Carbon dioxide in silicate melts: A molecular dynamics simulation study

Abstract

The distribution, recycling and storage of carbon in the Earth are of fundamental importance to understand the global carbon cycle between the deep Earth and near surface reservoirs. Degassing of CO2 at mid-ocean ridges may give information on the source region but the very low solubility of CO2 in tholeitic basalts has for consequence that near all Mid-Ocean Ridge Basalts glasses exsolve their CO2 rich vapor at shallow depth as they approach the ocean floor. Hence their CO2 contents mostly represent the pressure at eruption and not the source region. Recent petrological investigations have shown that the presence of carbonates at depth in the upper mantle has a large effect on the solidus of carbonated silicates by inducing incipient melting at much lower temperature. So the role of carbon-rich melts at great depth is now becoming a credible scenario to explain the extraction of CO2 from the source region to the surface. During the last three decades many studies have been devoted to measure the solubility of CO2 in silicate melts of various composition. But due to experimental difficulties these studies were generally restricted to low and moderate pressures (below ~20 kbar). By performing a series of molecular dynamics simulation where a supercritical CO2 phase is in contact with a silicate melt of various composition (from felsic to ultrabasic) at different temperatures (1473-2273K) and pressures (20-150kbar), we have been able to evaluate the solubility of CO2, the population of molecular and carbonate species, their diffusivity through the melt and the local structure. We show that this kind of molecular simulation is a useful theoretical guide to better understand the behavior of CO2 in magmas at depth.