Rapid fluid infiltration and permeability enhancement during middle–lower crustal fracturing: Evidence from amphibolite–granulite-facies fluid–rock reaction zones, Sør Rondane Mountains, East Antarctica

Permeability is a key control on fluid infiltration in the crust. However, quantitative geological constraints on crustal permeability are limited, particularly with regards to its temporal evolution. Here we constrain the permeability evolution in the middle–lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing. We investigated mafic granulite and orthopyroxene–hornblende schist (opx–hbl schist) samples from Mefjell, Sør Rondane Mountains, East Antarctica. Millimetre-scale amphibolite-facies reaction zones occur along fractures in these rocks. In the mafic granulite, two zones were identified: (i) reaction zones (1–2 mm thick); and (ii) mafic granulite host rock. The opx–hbl schist sample can be divided into the following three zones: (i) actinolite–cummingtonite zones (1.4 mm thick); (ii) actinolite–orthopyroxene zones (1.6 mm thick); and (iii) host rock. These zones are evident from the modal mineralogy, reaction textures, and trace element profiles. The P–T conditions of fluid infiltration are estimated to be 0.55 GPa and 620 °C for the mafic granulite, and 0.3 GPa and 450 °C for the opx–hbl schist, respectively. Chlorine concentrations in apatite grains show a gradual decrease from the fractures towards the host rocks. Chlorine concentration profiles suggest that the dominant processes of chlorine transport were advection with minor diffusion for both samples. Based on these results, the timescales of fluid infiltration are constrained to be ~8 h for the mafic granulite and ~10 h for the opx–hbl schist. The pressure gradient across the reaction zones was estimated from the H2O activity in the reaction zones to be 2–15 MPa/mm. The permeability of the host rock and fractures were estimated to be 10−21–10−23 and 10−8–10−9 m2, respectively. Our results show that rapid infiltration of Cl-bearing fluids (~10 h) occurred due to a limited fluid flux from low-permeability (10−21–10−23 m2) host rocks. The low-permeability media led to fluid accumulation and further fracturing. The spatio-averaged permeability then increased by more than several orders of magnitude (10−10–10−16 m2) and the fluid pressure decreased. The contrasting permeability between the host rocks and fractures reveals permeability enhancements associated with crustal fracturing on timescales comparable to geophysical observations. Compared with average long-term (Myr) permeability estimates from previous studies (e.g., 10−18 m2), the permeabilit