Reactive fluid infiltration along fractures: Textural observations coupled to in-situ isotopic analyses

Abstract

Fluid-rock interaction occurred in hydrothermal veins in dolomitic wall pendants during cooling of the Bruffione granodiorite (Adamello batholith, Italy). These veins formed through fluid infiltration along a central fracture, and the fluid reacted with the dolomite to precipitate forsterite + calcite in cm-wide replacement zones through SiO2 metasomatism. Textural observations have been coupled to in-situ stable isotope and chemical analysis of dolomite, calcite and forsterite to show that stable isotope exchange occurs only through mineral reaction and recrystallization. We identified two distinct reaction regimes; (1) a regime dominated by net transfer reactions, which is characterized by the complete dissolution of dolomite and the formation of the new vein minerals (i.e. forsterite + calcite) close to the open fracture. The driver of the reaction is the large chemical disequilibrium introduced by infiltration of fluid of magmatic origin. And (2) a regime dominated by the dissolution and reprecipitation of dolomite more distal to the central fracture. The dolomite protolith is partially dissolved along grain boundaries, but also trough the grains which are being fragmented, and reprecipitation of newer generations of dolomite rims occur. A progressive change in fluid composition is detected with increasing distance from the vein. In comparison to the first regime, the system is rock-dominated, and only small amounts of fluids and small chemical disequilibrium at the trace element level are suggested to be the drivers for this reaction. A fundamental understanding of mineral reactions in the presence of fluids is necessary for many areas of science and industry, and recrystallization reactions driven by very small chemical disequilibrium as described here are important to be identified and integrated into reactive transport models. In fact, for this case study, the fluid-rock interaction volume would be largely underestimated if only the net transfer reaction zone is considered. This could be crucial for safety evaluations in technical applications.