Mechanical re-equilibration of fluid inclusions in San Carlos olivine by power-law creep

Abstract

Fluid inclusions in San Carlos olivine stretch via plastic mechanisms when heat-treated at 1400° C and 0.1 MPa in controlled\(f_{O_2 } \) atmospheres for several days. Measurable changes in both inclusion dimensions and fluid densities occur; densities decrease from ≈1.0 to ≈0.7 g/cc. Stretching is fastest along 〈100〉, and slower along 〈001〉 and 〈010〉. The dislocation microstructure around the inclusions suggests that creep mechanisms operate. Uncertainties in the experimental determinations of stretching rates result from optical resolution limits, errors inherent in measuring homogenization temperatures, uncertainties in the fluid equation of state, and changes in fluid chemistry during the heat-treatment. Inclusion stretching by dislocation creep can be treated using a model developed for hot isostatic pressing. In this model, we assume spherical symmetry of plastic flow, that the material yields by steady-state power-law creep, and that the parameters for the constitutive law and fluid equation of state are known. Stretching rates are predicted to depend on the difference between the fluid pressure and the external pressure, the temperature, the constitutive law governing power-law creep, and geometry. Predicted stretching rates show fair, but not exact, agreement with experimentally measured rates. The amount of stretching predicted by the model is in rough agreement with estimates based on dislocation microstructures around natural inclusions, if xenolith ascent rates are of the order of 1 cm/s or faster.