Stability of CO2 Fluid in Eclogitic Mantle Lithosphere: Thermodynamic Calculations

Abstract

Findings of solid and liquefied CO2 in diamonds from kimberlites and placers have indicated its presence in the form of a fluid phase in the Earth’s mantle at depths of 150–250 km. However, this is inconsistent with the results of experiments and existing thermodynamic calculations. To clarify this, we carried out thermodynamic modeling of garnet–CO2 and bimineral eclogite–CO2 systems using the Perple_X v. 7.1.3 software package, which establishes the most thermodynamically favorable assemblages for a given bulk composition of the system, unlike previous calculations, for which the phase relationships were simply assumed. The key difference between our results and previously known data is the presence of a region of partial carbonation. In this region, the garnet and clinopyroxene of the new compositions, CO2 fluid, carbonates, kyanite, and coesite are in equilibrium. The calculations revealed that unlike endmember systems (pyrope–CO2 and diopside–CO2) in the eclogite–CO2 system, the carbonation and decarbonation lines do not coincide, and the Grt+Cpx+CO2 and Carb+Ky+Coe+Cpx fields are separated by the Grt+Cpx+CO2+Carb+Ky+Coe region, which extends to pressures exceeding 4.3–6.0 GPa at 1050–1200 °C. This should extend the CO2 stability field in the eclogitic mantle to lower temperatures. Yet, owing to the short CO2 supply in the real mantle, the CO2 fluid should be completely spent on the carbonation of eclogite just below the eclogite + CO2 field. Thus, according to the obtained results, the CO2 fluid is stable in the eclogitic mantle in the diamond stability field at temperatures exceeding 1250 °C and pressures of 5–6 GPa.