The quartz α → β 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 β-quartz. Here, we conducted an experimental study on the quartz α → β transition during which P-wave velocities were measured in-situ at pressure (from 0.5 to 1.25 GPa) and temperature (200 to 900 °C) conditions of the continental lower crust. Experiments were carried out on samples of microcrystalline material (grain size of 3-6 μm) 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 β-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 Dauphiné twinning in samples that underwent through 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 α and β quartz. If true, the velocity changes related to the α → β quartz transition at lower crustal conditions might be lower than that expected in thickened continental crust.

Deep-focus earthquakes occur at 300-660 km depth. Geophysical observations and deformation experiments propose the olivine-spinel (wadsleyite/ringwoodite) phase transformation as the faulting mechanism. While geophysical observations indicate that fault geometry influences the b values in the Gutenberg-Richter law for the phase transformation faulting, deformation experiments reveal that b values are also influenced by rock properties, including structural heterogeneity. Grain sizes play a crucial role in the rate of phase transformation, impacting the occurrence of faulting. Consequently, grain sizes may also influence b values. We conducted deformation experiments on germanate olivine, an analog silicate olivine material, with various grain sizes to reveal the effect of grain size on the difference in b value during the phase transformation faulting. We used a Griggs-type deformation apparatus and measured acoustic emissions (AE) with an AE transducer, which was calibrated by laser-doppler interferometry. This calibration enabled the acquisition of AE waveforms with a unit of velocity (m/s), facilitating comparison to natural earthquakes. b values in the fine-grained aggregates (a few μm) are smaller than those in the coarse-grained aggregates (hundreds μm) at the same deformation conditions. In the coarse-grained aggregates, the heterogeneous formation of spinel aggregates contributes to high b values. Conversely, in the fine-grained aggregates, the homogeneous formation of spinel grains inside olivine at the grain boundaries results in lower b values. Therefore, the homogeneity (or heterogeneity) of spinel formation appears to be a controlling factor for b values in phase transformation faulting associated with deep-focus earthquakes.