Compressive strength data for pure plagioclase and pure epidote aggregates deformed at high-pressure, high-temperature conditions

Despite their abundance in crustal rocks at elevated pressure and temperature conditions, the deformation behavior of epidote-group minerals is poorly studied. We experimentally investigated the relative strength of pure epidote and pure plagioclase aggregates at a confining pressure of 1 GPa, two different temperatures (550 and 650 °C) and two different strain rates (5.10^-5 and 5.10^-6 s^-1) using a Griggs apparatus. Furthermore, we investigated potential strength differences due to differences in grain size by deforming aggregates with grain-size ranges of either 90-135 μm or <25 μm. The chemical composition of the starting material is 64 % anorthite, 35 % albite, and 1 % orthoclase for plagioclase, and 69 % epidote. During deformation, most aggregates underwent buckling, i.e., the top part of the sample was sheared relative to its bottom part, which had a strong influence on the mechanical data. The mechanical data of the successful runs shows that epidote aggregates are either as strong as their plagioclase counterparts or moderately stronger, assuming an uncertainty in stress of 100 to 200 MPa.