On the sensitivity of inclusion pressures after entrapment: the drastic effect of aqueous fluid on garnet viscous relaxation

Elastic geo-thermobarometry has become an important technique in determining the pressure-temperature (P-T) conditions of entrapment during metamorphism. A prerequisite is that the inclusion’s over- or under-pressure is not reset during exhumation. This would be the case if the host-inclusion pair interacts elastically only, which is an oversimplification. It is thus not yet been fully understood how fast the inclusion pressure may become reset. In this study, we performed heating experiment on an almandine-rich (from an eclogite) and a spessartine-rich garnet (gem-stone) under 1) graphite (dry), 2) forming gas (5% H₂ and 95% N₂) and 3) water vapour (wet) buffered conditions at high T and room P. Raman spectroscopy is used to measure the same quartz and zircon inclusions at room T before and after different heating times. In wet and forming gas conditions, the Raman band wavenumber changes are dependent on time, decreasing for quartz and increasing for zircon inclusions. Under dry condition, the Raman band wavenumber exhibits a small amount of shift and becomes stable shortly. Raman mappings reveal that the stress heterogeneity of the garnet host develops stronger at the early stage of the wet heating experiments and fade away afterward, potentially indicating a diffusion-like behaviour of the dislocation density. A visco-elastic model is performed to fit the measured data. The calculated flow law parameters of garnet around quartz inclusions is comparable to the flow law extracted from deformation experiments, while zircon shows substantially faster relaxation rate. This study highlights that fluid can be an important trigger for fast viscous relaxation together with temperature, time and inclusion mineralogy. The study may have implications for elastic thermobarometry, garnet rheology, and the preservation of coesite inclusions.