IMPACTS OF FLUIDS ON THE MINERALOGICAL RECORD IN SUBDUCTION ZONES; IMPLICATIONS FOR PRESSURE-TEMPERATURE-TIME-DEFORMATION RECONSTRUCTIONS

Metamorphic rocks that formed at high-pressure conditions in subduction zones are common target to obtain detailed Pressure-Temperature-time-Deformation (P-T-t-D) paths. These rocks are incredible archives as they can preserve a mosaic of mineral relics, in favorable cases reflecting more than one stage of transformation. Detailed petrochronological studies allow subduction-related processes to be directly investigated and enable us to obtain constraints for thermo-mechanical models.

To obtain P-T-t-D paths, it is critical to link the P-T conditions retrieved from the mineral compositions to specific deformation phases (D) or fracturing events. In most instances, this link is empirical and based on micro-textural arguments such as the preservation of textural equilibrium features. One of the key assumptions is that the metamorphic minerals in each microstructure preserve their initial compositions without significant re-equilibration. However, recent studies have shown that metamorphic fluids promote re-equilibration through pseudomorphic replacement thus affecting the mineral record.

In this contribution, we will present a few case studies in which quantitative thermodynamic models are used to model the possible effects of fluids on the preservation of mineral relics. The systematic analysis of micro textures and compositional zoning in both garnet and muscovite shows that the fluids strongly affected the preservation of mineral relics. In some cases the original link between mineral composition and microstructure is distorted. Surprisingly, the inversion of equilibrium models has proven to be of a great help to predict the amount of replacement and thus to link a given re-equilibration stage to the presence of an internally- or externally-derived fluid. This method can be also used to identify single events of fluid-rock interactions occurring at high pressure and to retrieve those conditions.