Properties of Hydrous Aluminosilicate Melts at High Pressures

In
this study, we use first-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.