regulating long-term carbon storage and the habitability of Earth's deep biosphere. In particular, serpentinization of the subducting lithospheric mantle at outer-rise regions plays a pivotal role in shallow volatile cycling, facilitating both carbonation and the production of reduced volatiles such as hydrogen and methane. These reactions not only contribute to deep carbon storage but also provide chemical energy for sustaining subsurface microbial ecosystems. However, volatile fluxes associated with this process remain poorly constrained, primarily due to the inaccessibility of the outer-rise mantle, the scarcity of direct samples, and the inherent limitations of geophysical resolution at depth. Consequently, the partitioning and fate of slab-derived volatiles prior to deep subduction remain critical unknowns. Here, we present high-resolution two-dimensional visco-elasto-plastic models that simulate coupled serpentinization and carbonation within the faulted oceanic mantle seaward of the trench. Our results show that carbonation efficiency is primarily governed by the degree of serpentinization and the partial pressure of CO₂ in infiltrating fluids. These findings provide quantitative constraints on volatile processing in the shallow slab mantle and underscore the role of tectonically focused hydration in shaping deep carbon fluxes. More broadly, they highlight how slab deformation influences the geochemical and energetic architecture of Earth's deep subsurface.