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In this study, we use f irst-principles molecular dynamics simulations to explore the behavior of anhydrous aluminosilicate melt with a stoichiometry of NaAlSi2O6 up to pressures of ∼30 GPa and temperatures between 2500 and 4000 K. We also examine the effect of water (∼4 wt % H2O) on the equation of state and transport properties of the aluminosilicate melt and relate them to atomistic scale changes in the melt structure. Our results show that water reduces the density and bulk modulus of the anhydrous melt. However, the pressure derivative of the bulk modulus of the hydrous melt is larger than that of the anhydrous melt. The pressure dependence of the transport property exhibits an anomalous behavior. At a pressure of ∼12 GPa, anhydrous aluminosilicate melts exhibit maxima in diffusion and minima in viscosity. Dissolved water in melts also affects both diffusion and viscosity. In hydrous aluminosilicate melts, the maxima in diffusion and the minima in viscosity occur at ∼14 GPa. The anomalous behavior of transport properties is related to the pressure-induced changes in the melt structure. At shallower depths, i.e., up to 100 km, relevant for subduction zone settings, the lower density compounded by the lower viscosity of hydrous aluminosilicate melts is likely to provide buoyancy for upward migration. At greater depths of ∼180−200 km, greater compressibility of the hydrous aluminosilicate melts together with the minimum viscosity could hinder magma migration and may explain the presence of a partial melt layer at the lithosphere−asthenosphere boundary.
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This content will become publicly available on December 7, 2025
On the pressure dependence of viscosity, especially for fluids that have a tendency to form glasses
Understanding fluid viscosity is crucial for applications including lubrication and chemical kinetics. A commonality of molecular models is that they describe fluid flow based on the availability of vacant space. The proposed analysis builds on Goldstein’s idea that viscous transport must involve the concerted motion of a molecular ensemble, referred to as cooperatively rearranging regions (CRRs) by Adam and Gibbs in their entropy-based viscosity model for liquids close to their glass transition. The viscosity data for propylene carbonate reveal a non-monotonic trend of the activation volume with pressure, suggesting the existence of two types of CRR with different compressibility behaviors. This is proposed to result from a change in CRR free volume (<0.2 GPa) and a growth in its size (>0.2 GPa). We use Evans–Polanyi perturbation theory to develop an analytical model for the structural changes of the CRR in function of pressure and temperature and their effect on Eyring viscosity. This analysis shows that the activation energies and volumes scale with the CRR size. Using the compressibility data of propylene carbonate, we show that the activation volume of the CRR at low pressures depends on the compressibility of an ensemble comprised of the first coordination shell around a molecule. At higher pressures, we apply an Adam–Gibbs-type analysis to model the increase in CRR size and its effect on viscosity, where the increase in size is estimated from propylene carbonate’s heat capacity. However, this analysis also reveals deviations from the Adam and Gibbs model that will guide future improvements.
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- PAR ID:
- 10561253
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 161
- Issue:
- 21
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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