The effect of aqueous fluid on viscous relaxation of garnet and modification of inclusion pressures after entrapment

The elastic interaction between an inclusion and its host is often employed to study the entrapment conditions during metamorphism on the assumption that the host is not affected by creep. However, it is not well understood how fast creep-induced relaxation may occur and under what conditions the elastic regime holds for each crystalline inclusion-host system. In this study, we performed heating experiments on eclogite and spessartine garnets under 1) graphite, 2) N₂+H₂ and 3) H₂O+Ar fluxed conditions at different temperatures. Raman spectroscopy is used to measure the same quartz and zircon inclusions after different heating times. The Raman-band wavenumber undergoes a time-dependent decrease in quartz inclusions and increase in zircon inclusions under H₂O+Ar and H₂+N₂ conditions, but stabilises after the first heating step under graphite-buffered conditions. EBSD results reveal greater misorientation around the heated inclusions compared to unheated inclusions. Raman mapping reveals that stress heterogeneity in the garnet host develops first and fades away afterward, indicating dispersal of dislocations into the host. A visco-elastic model fit to the measured Raman data provides estimates of flow-law parameters for garnet. These results demonstrate the efficiency of H migration and its weakening effect on garnet. The data also indicate that the garnet can be stronger than previously thought under a dry and reduced environment, which is consistent with the high activation energy of Si diffusion in dry garnet. This study provides a critical temperature and water limit for elastic thermobarometry and criteria of determining whether an inclusion has been reset or not.