Constraining Rheology and Deformation Processes of Fine-grained Quartz Aggregate from High-Pressure and High-Temperature Experiments

Quartz is present in most rocks of the continental and oceanic crust. Because of the large data set, its rheology in the presence of H2O is the most important tool to understand the depth-dependent change in the strength of the crust. In coarse-grained quartz, grain size insensitive dislocation creep processes will be dominant, and most existing flow laws can be applied. There are a few studies that have focused on fine-grained quartz aggregates and determined stress exponent values (n) in flow laws from 1 to ~1.8, while the activation energy (Q) varies from ~170 to 220 kJ/mol (see Richter et al., 2018; Fukuda et al., 2018 for references). Consequently, there are indications that the deformation mechanisms may deviate from pure dislocation creep and may involve diffusion creep and grain boundary processes.

Here, we investigate the fine-grained (~ 3-5 μm) polycrystalline quartz rheology under high pressure-temperature conditions, using a hydraulically-driven Griggs-type apparatus. We performed coaxial constant-load experiments of high-purity, highly-dense novaculite from Arkansas. Our creep tests were carried out at 750 to 900 °C on the as-is (no added H2O) and H2O added samples under 1GPa of confining pressure. Microstructures from the deformed samples were characterized using light microscopy, SEM-CL, EBSD, and EPMA.

In comparison to our coarse-grained Tana quartzite (~ 200 μm), the novaculites (both as-is and 0.1 wt.% H2O-added) are ~5 times weaker at the same experimental conditions. Preliminary load-stepping experiments suggest a n-value of ~2, which is similar to that of the Tana quartzite. We recorded a low Q-value (∼100 kJ/mol) as in the studies of the Tana quartzite. We do not observe significant grain growth during our experimental conditions. EBSD mapping reveals some degree of crystal plasticity in the original quartz grains, but a significant CPO is not developed, presumably because of the low total strain (~19% in as-is and ~12% in 0.1 wt.% H2O-added). Aspect ratios of grains remain low, but SPO has developed. We observe porosity forming along triple junctions and grain boundaries. It appears that deformation processes are comparable to the coarse-grained Tana samples and a transition to dominant grain size sensitive deformation (diffusion creep with n = 1) at the tested grain size has not occurred.