Deep carbon cycling in subduction zones: 1. Thermally controlled metamorphic decarbonation of subducting oceanic slab

SUMMARY Oceanic subduction zone is the dominant pathway for transporting carbon into the interior of the Earth, and thus plays a critical role in deep carbon cycling. Despite being recognized as a key mechanism for slab carbon release in subduction zones, the metamorphic decarbonation outflux and efficiency remain subjects of ongoing debate. The thermal structure of subduction zone is widely recognized as a primary dynamic control on metamorphic decarbonation, however, the quantitative relationship between metamorphic carbon outflux and simplified thermal parameters of subduction zones (here defined as φ = slab age × subduction velocity / 100 in km) remains poorly constrained. On the other hand, previous studies on metamorphic decarbonation have been conducted within two distinct scenarios: the P–T-dependent decarbonation (PTD) system versus P–T–H2O-dependent decarbonation (PTHD) system, yet a quantitative comparison between these two scenarios remains lacking. In order to investigate the metamorphic decarbonation behaviour of subducting slab in the PTD versus PTHD systems, we develop a coupled thermo-petrological model by integrating the thermodynamic dataset of temperature–pressure–(H2O)-dependent CO2 content into the thermal model of subduction zones. Systematic numerical models indicate that the metamorphic carbon outflux in the PTHD system is about 50 percent lower than that predicted in the PTD system. Meanwhile, the quantitative functional relationship has been built between the metamorphic carbon outflux and φ, which reveals that the decarbonation outflux and efficiency decrease exponentially with increasing φ in both systems. Under present-day widespread subduction thermal conditions (φ ≈ 30 km), both PTD and PTHD system models yield low metamorphic decarbonation efficiency, suggesting that a substantial proportion of slab carbon is likely retained in the slab and transported into the deeper mantle.