Thermochemical calculation of log f_O2 - TP stability relations of diamond-bearing assemblages in the model system CaO-MgO-SiO₂-C-O₂-H₂O

Stability relations of coesite (Co), diopside (Di), diamond (Dm), dolomite (Do), enstatite (En), forsterite (Fo), graphite (Gr), magnesite (Ms) and CO₂-bearing fluid at X_CO2 = 0.1 in the model system CaO-MgO-SiO₂-C-O₂-H₂O were calculated at P = 4 to 8 GPá using available thermodynamic data already published. Isobaric log f_O2 -T stabilities of 4 decarbonation reactions and 4 oxidation reactions together with reaction Dm (Gr) + O₂ = CO₂ and diamond-graphite transition were determined. Parageneses of minerals as function of bulk composition were modeled for eclogite, clinopyroxenite, websterite, Iherzolite, and carbonate rocks. Reaction Co + Ms = En + Dm + O₂ stabilizes diamond in SiO₂-saturated rocks such as eclogite at f_O2 slightly higher than reaction En + Ms = Fo + Dm + O₂ (EMOD). Diamond in SiO₂-undersaturated rocks such as Iherzolites and websterites, is stable at f_o2 lower than EMOD. Bulk carbon content is also important for the appearance of diamond in silicate rocks. Diopside-rich and carbon-rich compositions are favorable for diamond occurrence in silicate rocks. Ca- and Mg-rich carbonate rocks occur 7 diamond-bearing assemblages at wide f_O2 conditions. Using calculated log f_O2 -T relations, our model PT-f_O2 paths during exhumation and corresponding changes in mineral assemblages were predicted.