Unravelling mechanisms and timescales of amphibolite-facies hydration of mafic crust

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.