Structural and transport properties of supercritical NaAlSi3O8-H2O fluids: Insight from ReaxFF molecular dynamics simulations

Abstract

With intermediate compositions between hydrous fluids and silicate melts, supercritical fluids are ideal agents for mass transfer from the subducting slab into the mantle wedge with fluid-like viscosity and melt-like elemental carrying capacity. However, the structure and transport properties of supercritical fluid have not been well understood. In this study, we investigated the structure and transport properties of supercritical NaAlSi₃O₈-H₂O systems by using the ReaxFF reactive molecular dynamics technique at 1000–3000 K and ∼ 2 GPa. The effect of water content and temperature on speciation, structure, diffusivity, and viscosity of supercritical fluids was explored. Si is primarily 4-coordinated, while Al exhibits coordination numbers of 3 to 6. As water content increases, Q⁴ and Q⁵ species decrease and Q⁰ species increase continuously, while the dominant species progressively transition from Q⁴ to Q³, followed by Q², Q¹, Q⁰. The elemental diffusivity and viscosity were both modeled as a function of water content and temperature. These models can be useful for the prediction of transport properties of hydrous silicate melts/fluids. Overall, the findings presented in this study formed a molecular level understanding of a typical supercritical fluid.