Acoustic Emissions and Damage during the Brittle/Ductile Transition and Dehydration of Gypsum

In most of rocks, the brittle/ductile transition is usually considered to occur when pressure and/or temperature increases. Dehydration reactions in rocks that contain hydrous minerals are supposed to play an important geological role by modifying this common behaviour: when such a reaction occur, the rock is generally more brittle whereas the temperature is still increasing. The purpose of our study is to better understand the processes acting during the deformation of a dehydrating rock, from the brittle to the ductile field, and during the dehydration itself. We have studied the deformation of gypsum aggregates at different temperatures (25-150°C) and different confining pressure ( 2-295 MPa) in a new triaxial apparatus at ENS, with measurements of ultrasonic waves velocities and Acoustic Emissions recording. At room temperature, the brittle/ductile transition occurs at around 20 MPa confining pressure. Below this value, the deformation is localized in a single shear band and stick slip is observed. Over this value, the deformation is still localized, but in multiple shear bands, and the differential stress still displays an oscillatory behaviour. We suggest that each oscillation of the stress is associated with the formation of one shear band. AE intensities and rate decrease with increasing confining pressure but never completely disappear. The P and S wave velocities strongly decrease with increasing deformation, indicating brittle microprocesses. At 70°C, the sample deformed at 10 MPa confining pressure is still purely brittle, and strong stick slip occurs after the peak stress. The sample deformed at 50 MPa confining pressure is macroscopically ductile, and multiple shear bands and stress oscillations are still observed. Again, the P and S wave velocities strongly decrease with increasing deformation. An interesting feature is that in both cases the AEs are more scarce but much stronger and at much lower frequency than in the room temperature experiments. We also performed experiments across the dehydration temperature threshold, for hydrostatic stress state. The P and S wave velocities dramatically drop at around 130°C, and compaction occur. This is due to the important solid volume change of the reaction (from gypsum to bassanite). No AE were observed during the dehydration reaction.