Simultaneous acoustic emissions monitoring and synchrotron X-ray diffraction at high pressure and temperature: Calibration and application to serpentinite dehydration
Résumé
We have successfully developed an experimental setup that allows collecting in situ X-ray powder diffraction and simultaneously recording full waveform acoustic emissions (AE) at high pressure and temperature (PT) in a DIA multi-anvil device. This setup is a powerful tool for investigating rock embrittlement at high PT due to phase transitions and/or mineral reactions since both reaction progress (and kinetics) and AE triggering can be simultaneously monitored. The dehydration of natural serpentinite samples (antigorite-rich) under deviatoric stress has been investigated by this method since antigorite dehydration is believed to trigger intermediate depth earthquakes through dehydration embrittlement. We performed, beforehand, a series of tests on the cold compression of reference materials with contrasted mechanical behaviors (quartz beads and kaolinite powder). Due to grain crushing, cold compression of quartz gave rise to numerous AE events (several hundreds), which were located within the sample. Cold compression and heating of kaolinite, a ductile material, yielded no AEs, demonstrating that the pressure assembly is noiseless. Unexpectedly, antigorite-rich serpentinite samples produced no detectable AEs in the course of their dehydration under the differential stress imposed by alumina waveguides. The only AEs that were recorded occurred during cold compression. Sample microstructures indicate that conjugate faults inherited from the cold compression stage are activated during or after dehydration of the sample. The "aseismic" slip along these faults could be attributed to the presence of talc (or a talc-like phase) or of fine-grained materials (dehydration reaction products) in the fault gouge. Furthermore, AE triggering can also be influenced by hydraulic diffusivity and the differential stress level on the sample, two parameters that are not controlled in conventional multi-anvil experiments. Our results highlight the fact that coupling between dehydration reactions and seismicity might not be as straightforward as previously thought. In fact, fast reaction kinetics or high reaction extent may inhibit the nucleation of mechanical instabilities through rapid stress relaxation of the solid matrix.