Tracking dehydration reactions and fluid flow in exhuming shear zones using garnet microstructures

Shear zones are preferential fluid pathways during prograde and retrograde stages of subduction cycles, but the drainage and permeability of subduction interfaces are poorly quantified. Analyzing exhumed rocks for preserved signatures of fluid production and flow provides insights into fluid circulation during burial and exhumation.

Here, we investigated fluid flow processes recorded by garnets in quartz-schists from the As Sheik shear zone (Saih Hatat window, NE Oman) that records evidence for burial during subduction and local overprinting during exhumation. Garnet occurs as equant, oblate, and honeycomb (i.e., skeletal) shapes, which each documents distinct fluid-related growth stages from peak-pressure to early exhumation associated with a thermal excursion, both occurring at broadly eclogite facies conditions. We show with thermodynamic models and microstructures that garnet first nucleated at 2.0–2.2 GPa and 500–550°C after the chloritoid-out dehydration reaction, which promoted dissolution–precipitation processes. We infer a pseudomorphic replacement of peak-pressure chloritoid by garnet, and based on the absence of internal lattice strain, we suggest that elongate garnet morphology reflects reaction-controlled growth rather than plastic deformation. Our microstructures and models suggest that subsequent decompression and heating (1.5–1.3 GPa, 600–650°C) promoted further fluid release and a renewed stage of honeycomb garnet growth.

We present a conceptual model in which dissolution, transport, and precipitation rates primarily influenced whether garnets grew as oblate grains  (i.e., as pseudomorphs on peak-pressure chloritoid grains), or as newly nucleated equant grains. In addition, we argue that honeycomb garnet represents a snapshot of the permeability network that allowed the fluids to escape from the shear zone using grain boundaries and through reaction-forming pathways.

Using measured maximum mass fraction of fluid released from all the hydrous phases modelled by thermodynamic modelling on a representative rock-scale column of 1000 meters, we estimate the time-integrated fluid flux of the studied shear zone was ~34 m³ m^-2 at eclogite facies conditions for the entire duration of garnet growth. This volume represents a limited time-range of the shear zone lifetime during garnet growth, i.e., from peak-pressure to incipient exhumation still at eclogite facies conditions. Therefore, the full lifetime of the shear zone during prograde and retrograde conditions would indeed provide a higher fluid flux.

The different garnet morphologies analyzed all resulted from the chloritoid dehydration reaction, but reflect different rates of dissolution–precipitation and efficiency of dissolution. 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.