Deformation and reaction of plagioclase-rich rocks at conditions of the lower continental crust

Abstract

Due to contrasting results between laboratory tests, geophysical data, and field observations, the strength of the plagioclase-rich lower continental crust remains a topic of debate. It has been shown that its strength highly depends on the presence of fluids as they trigger metamorphic reactions that can result in permanent weakening. An important metamorphic reaction in the lower continental crust is the breakdown or hydration of plagioclase and the associated growth of epidote-group minerals, kyanite, quartz, and jadeite/albite. To investigate the impact of this particular reaction on the strength of the lower continental crust, we combined experimental work, i.e., Griggs-deformation tests, with extensive microstructural observations of the recovered experimental samples. Experimental conditions were 1 to 1.5 GPa confining pressure, 550 to 950 °C, and for the deformation tests, we used strain rates ranging from 10-6 to 10-5 s-1. Our results reveal two main findings. First, deformed plagioclase aggregates as well as deformed granulite drill cores show that deformation-induced features in plagioclase grains, e.g., cleavage cracks and twin boundaries, act as nucleation sites for metamorphic reactions and melting. Consequently, reaction can progress faster in deformed samples as the effective reactive surface area is increased relative to undeformed counterparts. Second, when deformed under identical experimental conditions, pure epidote aggregates are consistently stronger or show equal strengths than pure plagioclase aggregates. Hence, a partial plagioclase breakdown, i.e., the exclusive growth of epidote-group minerals at low reaction progress, is not expected to result in permanent weakening. This result further strengthens the idea that a process akin to Zener pinning is a viable mechanism to cause long-term weakening in rocks.