Characterizing quartz rheology through load-stepping experiments, from diffusion to dislocation creep

Due to the abundance of quartz in the continental crust, quartz rheology is fundamental to our understanding of many geodynamic processes. Quartz rheology is commonly characterized using a dislocation creep flow law with a stress exponent equal to 4; however, several recent studies indicate that the stress exponent for quartz aggregates can be as low as 2 at conditions where it has been proposed to deform by a combination of dislocation creep and grain boundary sliding (GBS), known as dislocation accommodated grain boundary sliding (disGBS). To address these differing hypotheses, we conducted axial compression load-stepping experiments in a Griggs apparatus at temperatures ranging from 800-950°C, 1.5 GPa, and differential stresses ranging from ~40 MPa to ~1430 MPa with water added. Quartz samples were prepared with different grain sizes of ~3, 5, 10, and 20 μm. For each experiment ~25 load steps were conducted during which the strain rate achieved a mechanical steady state. At the finest grain size, the mechanical data show a stress exponent of n = 1, which then transitions to n ~ 1.8 with increasing stress; for a given stress, strain rate increases with decreasing grain size in both regimes. For larger grain sizes over the same stress range, the stress exponent transitions from n ~ 4 to n ~ 1.8 to n ~ 3 with increasing stress, where only the intermediate stress regime (n ~1.8) shows a grain size sensitivity. We interpret the lowest stress and finest grain size mechanical data to represent grain boundary diffusion creep and assume a grain size exponent of 3. With increasing stress, the samples are interpreted to represent disGBS, where dislocation creep and GBS act in series, where GBS is determined to have a grain size sensitivity of 1. The highest stress data represents dislocation creep. Microstructurally, we observe minimal variation in the starting and final grain sizes, suggesting that the grain size was nominally constant throughout the experiments. Experiments quenched in the GBS regime show microstructures with straight grain boundaries consistent with observations from previous studies. Flow laws have been constrained for all four deformation mechanisms. Plotting a deformation mechanism map using our new flow laws extrapolated to geologic conditions, we show consistent relationships between our flow law estimates and c-axis fabric relationships with naturally deformed quartzites. These new mechanical relationships improve our understanding and constraints on grain-size sensitive rheologies in quartz as well as our ability to model quartz rheology over a wide range of geologic conditions.