Mass transfer in the Earth’s interior: fluid-melt interaction in aluminosilicate–C–O–H–N systems at high pressure and temperature under oxidizing conditions

Abstract

Understanding what governs the speciation in the C-O-H-N system aids our knowledge of how volatiles affect mass transfer processes in the Earth's interior. Experiments with aluminosilicate melt + C-O-H-N volatiles were, therefore, carried out with Raman and infrared spectroscopy to 800°C and near 700 MPa in situ in hydrothermal diamond anvil cells. The measurements were conducted in situ with the samples at the desired temperatures and pressures in order to avoid possible structural and compositional changes resulting from quenching to ambient conditions prior to analysis. Experiments were conducted without any reducing agent and with volatiles added as H 2 O, CO 2 , and N 2 because both carbon and nitrogen can occur in different oxidation states. Volatiles dissolved in melt comprise H 2 O, CO 3 2-, HCO 3 -, and molecular N 2 , whereas in the coexisting fluid, the species are H 2 O, CO 2 , CO 3 2-, and N 2 . The HCO 3 -/CO 3 2-equilibrium in melts shift toward CO 3 2-groups with increasing temperature with ΔH = 114 ± 22 kJ/mol. In fluids, the CO 2 abundance is essentially invariant with temperature and pressure. For fluid/melt partitioning, those of H 2 O and N 2 are greater than 1 with temperature-dependence that yields ΔH values of − 6.5 ± 1.5 and − 19.6 ± 3.7 kJ/mol, respectively. Carbonate groups, CO 3 2-are favored by melt over fluid. Where redox conditions in the Earth's interior exceed that near the QFM oxygen buffer (between NNO and MW buffers), N 2 is the stable nitrogen species and as such acts as a diluent of both fluids and melts. For fluids, this lower silicate solubility, in turn, enhances alkalinity. This means that in such environments, the transport of components such as high field strength cations, will be enhanced. Effects of dissolved N 2 on melt structure are considerably less than on fluid structure.

Redox controls on H and N speciation and intermolecular isotopic fractionations in aqueous fluids at high pressure and high temperature: Insights from in-situ experiments

“…We may infer, as we did for O•••HO and O•••DO intermolecular bonds, that the presence of Si in experiment C favors isotopic exchanges, or that it favors the formation of Si-H bonds at higher temperatures reducing the abundances of CH 4 relative to CH x D y and of NH 3 relative to ND 3 . Another possibility is that oxidizing conditions prevailing in experiment C favors isotopic exchanges, which would be consistent with what was observed by Mysen (2018), who observed a greater temperature effect on D/H exchanges between an aqueous fluid and a silicate melt under oxidizing conditions than under reducing conditions.…”

Experimental Petrology Applied to Natural Diamond Growth

“…This solubility is higher than the 11.5 wt.% and 6.5 wt.% CO 2 dissolved in CaCO 3 and MgCO 3 melts, respectively, at liquidus conditions at 2.7 GPa (Huang and Wyllie 1976). To our knowledge, there are no spectroscopic data on solution mechanisms of either C or CO 2 in carbonate melts at diamond-stable P, T. This topic would of course be an ideal in situ study, such as those done at lower pressures (~700 MPa e.g., Mysen 2018), when technology has evolved sufficiently to make measurements at these conditions possible.…”

Nitrogen in the Earth: abundance and transport

“…14). The temperature-dependence results in a ΔH-value of − 5.9 ± 0.9 kJ/mol under the assumption of no pressure-dependence in the 0.6-1.4 GPa pressure range of those experiments (Mysen 2018a). This ΔH-value is significantly less than in the case of molecular N 2 with its enthalpy value at − 20 ± 4 kJ/mol (Mysen 2018b) suggesting, perhaps, greater structural similarity between reduced nitrogen species in fluids and melts than is the case for molecular N 2 .…”