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The amount of H 2 O and CO 2  that is carried into deep mantle by subduction beyond subarc depths is of fundamental importance to the deep volatile cycle but remains debated. Given the large uncertainties surrounding the spatio‐temporal pattern of fluid flow and the equilibrium state within subducting slabs, a model of H 2 O and CO 2  transport in slabs should be balanced between model simplicity and capability. We construct such a model in a two‐part contribution. In this Part I of our contribution, thermodynamic parameterization is performed for the devolatilization of representative subducting materials—sediments, basalts, gabbros, peridotites. The parameterization avoids reproducing the details of specific devolatilization reactions, but instead captures the overall behaviors of coupled (de)hydration and (de)carbonation. Two general, leading‐order features of devolatilization are captured: (1) the released volatiles are H 2 O‐rich near the onset of devolatilization; (2) increase of the ratio of bulk CO 2  over H 2 O inhibits overall devolatilization and thus lessens decarbonation. These two features play an important role in simulation of volatile fractionation and infiltration in thermodynamically open systems. When constructing the reactive fluid flow model of slab H 2 O and CO 2  transport in the companion paper Part II, this parameterization can be incorporated to efficiently account for the open‐system effects of H 2 O and CO 2  transport.

Devolatilization of Subducting Slabs, Part I: Thermodynamic Parameterization and Open System Effects