Quantification of thermally-controlled metamorphic decarbonation and carbonate dissolution in subduction zones

Oceanic subduction zone is the dominant (if not the only) pathway for transporting carbon into the interior of the Earth, and thus plays a critical role in deep carbon cycling. Several mechanisms have been proposed for slab decarbonation process, with two primary ones being metamorphic decarbonation and carbonate dissolution. The metamorphic decarbonation has been widely analyzed by numerical models in the closed system (i.e., with constant water content). However, the water and carbon evolutions in subduction zone are strongly coupled together, leading to an open system in which the water cycling not only affects the metamorphic decarbonation, but also controls the dissolution of carbonates. However, the decarbonation efficiency and the contributions of different decarbonation mechanisms to slab carbon removal remain controversial. Here, we develop a coupled thermo-metamorphic-dissolution model to investigate physicochemical decarbonation processes. Systematic numerical models with variable thermal parameters (Φ = slab age × subduction velocity / 100) have been conducted in both closed and open systems. The results indicate that the metamorphic carbon outflux in open system is lower than that in closed system, whereas the dissolved carbon outflux in open system is approximately three times higher due to fluid infiltration. Moreover, the metamorphic carbon outflux decreases exponentially with Φ in both closed and open systems. In contrast, the dissolved carbon outflux exhibits a nearly linear increase with Φ < 13 km, followed by an exponential decrease with Φ ≥ 13 km. The new models provide systematic and quantitative constraints for the deep carbon cycling in subduction zones.