Densities, structures and refractive indices of carbonate glasses at high pressure

• Applicants: Dr. Sergey Lobanov, Ph.D.; Professor Dr. Björn Winkler
• Subject Area: Mineralogy, Petrology and Geochemistry ; Geophysics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 513317185

Carbonate melts have remarkable physical properties at pressure-temperature conditions of the uppermost mantle. In particular, their low density and viscosity set them apart from silicate melts. These properties of carbonate melts, however, may undergo dramatic pressure-induced changes due to the expected crossover in carbon coordination by oxygen (CO3 → to CO4), due to the formation of pyrocarbonate (C2O5)-groups, or due to polymerization of the CO4-groups. However, the extant experimental evidence for the presence and role of four-fold and/or polymer-ized carbon in carbonate melts is scarce due to technical challenges associated with probing the melt structure at extreme pressure-temperature conditions. Here we propose to measure the density and the refractive index including its wave-length-dependence of carbonate glasses at high pressure using the capabilities of a setup we have recently developed. We will establish if the changes in the optical properties of carbonate glasses are due to pressure-induced changes in the carbon coordination by oxygen and/or changes in the polymerization of CO3- and CO4-groups. Specifically, we propose a study of K2CO3–MgCO3 glasses (with 40-60 mol.% MgCO3), which are among the few compositions quenchable from the carbonate melt. The refractive indi-ces and densities of these glasses at high pressure will be compared to that of crystalline car-bonates in the K2CO3–MgCO3–CO2 system with carbon in diverse bonding configurations. This comparison will constrain the pressure range of the anticipated crossover from three- to four-fold coordinated carbon and of a change in the polymerization of CO3- and CO4-groups in the car-bonate glasses. The local atomic structures and the polymerization of the crystalline and amor-phous carbonates in the K2CO3–MgCO3–CO2 system at high pressures will be characterized using X-ray diffraction, pair distribution function analysis, and Raman spectroscopy. We will also determine the partial molar volumes of K2O, MgO, and CO2 in the carbonate glass/melt as a function of pressure. These results will allow a comparison to the sparse literature data on the partial molar volume of CO2 in silicate melts under upper mantle conditions. The ex-tant and our new data will lay the foundation of a predictive density model of a carbonate-silicate melt under mantle pressure-temperature conditions.

• DFG Programme: Research Grants
• Co-Investigators: Dr. Lkhamsuren Bayarjargal; Dr. Sergio Speziale