P-Wave Velocities Across the Quartz Transition at Lower Continental Crust Pressure and Temperature Conditions

The quartz alpha -> beta transition is a displacive phase transition associated with a significant change in elastic properties. However, the elastic properties of quartz at high-pressure and temperature remain poorly constrained experimentally, particularly within the field of beta-quartz. Here, we conducted an experimental study on the quartz alpha -> beta transition during which P-wave velocities were measured in-situ at pressure (from 0.5 to 1.25 GPa) and temperature (200-900 degrees C) conditions of the continental lower crust. Experiments were carried out on samples of microcrystalline material (grain size of 3-6 mu m) and single-crystals. In all these, the transition was observed as a minimum in P-wave velocities, preceded by an important softening while P-wave velocities measured in the beta-quartz field were systematically lower than that predicted by thermodynamic databases. Additional experiments during which acoustic emission (AE) were monitored showed no significant peak of AEs near or at the transition temperature. Microstructural analysis nevertheless revealed the importance of microcracking while Electron Back-Scatter Diffraction (EBSD) imaging on polycrystalline samples revealed a prevalence of Dauphine twinning in samples that underwent the transition. Our results suggest that the velocity change due to the transition known at low pressure might be less important at higher pressure, implying a change in the relative compressibilities of alpha and beta quartz. If true, the velocity changes related to the alpha -> beta quartz transition at lower crustal conditions might be lower than that expected in thickened continental crust. Plain Language Summary Phase transitions occurring in minerals at high pressures and temperatures can cause seismological discontinuities, and they happen when the mineral structure transforms from its stable form at lower pressures to a different structure at higher pressures and temperatures. One such transition that has a significant seismic signature is the alpha-beta quartz transition. Seismologists use this signature to determine the temperature conditions in the Earth's crust at depths much greater than those achievable in laboratory experiments. However, the uncertainty arises due to the fact that the alpha-beta quartz transition has only been observed under low-pressure conditions, and it is unclear whether it actually occurs at greater depths. In order to address this, we conducted a series of experiments at increasing confining pressures up to 1.25 GPa (equivalent to a depth of 35 km) and found that the transition becomes less sharp at high pressure. Our results suggest that the transition should not be as sharp as previously thought in seismological images at these depths.