Modification of fluid inclusions in quartz by deviatoric stress. III: Influence of principal stresses on inclusion density and orientation

Extraction of useful geochemical, petrologic and structural information from deformed fluid inclusions is still a challenge in rocks displaying moderate plastic strain. In order to better understand the inclusion modifications induced by deviatoric stresses, six deformation experiments were performed with a Griggs piston-cylinder apparatus. Natural NaCl-H ₂O inclusions in an oriented quartz crystal were subjected to differential stresses of 250-470 MPa at 700-900 °C and at 700-1,000 MPa confining pressure. Independently of the strain rate and of the crystallographic orientation of the quartz, the inclusions became dismembered and flattened within a crystallographic cleavage plane subperpendicular to σ ₁. The neonate (newly formed) inclusions that result from dismemberment have densities that tend towards equilibrium with P _fluid = σ ₁ at T _shearing. These results permit ambiguities in earlier deformation experiments on CO ₂-H ₂O-NaCl to be resolved. The results of the two studies converge, indicating that density changes in neonate inclusions are promoted by high differential stresses, long periods at high P and high T, and fluid compositions that maximize quartz solubility. Neonates spawned from large precursor inclusions show greater changes in density that those spawned from small precursors. These findings support the proposal that deformed fluid inclusions can serve as monitors of both the orientation and magnitude of deviatoric stresses during low-strain, ductile deformation of quartz-bearing rocks.