Alpha-Beta quartz transition in the lower continental crust: perspective from diffraction and acoustic data at high P-T conditions

Quartz is a common constituent of most rocks in the Earth continental crust and it undergoes the α-β transition at depths controlled by the geotherm. Despite the α-β quartz transition representing one of the most well-known and largely studied phase transitions in geological sciences, only few works report the behaviour of this transformation at high pressure (i.e. in the relevant conditions of the deep crust). Hence, it is important to investigate this transformation through an experimental approach at lower-crust pressure and temperature (P-T) conditions.

In this study, we performed deformation experiments at high P-T conditions on novaculite (quartzite) samples using a Griggs apparatus equipped with acoustics and a multi-anvil press at the European Synchrotron Radiation Facility (ESRF, beamline ID06). Experiments were performed at 1-3 GPa and up to 1000°C.

Measurements in the Griggs apparatus indicate that the expected P-wave velocity increase in the β-field is not observed at high pressure. Diffraction data from ESRF show that the transition becomes smoother at high pressure and results in a smaller crystal lattice change than it does at low pressure, consistently with the P-wave velocity measurements in the Griggs apparatus.

In addition, on the temperature-up path we are able to observe the expected negative thermal expansion of β-quartz but, interestingly, this behaviour is not visible on the cooling path. As a possible explanation, we suggest a competing effect of stress and temperature on the crystal lattice parameters. Moreover, at the transition, in a short temperature range, the intensity of quartz diffraction peaks decreases significantly. Acoustic measurements seem to indicate that this could be also related to a transient increase in attenuation. Further experiments will be performed at the ESRF coupling X-ray diffraction and acoustic measurements to assess the relationship between crystal structure and Vp changes.

Our results question the interpretation of seismic contrasts in the deep crust as due to the α-β quartz transition. However the existence of a high attenuation region might reflect the presence of this transformation.