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<p>Supercritical fluids play a crucial role in material transport within Earth's deep interior. Investigating the pressure-dependent atomic structures and transport properties of such fluids is essential for understanding their petrological, chemical, and geophysical behaviors. In this study, we employed first-principles molecular dynamics simulations to explore the structures, self-diffusion coefficients (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(D\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>D</mi> </math></EquationSource> </InlineEquation>), and viscosities (<i>η</i>) of supercritical NaAlSi<sub>3</sub>O<sub>8</sub>–H<sub>2</sub>O fluids under conditions of 2000&#xa0;K and 3–10 GPa, with water contents of 30 wt% and 50 wt%. Our calculations indicate that at a water content of 30 wt%, Q<sup>2</sup> and Q<sup>3</sup> exhibit a certain degree of positive and negative pressure dependence, respectively, while other Q<sup>n</sup> species (n represents the number of bridging oxygens connected to Si/Al) show minimal changes. At a water content of 50 wt%, Q<sup>2</sup> and Q<sup>0</sup> exhibit a certain degree of positive and negative pressure dependence, respectively, while other Q<sup>n</sup> species show minimal changes. At both water contents, Si–O–H and molecular water in the system exhibit negative pressure dependence, suggesting that the migration of supercritical fluids from deep to shallow regions is accompanied by the release of water. The self-diffusion coefficients in the supercritical NaAlSi<sub>3</sub>O<sub>8</sub>–H<sub>2</sub>O fluid follow the order <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(D\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>D</mi> </math></EquationSource> </InlineEquation><sub>Na</sub> ≈ <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(D\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>D</mi> </math></EquationSource> </InlineEquation><sub>H</sub> &gt; <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(D\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>D</mi> </math></EquationSource> </InlineEquation><sub>O</sub> &gt; <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(D\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>D</mi> </math></EquationSource> </InlineEquation><sub>Al</sub> ≈ <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(D\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>D</mi> </math></EquationSource> </InlineEquation><sub>Si</sub>, with an overall weak negative pressure dependence. By comparing the viscosities of anhydrous and hydrous silicate melts from previous studies, we found that the addition of water caused a transition from negative to positive pressure dependence of viscosity, corresponding to a structural change from polymerization to depolymerization. Additionally, we calculated the fluid mobility Δ<i>ρ</i>/<i>η</i> of supercritical NaAlSi<sub>3</sub>O<sub>8</sub>–H<sub>2</sub>O fluids and found that their mobility is several orders of magnitude higher than that of basalt melt and is also significantly greater than that of carbonate melt. As supercritical fluids ascend from deeper to shallower regions, their mobility is further enhanced, significantly contributing to the transport of elements from subducting slabs to the overlying mantle wedge.</p>

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Pressure dependence of the structures and transport properties of supercritical NaAlSi3O8–H2O fluids

  • 期刊论文

  • 发表日期：

    2025年5月29日

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  • Ziteng Long,
  • Yicheng Sun

摘要

Supercritical fluids play a crucial role in material transport within Earth's deep interior. Investigating the pressure-dependent atomic structures and transport properties of such fluids is essential for understanding their petrological, chemical, and geophysical behaviors. In this study, we employed first-principles molecular dynamics simulations to explore the structures, self-diffusion coefficients ( \(D\) D ), and viscosities (η) of supercritical NaAlSi3O8–H2O fluids under conditions of 2000 K and 3–10 GPa, with water contents of 30 wt% and 50 wt%. Our calculations indicate that at a water content of 30 wt%, Q2 and Q3 exhibit a certain degree of positive and negative pressure dependence, respectively, while other Qn species (n represents the number of bridging oxygens connected to Si/Al) show minimal changes. At a water content of 50 wt%, Q2 and Q0 exhibit a certain degree of positive and negative pressure dependence, respectively, while other Qn species show minimal changes. At both water contents, Si–O–H and molecular water in the system exhibit negative pressure dependence, suggesting that the migration of supercritical fluids from deep to shallow regions is accompanied by the release of water. The self-diffusion coefficients in the supercritical NaAlSi3O8–H2O fluid follow the order \(D\) D Na ≈ \(D\) D H > \(D\) D O > \(D\) D Al ≈ \(D\) D Si, with an overall weak negative pressure dependence. By comparing the viscosities of anhydrous and hydrous silicate melts from previous studies, we found that the addition of water caused a transition from negative to positive pressure dependence of viscosity, corresponding to a structural change from polymerization to depolymerization. Additionally, we calculated the fluid mobility Δρ/η of supercritical NaAlSi3O8–H2O fluids and found that their mobility is several orders of magnitude higher than that of basalt melt and is also significantly greater than that of carbonate melt. As supercritical fluids ascend from deeper to shallower regions, their mobility is further enhanced, significantly contributing to the transport of elements from subducting slabs to the overlying mantle wedge.