Deforming rocks in mylonitic shear zones are first-order fluid conduits in the crust, where fluids are hosted by grain boundaries, cracks and pores, all of which evolve during deformation and synkinematic reactions. This evolution, which entails coupled chemical, hydraulic and mechanical processes on multiple scales, controls where fluids will migrate and interact with rocks. However, we have no systematic and comprehensive knowledge of how these processes, and feedbacks between them, combine to form the dynamic transport properties of natural mylonitic shear zones. This shortcoming severely affects our multi-scale assessments of fluid-rock interaction at plate boundaries and at other societal interfaces with the geosphere.
In this presentation, I will provide a brief overview of mechanisms that produce (and destroy) porosity in mid-crustal mylonites. I will use high-resolution µCT and SEM data from natural shear zones and operando experiments to derive a conceptual model for how porosity and permeability evolve as a function of shear strain. This model highlights critical but measurable parameters that enable its integration into forward simulations of fluid migration in deforming crustal rocks.

Carbonation of ultramafic and mafic rocks is an important process that can occur in subduction zones, trapping and/or mobilizing carbon, sulfur, and fluid-mobile elements. Here we investigate two exceptional sequences of carbonated ultramafic and mafic rocks from the Point-Rousse Complex (Newfoundland, Canada) to understand the element mass-transfer, redox processes and physicochemical conditions triggered by fluid-flow along a major shear zone.

The complex comprises ophicarbonates (≤5.2 wt% CO2), soapstones (3.4–26.1 wt% CO2), carbonate-bearing greenschists (6.2–8.5 wt% CO2) and albite-dolomite rocks (29.1–35.4 wt% CO2). Petrography and chemical mapping reveal replacement and dissolution/precipitation textures of carbonation reactions. In ophicarbonate, serpentine is pseudomorphically replaced by magnesite and dolomite, preserving relict bastite and mesh textures. Foliated ophicarbonate exhibits the replacement of serpentine by coarse-grained talc and magnesite. Soapstone is characterized by corona-shaped reaction rims of quartz and talc surrounding dolomite crystals. Coarse-grained magnesite displays chemical zoning with increasing Fe content and core-to-rim decreasing of magnetite inclusions. Albite-dolomite rocks, composed primarily of dolomite, albite and muscovite are crosscut by centimetric albite veins. Carbonated greenschist, composed of albite, chlorite, dolomite, and pyrite, exhibits dissolution/precipitation structures associated with the formation of albite and carbonate veins. Pyrite mineralization is concentrated along albite vein margins in carbonated mafic rocks (bulk-rock S ≤3.3 wt%).

The reported carbonation occurred simultaneously in mafic and ultramafic rocks. Thus, this rock assemblage represents an ideal natural laboratory to investigate CO2 entrapment, sulfur mobility, and the sources and compositions of involved fluids.

Funding: FPI2022/PRE2023_IACT_059 linked to Grant PID2022-136471NB-C21 (RUSTED), granted by MCIN/AEI/10.13039/501100011033 and FSE+, Spain.

Porosity generation and permeability maintenance in low-porosity rock systems are crucial for Enhanced Geothermal Systems (EGS), as they enable efficient fluid flow and transport while enhancing heat exchange in low-permeability reservoirs. In EGS, reactive transport processes, triggered by chemical stimulation, drive dynamic changes in mineral composition and petrophysical properties. However, the parameters that control the efficiency of chemical stimulation of granitic rocks are incompletely understood and experimental studies are still scarce.

In this study, we investigate reactive transport processes by performing batch and flow-through experiments, analyzing the interactions of granites with acidic F-bearing aqueous fluids under simulated geothermal reservoir conditions. Our experiments aim to understand and quantify the reaction-induced porosity and permeability increase of low-porosity granite systems using fluid compositions relevant for the chemical stimulation of EGS.

We used X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) to characterize and quantify mineralogical changes while assessing the microstructural evolution of granites exposed to reactive fluids.

Our first experiments have demonstrated that significant porosity is created through chemical stimulation of low-permeability granite, driven by preferential dissolution of feldspar and mica in the host rock and the precipitation of amorphous silica and denser F-bearing phases that pseudomorphically replace the original mineral assemblages.

The chemical stimulation in the laboratory can be reproduced by reactive transport modeling. Our one-dimensional (1D) numerical model with different experimental parameters aim to recreate results from experiments, providing insights into the evolution of the system’s composition and petrophysical properties and predicting the potential for EGS reservoir development.

During the collision of continental plates, the presence of fluids can be crucial for the progress and outcome of specific metamorphic mineral reactions, mass transport or tectonic deformation. These fluid-mediated processes require the pre-existence or formation of fluid pathways. In this study, we examined the underlying mechanisms and timescales of amphibolitization of mafic crust. The initially dry and nearly impermeable Kråkeneset Gabbro in the Western Gneiss Region, Norway was hydrated and transformed into an amphibolite under amphibolite-facies conditions, while the amphibolitization process was triggered by fluid infiltration through a newly opened N–S striking fracture network and allowed the fluid to pervasively infiltrate the rock. We performed a detailed mineralogical, petrophysical and thermodynamic analysis of a sample profile perpendicular to a vein, which includes sample material from the fully reacted amphibolite, the transition zone and the most pristine gabbro. Petrological data and thermodynamic modelling show that the transition from gabbro to amphibolite was accompanied by densification and related porosity formation. We observe that the progress of the amphibolitization was controlled by fluid availability and that besides the uptake of H₂O, no significant mass exchanges were necessary for this transformation, at least down to the thin-section scale. To estimate the duration of the amphibolitization we set up a reactive transport model based on local equilibrium thermodynamics, mass balance and Darcy flow, which addresses the mineralogical and petrophysical changes of the rock along the sampled profile at constant ambient amphibolite-facies P–T conditions. Furthermore, we calculated the evolution of lithium concentrations and isotopic compositions along the profile to better determine the timescales of the fluid-rock interaction. Starting from a fully dry rock, the model calculated reaction-induced porosity, permeability, and fluid pressure evolution based on the local bulk composition and the evolving mineral paragenesis. We used measured lithium data and amphibole content as fitting parameters for additional consistency and robustness of the model. This case study demonstrates how variations in crustal properties affect the rates of fluid infiltration and reaction front propagation, and that fluid–rock interactions can be efficiently maintained in near impermeable, dry and mafic crust.

Garnet textures in metamorphic rocks provide valuable insights into fluid–rock interactions, metamorphic reactions, and the mechanisms driving mineral transformations during retrogression. This study focuses on metapelitic samples from the Blåhø Nappe on Fjørtoft Island (Western Gneiss Region, Norway), part of the Caledonian allochthon. The aim of this work is to investigate how local mineralogical composition and fluid availability during retrogression influence garnet textures. Chemical mapping reveals a systematic association between grossular-enriched garnet rims and the presence of plagioclase and biotite, suggesting a coupling between garnet zoning and the surrounding mineralogy. Geothermobarometric and thermodynamic constraints, support amphibolite overprinting over older eclogite facies. Thermodynamic calculations based on Gibbs free energy minimization were performed to better understand the reactions involved. In particular, the breakdown of phengite in the presence of quartz and fluid is considered a key process leading to the formation of biotite and plagioclase, with implications for garnet zonation. This case study highlights how microscale equilibrium conditions and mineral interfaces can interact with high-grade metamorphism garnets.