P‐T‐H ₂ O Dependence on Structure and Transport Properties in Supercritical SiO ₂ ‐H _{2 Yifan Lu, Yicheng Sun, Kai Wang, Yingchun Zhang, Guo‐Guang Wang, Xiancai Lu}

Abstract

Supercritical fluids play a key role in the transport of trace elements (such as high‐field‐strength elements, HFSE) in subduction zones, yet their structure and transport properties across temperature, pressure, and water content ( P‐T‐H ₂ O ) remain lacking. Using machine learning force field (MLFF) and reactive force field (ReaxFF), we investigate the structure and transport properties of SiO ₂ ‐H ₂ O system at 1500–3000 K, 1–10 GPa, and 0–80 wt% H ₂ O. Water content primarily controls silicate network structure, while temperature and pressure have minor effects. The pressure dependence of transport properties reverses at 30–40 wt% H ₂ O, which corresponds to the disappearance of compositional heterogeneity. A modified Arrhenius equation is proposed to describe viscosity with P , T , and H ₂ O , showing good compositional scalability. Using this model, we quantified the mobility and ascent velocities of supercritical SiO ₂ ‐H ₂ O fluids along isotherms and geotherms. In the SiO ₂ ‐H ₂ O system, ascent velocities along isotherms range from 3.16 × 10 ⁻¹ to 5.70 × 10 ⁵  m/y at 1500 K, revealing a potential “isothermal acceleration” mechanism in subduction zones. At the source depth of ocean island basalts (OIB), supercritical SiO ₂ ‐H ₂ O fluids have maintained stable migration capabilities since 70 Ma, with ascent velocities ranging from 1.69 × 10 ³ to 7.66 × 10 ⁴  m/y, which are 8.5–383 times those of basaltic magma. This significant flux disparity between fluids and rocks provides quantitative evidence that supercritical fluids can act as efficient carriers to transport sufficient quantities of HFSE, explaining the unique geochemical characteristics of OIB.