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

Abstract

The elastic interaction between an inclusion and its host is often employed to study metamorphic processes based on the assumption that the host is not affected by processes such as creep that irreversibly releases stress. However, it is not well understood how fast inelastic relaxation of stress may occur and under what conditions the elastic regime holds for each inclusion-host system. To provide new constraints for the widely used systems of quartz and zircon inclusions in garnet, we performed heating experiments on almandine-pyrope and spessartine garnets under graphite, N2+H2, or H2O+Ar fluxed conditions at different temperatures. Raman spectroscopy was used to measure the same quartz and zircon inclusions after different heating times. The Raman-band wavenumbers undergo time-dependent decreases in quartz inclusions and increase in zircon inclusions under H2O+Ar conditions and exhibit a greater final shift than under graphite and N2+H2 buffered conditions. Under graphite-buffered conditions, the wavenumbers of Raman bands measured on zircon and quartz stabilise after the first heating step, after which no change was observed. Electron backscatter diffraction results reveal greater misorientation around the heated inclusions compared to unheated inclusions, implying a greater dislocation density after heating. Raman mapping reveals that stress heterogeneity in the garnet host develops at an early stage of heating and fades away afterward, indicating dispersal of dislocations into the host. Fitting a visco-elastic model to the Raman data of garnet fluxed with N2+H2 or Ar+H2O allows an estimate of flow-law parameters for garnet around quartz inclusions, similar to those obtained by conventional deformation experiments. The results demonstrate the weakening effect of aqueous fluid on garnet. The data also indicate that the garnet can hold inclusion pressure at elevated temperatures under a dry and reducing environment. This study provides information on the relaxation rate of pressurized inclusions in garnet at different temperatures and within different external environments. Furthermore, the use of inclusion-host pairs for studying creep processes offers a complementary approach to conventional deformation experiments to better understand the rheological behaviour of earth materials.