Salinity and density modifications of synthetic H2O and H2O-NaCl fluid inclusions in re-equilibration experiments at constant temperature and confining pressure
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Experiments have been performed to elucidate post-entrapment modifications of natural fluid inclusions. Synthetic H2O and H2O-NaCl fluid inclusions in quartz were re-equilibrated at 599.7–600.7 °C and 332.6–338.3 MPa in hydrothermal autoclaves, and subjected to H2O fugacity gradients at similar temperature and hydrostatic pressure conditions to those of the original syntheses. Individual inclusions in specific assemblages were analysed in detail, i.e. size, shape, depth, homogenization and ice-dissolution temperatures, before and after the re-equilibration experiments. Accurate loading and unloading of the samples along isochoric T–p paths requires monitoring the temperature at the sample within the autoclaves. Multiple loading and un-loading of the samples along these paths does not affect the properties of fluid inclusions. Synthetic H2O fluid inclusions are not modified after re-equilibration in a pure H2O fluid, but reveal significantly higher homogenization temperatures after re-equilibration in 20 mass% NaCl solution. Synthetic NaCl-H2O fluid inclusions with 10, 16.3 and 19.8 mass% NaCl reveal higher salinities after re-equilibration in a pure H2O fluid, and highly variable homogenization temperatures: positive as well as negative modifications. The magnitude of modifications indicates that two processes must have operated simultaneously to obtain the observed homogenization and dissolution temperatures in individual inclusions: (1) preferential H2O loss via diffusion; and (2) total volume loss by diffusion of quartz into the former inclusion volume. These processes are inconsistent with the expected H2O diffusion into inclusions according to the applied fugacity gradients in the experimental setup. These simultaneously operating processes are suggested to be the main modification method of natural fluid inclusions in a variety of experimental settings and in geological environments.