Water-added experiments of simulated quartz-feldspar shear zone at brittle-ductile transitional condition

Crustal strength has been estimated to become the largest at the brittle-ductile condition^[1]. Previous experiments have shown that water reduces the crustal strength not only at shallower depth regions where frictional slip becomes dominant^[2] but also at greater depth regions where viscous flow becomes dominant^[3]. However, the microphysical process of how water alters deformation mechanisms and reduces rock strength at the brittle-ductile transition zone remains unclear. To investigate the effect of water on controlling deformation mechanisms at the brittle-ductile transition, we perform a series of shear deformation experiments with a trace amount of water (either 0.2 wt % or 0.4 wt %). We deformed a quartz-albite mixture using a Griggs-type solid salt assembly. Each experiment uses ~ 0.1 g of the sample mixture. The shear strain rate is sequentially changed between ~ 10^-3 /s and 10^-4 /s to investigate the strength dependence on velocity. We further conducted microstructural observations using electron microscopes.

Here, we report a preliminary result of a series of water-added experiments conducted with 0.4 wt% water (i.e., 0.4 μL) at a confining pressure P_C of 760 MPa and a temperature T of 720 °C. Mechanical results show that the peak shear stress is 790 MPa at a shear strain of 1.4, followed by a strain weakening by 200 MPa towards a final shear strain of 4.9. This peak stress is much weaker than a previous result of a room-dry experiment performed at a similar experimental condition (P_C = 750 MPa and T = 720 °C)^[4]. In the dry experiment, the peak shear stress was 1280 MPa, followed by a strain weakening of 230 MPa^[4]. Microstructural analyses showed that the water-added sample is pervasively covered with microcracks. A transmission electron microscopy revealed that nano-grains as small as 50 nm are distributed in the areas between the microcracks. Meanwhile, a sample from the dry experiment exhibits fewer microcracks and contains nano-grains similar in dimensions to those in the sample of the wet experiment^[5].

Our results suggest that water enhances fracturing in the sample layer, and nano-grains are formed regardless of the addition of water. This indicates that the reduction in the peak stress of wet conditions is due to the fracturing promoted by water, while the strain weakening after peak stresses is controlled by nano-grain domains in both conditions. We propose that water reduce the crustal strength by fracturing, that is brittle deformation, accompanied with weakening mechanisms in nano-grain domains such as grain boundary sliding. Furthermore, this suggests that brittle deformation remains dominant even at a greater depth in wet conditions, compared with in dry conditions.

[1] Kohlstedt et al., 1995JGR. [2] Blanpied et al., 1995JGR. [3] Kronenberg & Tullis, 1984JGR. [4] Furukawa et al., 2023 WRI-17. [5] Furukawa et al., 2025 in preparation.