The influence of the α–β phase transition of quartz on fluid inclusions during re-equilibration experiments

•The effect of the α–β quartz phase transition on the properties of H2O-rich synthetic fluid inclusions is investigated.•Fluid inclusions have been synthesized and were re-equilibrated close to the α–β quartz phase transition.•Re-equilibration experiments within the α-quartz stability field illustrate only minor changes in fluid inclusion properties.•Fluid inclusion properties did significantly change to lower densities after re-equilibration in the β-quartz field.•The observed changes in homogenization temperatures illustrate that density-loss due to the α−β quartz phase transition.

The influence of the α–β quartz phase transition on the properties of fluid inclusions was investigated experimentally. The experiments were designed to have no gradients in pressure and fugacity between fluid inclusions and pore fluid. Deformation due to pressure differences were also excluded in this study. H2O-rich fluid inclusions with similar densities were synthesized in quartz at approximately 625 °C and 280 MPa in the α-quartz stability field, and at approximately 675 °C and 320 MPa in the β-quartz stability field. The experimental set-up prevented any pressure differences during loading and unloading of the experiments. These inclusions were re-equilibrated at the same temperature–pressure–fluid conditions, and changes in total homogenization temperature and ice melting temperature were recorded. Fluid inclusions are sensitive monitors of fluid conditions during entrapment, which record minor variation in temperature and pressure and display a corresponding distribution pattern in homogenization temperatures. Fluid inclusions re-equilibrated in the α-quartz stability field were not affected by changes in density. Fluid inclusions re-equilibrated in the β-quartz stability field revealed density loss of − 0.6% at 320 MPa to − 3.0% at 280 MPa, which was caused by the α–β quartz phase transition. Consequently, density loss, which is probably caused by the formation of micro-cracks at the transition from α- to β-quartz is more efficient at lower pressures. In addition, re-equilibration experiments were performed with a pure D2O pore fluid, to investigate diffusion processes at these experimental conditions. Diffusion of D2O is more efficient in β-quartz stability field, and may result in near total exchange of the original H2O content within only 19 days. Fluid inclusions re-equilibrated in the α-quartz stability field contain only half the amount of D2O within the same experimentation run time, up to 53 mol% D2O.