Water Dissolution Driving High Mobility of Diopside-H2O Supercritical Fluid

Supercritical geo-fluid serves as an ideal agent for chemical transport in the subduction zone. Yet the nature of its structure and transport properties remains elusive. Here, we provide comprehensive investigations on the atomic structures and transport properties of diopside-H₂O system (with 0–78 wt% H₂O) at 0–12 GPa and 3,000 K, based on first-principles molecular dynamics simulations. Our results reveal the prevailing coexistence of both Si-OH and Mg/Ca-OH, with the latter arising from charge compensation by the more predominate Mg/Ca-OH₂ species. The incorporation of water constantly disrupts the silicate network by converting the bridging oxygens (BOs) to non-bridging oxygens (NBOs), generating more isolated, diffusible, yet stable silicate clusters (such as monomers or oligomers), with lower coordination numbers, longer species lifetimes and more stretched bond angles. The dissolution of water significantly facilitates the diffusivities of all species, while reducing the shear viscosity. The strong linear correlations between the diffusivities/viscosity and the degree of polymerization underscore the water-induced depolymerization as the primary mechanism driving the high mobility of supercritical fluids. The viscosity contrast between anhydrous and hydrous melts could cause the substantial differences in magma mobility and ascent rates, leading to the diverse radioactive isotope patterns between the melt-source and fluid-source arc lavas. Our results highlight the critical role of water in shaping the structure and transport properties of silicate-H₂O system, and emphasize the potential importance of supercritical fluids within the subduction-related magmatism and mineralization processes.