C–O–H fluid solubility in haplobasalt under reducing conditions: An experimental study

We conducted an experimental study to constrain the C–H–O solubility and speciation in hydrous silicate melts equilibrated under reduced fO2 conditions. Haplobasaltic glasses in the NCMAS-C–O–H system were synthesized using IHPV at 1250 °C, 200–300 MPa with variable applied fH2 so as to vary fO2. Recovered rapidly quenched glasses were characterized using various spectroscopic methods: Micro-FTIR, Raman and 13C-MAS NMR. Glass CO2 content changes from 680 to 1320 ppm between ΔFMQ − 2.6 and ΔFMQ + 2.6 independently of H2O content changing from 1.3 to 4.0 wt.%. Recent thermodynamic modeling of isobaric CO2–H2O solubility fails to reproduce our CO2–H2O solubility trend under reducing conditions. The lower CO2 solubility in the melt as compared to more oxidized conditions is directly correlated to the decrease of fCO2 within the fluid phase under reducing conditions. Carbonate groups (CO32−), OH− and H2Omol are the volatile species in the glasses. No evidence for CH4, carbides or organic compounds was observed. 13C-MAS NMR analysis suggests that several carbonate units are coexisting in the glasses. {1H} 13C-CPMAS NMR suggests that all CO32− units are surrounded by OH groups. Those environments appear to slightly change with changing fO2 conditions suggesting a different degree of hydrogenation in the vicinity of the carbonate groups. Our data show that the presence of a significant amount of dissolved does not increase the solubility of species such as CO or CH4. In other words, such species remain insoluble in basaltic melts, as established under dry conditions. Altogether, our CO2 solubility results show that a wet but reduced basalt will degass more C-species than if oxidized, owing to the lower prevailing fCO2 and insoluble character of CO. The presence of an important fraction of CO in the fluid phase will have a large impact on the primitive atmospheric compositions of Mars and the Earth.

Research Highlights
► Experimental investigation of C–H–O solubility in silicate melts.
► CO2 and H2O are the only dissolved species in melt under moderately reducing fO2.
► Applying low fO2 conditions induces a decrease in the CO2 solubility.
► CO2 speciation shows different hydrogenated environments.