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 is then shifted to colder regions where more H2O is available along GB. Plain Language Summary The mechanical behavior of rocks is reflected by deformation microstructures, and it is usually dependent on available H2O in the deformation environment. We tested influence of H2O in quartzite samples by adding various amounts of H2O prior to deformation experiments, in the range from 0 to 0.5 wt.%. The results showed that samples with up to 0.2 wt.% of added H2O are generally weaker than the as‐is samples, while with H2O addition of more than 0.2 wt.% mechanical behavior becomes more erratic. These samples showed either weaker or stronger behavior. While plastic deformation in weaker samples is localized mostly in sample regions with the highest temperature, deformation in stronger samples is unexpectedly localized in colder regions. Quartz grains are f