Glaucophane plasticity and scale-dependent yield strength from nanoindentation experiments

Subduction interface shear zones localize deformation and sustain plate-boundary weakness on million-year timescales, as well as host a variety of enigmatic seismicity and slow slip transients. A physical understanding of the steady-state and transient mechanics of subduction shear zones requires quantitative constraints of the plastic yield strength and deformation mechanism(s) of metamorphic rocks and minerals that occupy the plate interface. However, very little is known about the rheology of many hydrous minerals that occupy the plate interface, such as glaucophane (end-member sodic amphibole). This is partly because conventional deformation experiments meet technical challenges when trying to measure plasticity in the laboratory due to the stability field of glaucophane, the confining pressure needed to suppress fracture, and the limited range of trade-off between temperature and strain rate in experiments.

 

Here, we present preliminary results from room-temperature nanoindentation experiments on thin sections of glaucophane-rich rocks that produced crystal plasticity by dislocation glide under high-stress conditions. Nanoindentation produces in-situ confining pressure that typically inhibits brittle fracture during loading in favor of plastic deformation. Since the volume of deformation beneath the tips is very small compared to the grain size, each indent is essentially a single-grain mechanical test (i.e., effects of grain boundaries can be ignored). We convert load-depth data from two spheroconical tips of different radii to stress-strain curves to quantify the elastic-plastic transition and characterize post-yield behavior. We measure yield stress as a function of grain orientation. Both post-yield weakening and post-yield hardening occur, which likely reflect brittle fracture along micro-faults/cleavage planes, and dislocation bursts and pile-ups, respectively. Glaucophane hardness decreases with increasing length scale of deformation (i.e., indentation radius), capturing a “size effect” that may reflect an effective decrease in dislocation density as the volume of plastic deformation increases beneath the indent tip. This effect is well-constrained for many metals and some geologic materials, including olivine.

 

The mechanical tests provide a basis for interpreting microstructures of naturally-deformed blueschists, which suggest that glaucophane exhibits recovery-limited dislocation glide and dynamic recrystallization. Low-temperature plasticity may provide a micro-physical framework for long-term strain localization and transient brittle shear when meta-mafic rocks are deformed to high strain.