Mechanical Impact of Heterogeneously Distributed H2O on Quartz Deformation

In order to identify relations between mechanical behavior, deformation mechanisms, microstructural properties, and H₂O distribution, Tana‐quartzite samples with added H₂O ranging from 0 to 0.5 wt.% were deformed by axial shortening at constant displacement rates, at 900°C and 1 GPa, reaching up to ∼30% bulk strain. Samples with lower quantities of added H₂O (0.1 and 0.2 wt.%) were in average ∼30 MPa weaker than the as‐is samples with no added H₂O. In contrast, samples with more than 0.2 wt.% added H₂O revealed more variable mechanical behavior, showing either weaker or stronger trend. The weaker samples showed strain localization in their central parts in the vicinity of the thermocouple, that is, the hottest parts of the samples, whereas the stronger samples showed localization in their upper, slightly colder parts. Bulk deformation is accommodated by crystal plasticity and dissolution‐precipitation processes. Distribution of H₂O in our samples revealed systematic decrease of H₂O content in the interiors of original grains, caused by increasing strain and H₂O draining into grain boundary regions. With increasing content of added H₂O, the quartz recrystallization gradually changes from subgrain‐rotation‐dominated to crack‐induced nucleation, along with increasing quantity of melt/fluid pockets. The unexpected strain localization in the upper parts of stronger samples most likely results from mode‐1‐cracking that led to drainage of grain boundaries (GB) due to the crack dilatancy effect, and inhibited dissolution‐precipitation in the hottest part of the samples next to the thermocouple. The locus of deformation isthen shifted to colder regions where more H₂O is available along GB.