Carbon bearing aluminosilicate melt at high pressure

Abstract

It is well known that volatiles such as water has significant influence on the properties of silicate melts. Carbon dioxide (CO2) is also an abundant volatile in deep Earth, however the effect of CO2 on the properties of polymerized melts, particularly the transport properties, are poorly understood. This is crucial for better understanding of the generation and migration of carbon bearing silicate magma in deep crustal and mantle settings. In this study, we explore the structure and properties of carbon bearing aluminosilicate melt up to a pressure of ~25 GPa and temperature range of 2500–4000 K using first principles molecular dynamics (FPMD) simulation. Our results show that CO2 in the aluminosilicate melts dissolves as molecular CO2 and carbonate (CO32-) at lower pressures (~0–3 GPa). However, at higher pressures (>3 GPa) relevant to most of the upper mantle, CO32- is predominant carbon species along with carbon in 4-fold coordination (CO4). Fraction of CO32- increases with decreasing temperature and increasing pressure. We find that at the reference isotherm (2500 K), the density of the aluminosilicate melt is reduced by addition of CO2 (in wt.%) with dρdXCO2 = −0.0214. Effect of water on the density of melt is more pronounced with dρdXH2O = −0.0422. Thus, the gravity-driven buoyancy of volatile rich magma will be greater than that of the magma without volatile components. The compressibility of the aluminosilicate melt is also affected by volatiles. For instance, both bulk modulus (KT0) and its pressure derivative (KT0') for volatile bearing melts are respectively lower and higher than that of dry melt. Aluminosilicate melts are highly polymerized and show an anomalous pressure dependence of melt viscosity. Pressure dependence of viscosity at low pressure regime (P < 5 GPa) is significant with dlog(η)dP=-2.06. Melt viscosity is further reduced by the addition of volatiles. Effect of volatiles on viscosity is more pronounced at low pressures and it minimizes at pressures where the melt viscosity exhibits minima. Due to the lower viscosity and larger buoyancy for both the H2O and CO2 bearing aluminosilicate melts at crustal and upper mantle depths, magma mobility (Δρη) is greater for volatile bearing melts. Thus, the residence time of such melts are expected to be shorter than volatile free melts in those settings. Like the mobility, electrical conductivity of aluminosilicate melts also increases when volatile is present in the silicate melt. We find that that as low as <1 vol.% volatile bearing aluminosilicate melt mixed with mantle matrix could be sufficient to explain the observed electrical conductivity anomalies in the upper mantle.