Antigorite-ophicarbonates: Phase relations in a portion of the system CaO-MgO-SiO2-H2O-CO2

The occurrence of critical assemblages among antigorite, diopside, tremolite, forsterite, talc, calcite, dolomite and magnesite in progressively metamorphosed ophicarbonate rocks, together with experimental data, permits the construction of phase diagrams in terms of the variables P, T, and composition of a binary CO2-H2O fluid. Equilibrium constants are given for the 30 equilibria that describe all relations among the above phases. Ophicalcite, ophidolomite, and ophimagnesite assemblages occupy partially overlapping fields in the \(T - X_{{\text{CO}}_{\text{2}} } \) diagram. The upper temperature limit of ophicalcite rocks lies below that of ophidolomite and ophimagnesite. The fluid phase in ophicarbonate rocks has \(X_{{\text{H}}_{\text{2}} {\text{O}}} > 0.8\), and there are indications that \(P_{{\text{H}}_{\text{2}} {\text{O}}} + P_{{\text{CO}}_{\text{2}} } \) during their progressive metamorphism is approximately equal to Ptotal.