How Strong/Weak Is Epidote Relative to Plagioclase?

Abstract

Abstract In natural lower crustal rocks, we observe that plagioclase breakdown is often partial as evidenced by the presence of epidote‐group minerals and the absence of the remaining reaction products for example, kyanite and quartz. Due to the lack of experimental data on epidote deformation, it is unclear if this partial reaction would affect the strength of the plagioclase‐rich lower continental crust. 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. Under identical conditions, epidote aggregates are either as strong as their plagioclase counterparts or moderately stronger, suggesting that the partial replacement of plagioclase by epidote‐group minerals would not have a permanent weakening effect on the strength of the lower continental crust.