Phase Equilibria Analysis for Metacarbonate With Applications to Zoned Calc-Silicate Aureoles

Abstract

Carbonate rocks react with infiltrating hydrothermal fluids to produce zoned calcsilicate assemblages in contact aureoles. Petrogenetic grids provide valuable insights into phase relations, metamorphic temperature (T) and the fluid composition (X) of the metacarbonate systems, as well as semi-quantification of the prograde decarbonation at convergent boundaries. In this study, we constructed T-X_CO2 (composition of H₂O–CO₂ binary fluid) grids in the system CFMASHc (CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–CO₂), supplemented with Fe₂O₃ or TiO₂, and its subsystems (CMASHc, CMSHc, CFSHc and CASHc). The grids were constructed to encompass upper crustal conditions, with temperatures ranging from 300°C to 1000°C at 2 kbar and 4 kbar, and X_CO2 from 0 to 0.8 (0 = pure water). We adopted internally consistent thermodynamic datasets and compatible activity–composition models for solid solutions. The grids illustrate the index minerals and field gradients observed in classical aureoles. Typical calcsilicate assemblages in these contact aureoles appear along a heating trajectory at a relatively low X_CO2, in the sequence of talc, tremolite, diopside (±olivine), garnet and wollastonite. The grids in the CASHc, CMSHc and CMASHc subsystems are sufficient to cover important reactions that lead to the formation and decomposition of these minerals. The grids with an additional TiO₂ component help interpret phase relations involving rutile, titanite and ilmenite. In addition, we note that phase relations calculated with endmember carbonates are practically similar to those calculated for a complete ternary solid-solution model at low-to-mid temperatures (< 600 °C). In this study, we recalculated reactions in subsystem grids from previous studies across various P-T-X_CO2 conditions within a consistent framework. These results are contextualized with natural assemblages and applied to constrain the field gradient of a representative contact aureole. By incorporating additional components, the grids accommodate a broader range of assemblages observed in metacarbonate rocks. Together, these expanded grids provide a robust framework for future studies of contact metamorphism in metacarbonate systems. The calculated phase equilibria were specifically applied to a contact aureole in southern Tibet, with temperature estimations derived from the phase equilibria aligning closely with a conduction model based on the timescales from diffusion speedometry.