Artificial fluid inclusions were hydrothermally synthesized by crack healing in natural Brazilian quartz. Two original experiments E421 and E679 with a H2O-CO2 fluid were carried out at 835 K, 200 MPa, over 38 days, and at 856 K, 211 MPa, over 35 days, respectively. In both experiments homogeneous three-phase (a vapor and two liquids) fluid inclusions were synthesized with 22 and 20 mol% CO2, respectively. The CO2 phases homogenize to the vapor phase at 302.2 +/- 0.1 K (E679, cores 1 and 2), and to the liquid phase at 303.6 +/- 0.3 K (E421, core 3), 303.9 +/- 0.2 K (E421, core 4). The CO2-H2O phases homogenize to the vapor phase at 573.6 +/- 0.4 K (E679, core 1 and 2), 575.6 +/- 1.5 K (E421, core 3), and 576.1 +/- 0.8 K (E421, core 4). Micro cracks and the new hydrothermally precipitated quartz, which directly surrounds the inclusions, were studied with TEM and SEM. The healed cracks have numerous growth imperfections that provide many possible routes for fluid transport. Dislocation arrays and small channels were observed and are often connected to the inclusions. Quartz cores 3 and 4 were subsequently re-equilibrated for 21 and 27 days respectively under hydrothermal conditions with pure H2O, at both a lower pressure of 100 MPa (E463) and a higher pressure of 365 MPa (E490) than that of the original experiment E421. The temperatures of the re-equilibration experiments were equal to the original (835 K). In E463, the internally overpressured re-equilibration, only traces of solution and precipitation of quartz were evident with minor transformation of the angular walls to more rounded forms. Volume increase for some inclusions resulted from decrepitation. The homogenization of the CO2-H2O phases to the gas phase occurred at higher temperatures, up to 604 K. In E490, the internally underpressured re-equilibration, major solution and precipitation resulted in the transformation of irregular shaped inclusion walls and formation of secondary inclusions halos. The homogenization of the CO2-H2O phases to the gas phase occurred at lower temperatures, down to 565 K. The fluids in inclusions from both re-equilibration experiments were found to have lower densities than the original fluids synthesized. This is quantified by the increased volumetric proportions of CO2 vapor. The CO2 fraction in inclusions was found to have increased, by up to 54 mol%. The change in homogenization temperatures and the decrease in the proportions of H2O in the original inclusions favours a model in which preferential transport of H2O occurs along mobile dislocation lines, small intercrystalline nanocracks, and/or channels. Results from experiment V1 and V4, using cores 1 and 2, indicate that the changes observed in the re-equilibration experiments are not artifacts of the experimental method.
