The Role of Fluid Salinity in Wollastonite Reaction Rim Formation

Mineral reaction textures such as coronae are commonly observed features in metamorphic rocks and serve as valuable indicators for quantifying metamorphic processes. Their formation is typically controlled by pressure, temperature, chemical composition, as well as the presence/absence of fluids and their composition. While highly saline fluids have been reported from various metamorphic environments, their impact on reaction rim growth remains elusive.

In this study, we experimentally explored the role of fluid salinity on the formation of wollastonite reaction rims between quartz and calcite using a piston-cylinder apparatus. The starting material comprised a mixture of fine-grained calcite and coarse-grained quartz (95:5 by mass), along with 1 wt% fluid. Fluid composition was set at an XCO₂ of ~0.3 with variable CaCl₂ between 0 and 40 wt%.

Experiments involved two steps: (1) annealing at 1 GPa and 800 °C for ~2 days to promote recrystallization and produce a synthetic impure carbonate, followed by (2) reaction at 1000 °C for 24 hours to initiate wollastonite formation. Results reveal wollastonite rim formation on calcite surfaces at low CaCl₂ concentrations (0-6 wt%), while wollastonite growth on quartz surfaces has been observed at high CaCl₂ concentrations (up to 40 wt%).

We interpret our finding that fluid salinity affects solubility of minerals and therefore element mobility in grain boundary fluids, which in turn controls local mineral growth/dissolution recorded in metamorphic textures. Hence, the role of fluid composition in interpreting natural metamorphic microstructures provide potential for future studies.