Modification of fluid inclusions in quartz by deviatoric stress I: experimentally induced changes in inclusion shapes and microstructures

Fluid inclusions in quartz are known to mod- ify their shapes and microstructures (textures) during weak plastic deformation. However, such changes have not been experimentally demonstrated and criteria are not available to relate them to paleostress conditions. To address these issues, quartz crystals containing natural CO2-H2O-NaCl fluid inclusions have been experimen- tally subjected to compressive deviatoric stresses of 90-250 MPa at 700C and *600 MPa confining pres- sure. Strains of up to 1% cause the inclusions to develop irregular shapes and to generate microcracks in crystal- lographic planes oriented subperpendicular to the major compression axis, r1. The uniform alignment of the microcracks imparts a planar fabric to the samples. The microcracks heal and form swarms of tiny satellite inclusions. These new inclusions lose H2O by diffusion, thereby triggering plastic deformation of the surrounding quartz via H2O-weakening. Consequently, the quartz samples deform plastically only in domains originally rich in inclusions. This study shows that fluid inclusions deformed by deviatoric stresses may indeed record information on paleostress orientations and that they play a key role in facilitating crystal-plastic deformation of quartz.