The influence of Cr on the garnet-spinel transition in the Earth's mantle: experiments in the system MgO-Cr 2O 3-SiO 2 and thermodynamic modelling

The position of the transition from spinel peridotite to garnet peridotite in a simplified chemical composition has been determined experimentally at high pressures and high temperatures. The univariant reaction MgCr ₂O ₄+2Mg ₂Si ₂O ₆=Mg ₃Cr ₂Si ₃O ₁₂+Mg ₂SiO ₄, has a negative slope in P- T space between 1200 °C and 1600 °C. The experimental results, combined with assessed thermodynamic data for MgCr ₂O ₄, MgSiO ₃ and Mg ₂SiO ₄ give the entropy and enthalpy of formation of knorringite garnet (Mg ₃Cr ₂Si ₃O ₁₂). Thermodynamic calculations in simplified chemical compositions indicate that Cr shifts the garnet-in reaction to much higher pressures than previously anticipated. Moreover, in Cr-bearing systems a pressure-temperature field exists where garnet and spinel coexist. The width of this divariant field strongly depends on the Cr/(Cr+Al) of the system.