Experimental study of the impact of hydration extent on the strength of the lower continental crust

Previous experimental data and field observations demonstrate that fluids have a significant influence on rock strength. The relation between strength and hydration extent of the lower continental crust is still poorly constrained and thus a matter of an ongoing debate. We tested the impact of hydration extent on the strength of rocks representing the lower continental crust by performing deformation experiments on various plagioclase-epidote mixtures as well as on natural granulite samples in a Grigg's type deformation apparatus. In these samples, the plagioclase component represents rocks of the lower continental crust and epidote reflects hydration extent, because, alongside with quartz, kyanite and jadeite or albite, it forms as a decomposition product of plagioclase at high-pressure/ high-temperature conditions in the presence of even small amounts of fluids. To quantify the relation between strength and epidote content, we conducted the tests on plagioclase-epidote powders with a grain size of 90-135 mm and plagioclase-epidote ratios of 100:0, 99:1, 98:2, 95:5, 90:10, 85:15, and 0:100. The pre-dried powders were first hot-pressed at 550 °C and a confining pressure of 1 GPa for 3 h in the Griggs apparatus. Mixtures were subsequently deformed at 1 GPa and 550 to 650 °C at strain rates of 5·10^-6 to 5·10^-5 s^-1. All stress-strain curves show pronounced maxima followed by strain softening towards a final strength. The deformation data yield an exponential decrease of the ultimate strength with increasing epidote content. Investigations of the microstructures of samples deformed at 550 °C and 5·10^-5 s^-1 using the SEM and polarized light microscopy reveal cataclastic flow by grain-scale fracturing of both epidote and plagioclase and the rotation and alignment of epidote grains at angles between 60° and 70° to the maximum principal stress  σ₁. In addition, plagioclase grains show pronounced undulatory extinction but we found no evidence for deformation twinning. Some samples exhibit networks of conjugate bands of fine-grained plagioclase surrounding larger plagioclase grains oriented at an angle of around 50° towards σ₁. These bands are mostly visible in samples without epidote.