PROPOSED EXPERIMENTAL DETERMINATION OF CALCITE-DOLOMITE-ANKERITE PHASE RELATIONS

Experimental phase equilibria studies provide key information for thermobarometry. There are few studies of the important Ca-Mg-Fe carbonate ternary phase relations using modern experimental techniques. The main phases are calcite (CaCO₃), dolomite (CaMg(CO₃)₂), ankerite (CaFe(CO₃)₂), magnesite (MgCO₃), and siderite (FeCO₃). Compositions of co-existing calcite solid solution (calcite ss) and dolomite solid solution (dolomite ss) have been calibrated as a geothermometer (Anovitz & Essene (1987, J. Petrol., 28, 389-414). This is a crucial geothermometer for contact metamorphic aureoles, and the calibration affects related studies of heat and fluid flow; pressure-temperature-time paths; diffusion studies; and deformation studies. General phase relations in the ternary system (Rosenberg (1967, Amer. Mineral., 52, 787-796); Goldsmith et al. (1962, J Geology, 70, 659-688) are mapped out but not known in detail. Limited work considers the addition of iron to the system.

Anovitz & Essene collated available reversed experimental data with carefully considered natural parageneses to devise a carbonate geothermometer appropriate for some iron bearing compositions. The ternary tie lines between calcite ss and dolomite ss are, by their own admission, rather poorly known. In my analysis there is a frighteningly small amount of experimental data used to calibrate this geothermometer. Experimental techniques have vastly improved since the 1960's.

I propose piston-cylinder experiments over a range of temperatures to determine the equilibrium compositions of coexisting calcite ss and dolomite ss in the iron-bearing part of the system. Results would include a greatly improved geothermometer, appropriate for Fe-bearing carbonates, expanding applications; and clarified phase relationships on the iron-rich side, especially the position of the calcite ss-ankerite ss-siderite ss 3 phase triangle. Better geothermometry will improve models of: heat flow, especially in contact aureoles; fluid flow; and stress and deformation in metamorphic rocks.