Properties of Hydrous Aluminosilicate Melts at High Pressures

Abstract

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.

Density of NaAlSi2O6 Melt at High Pressure and Temperature Measured by In-Situ X-ray Microtomography

“…The high-pressure densities determined in this study are consistent with the room-pressure density calculated from the ideal mixing model using partial molar volumes of the oxide components [54]. Figure 4 compares our results with previous studies on jadeite melt including X-ray absorption measurements from Sakamaki [33], FPMD simulations from Bajgain et al [38], and classical molecular dynamics (MD) simulations from Suzuki et al [21]. After correcting for thermal effects, our data are mostly consistent with the results from the FPMD [38] over the entire pressure range of our experiments.…”

“…Figure 4 compares our results with previous studies on jadeite melt including X-ray absorption measurements from Sakamaki [33], FPMD simulations from Bajgain et al [38], and classical molecular dynamics (MD) simulations from Suzuki et al [21]. After correcting for thermal effects, our data are mostly consistent with the results from the FPMD [38] over the entire pressure range of our experiments. Although our results also agree with those of classical MD simulations from Suzuki et al [21] between~1-3 GPa, a deviation in the pressure effects on density can be observed.…”

“…The high viscosity, high reactivity, and low density of alkali-rich melts hinder the application of some of the most widely used melt density measuring techniques, such as the sink-float [1,[26][27][28][29] and shock-wave [30][31][32] techniques. Until now, the only experimental data on the density of jadeite melt at high pressures were obtained by Sakamaki [33] using the X-ray absorption method [34][35][36][37], in which the density was calculated from the X-ray absorption contrast between the sample and diamond lid using the Beer-Lambert law and the mass absorption coefficients, but his results are not in agreement with a recent first-principles molecular dynamics (FPMD) study on jadeite melt [38].…”

“…This would result in a deviation of about 4% in the estimated X-ray travel lengths in the sample and hence the absolute density at a given mass absorption coefficient. [38], Sa17-Sakamaki [33] and Su11-Suzuki et al [21].…”

“…It should be noted that although a very small ′ (<3) is not compatible with the BM-EOS and gives no reasonable solutions, the compression curve calculated from the BM-EOS using a very small ′ (e.g., 1.3) can pass through the data (the thin black dash-dotted line in Figure 4). In order to examine the possibility of a very small ′ , we also fit the data to the Murnaghan [38], Sa17-Sakamaki [33] and Su11-Suzuki et al [21].…”