Deformation of epidote and plagioclase in the semi-brittle regime

In the plagioclase-rich lower continental crust, hydrous epidote-group minerals will, among other phases, replace plagioclase in the presence of minor amounts of fluids. It has previously been shown that this reaction has a significant impact on the strength of plagioclase aggregates, with reacting aggregates being much weaker than their unreacted counterparts (Stünitz and Tullis, 2001). Hence, reactions taking place in the lower continental crust may have a strong influence on its deformation behaviour and thus on its strength. Yet, it still remains unclear if the observed weakening is due to the nucleation and growth of inherently weaker product phases, e.g., epidote-group minerals, or due to inhibited grain growth in a polyphase aggregate as a result of Zener pinning. 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 solid-medium Griggs-deformation apparatus. Furthermore, we also investigated potential strength differences due to differences in grain size by deforming aggregates with a grain-size range of either 90-135 μm or <25 μm. After deformation under 650 °C, the epidote aggregates reveal the nucleation and growth of new phases indicating that epidote was no longer stable. The amount of product phases found in the epidote aggregates scales with the duration of deformation. At the explored experimental conditions, the compressive strength of plagioclase and epidote aggregates depends on temperature and strain rate with a decrease in strength with an increase in temperature or a decrease in strain rate. At identical conditions, the epidote aggregates are either significantly stronger or show a similar strength as the plagioclase aggregates. Microstructural analyses of the recovered samples reveal that deformation in both aggregates was almost exclusively accommodated by grain fracturing and occasionally slip along cleavage planes, and remained non-localized except for the epidote aggregate deformed at 650 °C with a strain rate of 5·10^-6 s^-1, exhibiting kinetically-controlled faulting due to reaction.

 

Stünitz, H. and Tullis, J. (2001). Weakening and strain localization produced by syn-deformational reaction of
plagioclase. International Journal of Earth Sciences, 90(1):136{148.