New experimental constraints on seismic velocities and densities across the \(\alpha \)–\(\beta \) quartz transition at deep crustal conditions

At the pressure and temperature conditions of the lower crust, quartz undergoes a displacive phase transition from a trigonal (\(\alpha \)) to a hexagonal phase (\(\beta \)). At room pressure, the \(\alpha \)–\(\beta \) quartz transition occurs at 574.1 °C and it is associated with large changes in the thermodynamic and elastic properties. For that reason, it is interpreted as the cause of significant seismic velocity contrasts in the crust seen by seismic tomography. Existing thermodynamic models and Equations of State (EoS) of quartz are mostly constrained by data collected at room pressure (or at high pressure and room temperature). In this work we characterized the \(\alpha \)–\(\beta \) quartz transition experimentally at simultaneous HP–HT conditions using synchrotron X-ray diffraction and acoustic measurements, and derived values of \(V_p\), \(V_s\), the adiabatic bulk modulus (\(K_s\)) and the shear modulus (G). The data collected in the \(\alpha \) field agree with the models from the literature, so entrapment pressures of \(\alpha \)-quartz inclusions calculated via elastic barometry with these EoS should be reliable. However, our measured \(V_p\), \(V_s\), and \(K_s\) are significantly lower than those predicted for \(\beta \)-quartz. Whatever the cause of this discrepancy, interpretations of seismic data in terms of the properties of \(\beta \)-quartz in the lower crust and calculations of entrapment conditions of quartz inclusions in the stability field of \(\beta \)-quartz should be treated with caution.