Dislocation networks facilitate element diffusion in deformed garnet

Abstract

Garnet is a key mineral in constraining the conditions and timing of metamorphism. Changes in its elemental composition can record distinct pressure (P) and temperature (T) conditions, and information on timing can be retained by its isotopic systems. Due to its mostly rigid behavior during deformation and high closure temperature for Lu–Hf diffusion, garnet geochemistry is interpreted to reflect garnet growth undisturbed by subsequent ductile deformation. However, nanoscale observations demonstrated element mobility into intracrystalline defects and suggested that garnet may not be as geochemically robust as commonly thought. Here, we assess the efficiency of dislocations in promoting grain-scale element mobility in garnet porphyroclasts naturally deformed under high-T conditions using a range of high-spatial resolution microstructural and chemical-isotopic techniques. We show that the development of low-angle subgrain boundaries in response to dislocation creep is insufficient to promote grain-scale element mobility. However, we find that Ca, Mg and trace elements (e.g., La, Ce, Lu, Hf, Sm, Ti, Zr and U) are mobilised on the grain-scale when the dislocation density exceeds the threshold to form a dislocation network. Although dislocation networks can enhance element mobility, the differences in P–T conditions from ‘low’- and ‘high-strain’ domains are negligible and unresolvable, reinforcing its geochemical robustness to estimate the prograde garnet growth conditions. Nevertheless, dislocation networks facilitate syn-kinematic diffusional Hf loss and Lu gain, demonstrating that garnet intracrystalline deformation can impact the Lu–Hf geochronometer. These observations indicate that isotopic resetting in garnet is more complex than previously assumed in rocks that underwent high-strain and temperature deformation.