Tracking Dehydration Reactions and Fossil Fluid Flux in Shear Zones Using Garnet Microstructures

Abstract

Shear zones act as preferential fluid pathways during prograde and retrograde stages of metamorphism. Nonetheless, we still have limited knowledge of the drainage and permeability of natural settings. The preserved signature of fluid in exhumed rocks provides insights into fluid flow during burial and exhumation. This study investigates fluid flow processes recorded by garnet in quartz-schists from the As Sheik shear zone (Saih Hatat window, NE Oman). We observed garnet as equant, oblate and honeycomb (i.e., skeletal) shapes that document distinct fluid-related growth stages from peak-pressure to early exhumation, both occurring at eclogite facies conditions. Garnet nucleated at 2.0–2.2 GPa and 500°C–550°C after the chloritoid-out dehydration reaction. Subsequent decompression and heating (1.3–1.5 GPa, 600°C–650°C) promoted further fluid release and honeycomb garnet growth. We infer that dissolution, transport and precipitation rates primarily influenced whether garnets grew as oblate grains (i.e., as pseudomorphs on peak-pressure chloritoid grains) or newly nucleated equant grains. Honeycomb garnet was interpreted to show the permeability network that records migration and drainage of fluid. This internally produced fluid escaped from the shear zone using pre-existing grain boundaries and reaction-formed pathways. Assuming 1 km of shear zone thickness, we computed a time-integrated fluid flux of ~34 m³ m⁻² for the entire duration of garnet growth. This study highlights garnet morphology as a tracer of transient fluid pathways during a burial–exhumation cycle of an eclogitic shear zone. The close connection between garnet morphology and fluids calls for a re-evaluation of similar microstructures in different tectonic settings.