Transformational faulting: Is olivine special? Evidence from quartz-coesite phase transition

Phase transitions in olivine are considered as a key mechanism for triggering faulting at depths greater than 300 km, leading to the nucleation of deep-focus earthquakes (DFEs), under conditions where ductile flow should dominate. Olivine phase transitions are characterized by three fundamental features thought to promote faulting: an exothermic reaction, a large negative volume change, and a strong mechanical contrast. However, it is unclear whether faulting at depth requires all three characteristics or can be triggered by just one.

To address this question, we conducted a series of deformation experiments using a large-volume press at the P61b beamline at DESY synchrotron (Hamburg, Germany). Experiments were performed on novaculite, while samples were transforming into coesite. This phase transition is dominated by a significant volume reduction but involves only minor latent heat release, allowing us to investigate the role of volume change. Throughout the experiments, we simultaneously collected X-ray diffraction patterns and images, together with acoustic emission (AEs) monitoring. Our results show that the growth rate of the high-pressure phase varies strongly with pressure–temperature (P-T) conditions and equilibrium overstep. All experiments were conducted under high differential stress. Thousands of AEs were collected in each experiment, whose locations were reconstructed using arrival times from six acoustic transducers placed around the sample assembly. In experiments characterized by lower transformation rates, AE locations mark fault planes that developed within initially intact sample volumes. Analysis of the AE catalogs reveals magnitude–frequency distributions spanning a wide range of b-values, which vary with P-T conditions and transformation kinetics. We observed that brittle faulting yields an expected b-value of about 1 and was related to the nucleation of coesite grains. 

These experiments represent the first example of transformational faulting in deforming, polycrystalline quartz undergoing a high-pressure phase transition under elevated differential stress. Our findings indicate that a volume-changing phase transition with minor latent heat release can promote brittle failure at high pressure, providing new constraints on the mechanisms of deep faulting and expanding the range of mineral phase transitions potentially relevant to crustal and mantle seismicity.