Structures and transport properties of Mg2SiO4-H2O system under high temperature and pressure: insights for supercritical fluids in deep subduction zones

Understanding the structure and transport properties of supercritical fluids is crucial for gaining insights into their behavior in subduction zones. While previous research has provided an understanding of the properties of supercritical fluids derived from felsic melts, our knowledge regarding supercritical fluids formed from ultramafic melts in deep subduction zones remains limited. In this study, we employed first-principles molecular dynamics to systematically investigate the structure, speciation, self-diffusion, and viscosity of the Mg₂SiO₄-H₂O system at temperatures of 2000 K and 3000 K under a pressure of approximately 10 GPa, covering water contents ranging from 0 to 70 wt%. Our study confirmed the previous experimental observation that ultramafic melts can undergo polymerization due to dissolved water. The cause of the polymerization is the increase in 5-fold Si-O coordination, and it only occurs within specific ranges of water content at a given temperature. In the Mg₂SiO₄-H₂O system, water and OH are primarily bonded to Mg, forming Mg-H₂O_m and Mg-OH species. The results demonstrate that as the water content increases, the viscosity of the Mg₂SiO₄-H₂O system exhibits a rapid initial decrease followed by a gradual reduction. The rapid decrease in viscosity observed at 2000 K is not due to the depolymerization of the structure; on the contrary, the structure of the system becomes more polymerized during this process. The crucial factor driving the rapid viscosity decrease is the increasing proportion of protonated silicate units. The low viscosity of supercritical Mg₂SiO₄-H₂O fluid allows its mobility to reach 2 to 3 orders of magnitude greater than that of basalt melt and 1.7 to 20 times greater than that of carbonate melt. By comparing these findings with supercritical fluids derived from felsic melts, we propose that supercritical fluids formed from different silicate components in subduction zones exhibit similarly low viscosities with minor differences. The ability of supercritical fluids to facilitate element migration primarily depends on the solubility of these elements within the supercritical fluids. In the supercritical Mg₂SiO₄-H₂O fluid, we observed that Q⁰ and Q¹ species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Qⁿ species in the supercritical Mg₂SiO₄-H₂O fluid is: Q¹ > Q⁰. However, when the water content exceeds 30 wt%, the order of Qⁿ abundance shifts to: Q⁰ > Q¹. The content of Q² species is relatively low, while the concentrations of Q³ and Q⁴ species are negligible. This study reveals that the supercritical Mg₂SiO₄-H₂O fluid not only contains a significant amount of low-Qⁿ (n ≤ 1) species, but also a large proportion of OH^– species. In deep subduction zone, the supercritical fluids generated by the release of fluids from the subducting oceanic crust will be more enriched in monomers [SiO₄]. This structure is favorable for the formation of complexes with specific elements (such as high field strength elements) and promotes their mobilization by supercritical fluids. Furthermore, the substantial release of OH^– during the formation of supercritical fluid from ultramafic melts serves as an important source of OH^– in deep subduction zones.