Effects of Surface Mobility on Relevant Mantle H2O - C Fluxes and Distribution

Through Gibbs free energy solvers combined with published experimental data, we assess the structurally bound water capacity (s^H2O) of nominally anhydrous minerals, together with low and high pressure hydrous phases. These maps are implemented into a global mantle convection model to investigate the long-term evolution of the mantle water content (c^H2O). A parameter study spanning a range of yield stresses is performed, with particular emphasis on the role of surface mobility in controlling volatile exchange fluxes between the mantle and the atmosphere. Across multiple simulation ensembles, surface mobility emerges as the primary control on the intensity of ingassing between the two reservoirs. Time-series autocorrelation analysis of reservoir H₂O mass indicates that the mantle transition zone (MTZ) behaves as a transient, high-s^H2O layer that is unable to sustain long-lived hydrated states in the absence of frequent water-rich slabs penetrating beyond 410 km depth. Principal component analysis reveals divergence in simulation evolution as a function of surface yield stress, leading to distinct H₂O partitioning regimes between the MTZ and the lower mantle, with coupled increase in upper mantle c^H2O dominance. This highlights the tendency of episodic or stagnant-lid regimes to sequester water at greater mantle depths relative to tectonically active planets. Bottom-up integration of our model profiles suggests a total stored mantle H₂O in the order of 1–1.5 ocean masses, an amount significantly lower than previous estimates, resulting from the rapid decrease of s^H2O beyond 660 km depth and subsequent ease of outgassing. Because supercriticality-enhanced extraction processes are not included and a depth-dependent background permeability restricts vertical transport, this estimate should be regarded as an upper bound. We further find that the s^H2O associated with the perovskite phase is of first-order importance in determining total mantle water storage. Low convective velocities maintain relative water enrichment within the perovskite-dominated region, implying that deviations from the commonly assumed dry-perovskite composition may increase estimated storage by non-negligible amounts.

In addition, recent advances in high-pressure thermodynamic databases enable the assessment of oxygen fugacity profiles down to core–mantle boundary depths. Building on this framework, a separate suite of simulations explores a new carbon-tracking scheme that accounts for solid and molten reservoirs, redox-dependent melting interactions, and enhanced shallow magmatism, with the ultimate objective of coupling the deep carbon and water cycles.