Experimental study of Mg-rich silicates carbonation at 400 and 500 °C and 1 kbar

Abstract

The reactivity of olivines, orthopyroxenes and serpentines (chrysotile) with CO 2 rich fluids was experimentally studied at 400–500 °C and 1 kbar (+/− NaCl). Levels of formation of solid carbonate phases were measured both by analysing the recovered gas phase and by step heating of the solid samples. Carbonation levels of several percents per hour (from 3 to 57% in 4 h depending upon experimental conditions) were measured with increasing efficiency in the order: orthopyroxene, chrysotile, olivine. In the case of olivine, a positive impact of water fugacity and salinity on carbonation levels was evidenced. Microstructures of the samples showed that the carbonation reaction proceeded by dissolution/precipitation. A coupling between solid carbonate production, mostly as magnesite, and olivine serpentinisation was demonstrated. Isotopic compositions of carbon in gas phases and carbonates were measured. They were found to be consistent with mass balance calculations, additionally suggesting the existence of an accessory carbon-bearing phase carrying negative 13C signatures. This phase was identified by transmission electron microscopy, as an ill ordered graphite phase, which would account for about 15% of carbon mineral under these conditions.