Effect of phlogopite on the strength of mica-quartz assemblage and underlying chemical processes

While mineral plasticity is often seen as the most important process to deform rocks in the deep, viscous realm, but an increasing number of studies is highlighting the contribution of mineral reactions and chemical processes to bulk strain. This issue is especially acute at the brittle-viscous transition, where plasticity is hampered by the low temperature conditions. In this work, we studied the deformation processes operating in mica+quartz assemblages, representative of greenschist to amphibolite-facies mylonites, with a special focus on the respective contributions of plasticity and dissolution-precipitation processes.

Assemblages made of phlogopite (Phl) and quartz (Qz) were experimentally deformed in simple shear (Griggs-type apparatus) at 800°C, 10kbar, shear strain rate ~10^-5 s^-1 and 0.1 wt. % added H₂O.  For well-constrained grain sizes (63μm < Phl < 125μm, 10μm < Qz < 20μm), we varied the phase proportions of phlogopite (10, 20, 30, 50, 70 and 100% vol. Phl).

Mechanical results indicate at first order a decrease in strength as the proportion of mica is increased. Samples for 10% vol. Phl deform at differential stresses of ∼1200 and at 20% Phl at ~900MPa, while 100% vol. Phl sample deform for differential stresses as low as ~300MPa. However, the weakest behavior is observed for 30% vol. of mica. The strength of the latter sample lies well out of the iso-strain/iso-stress curves, pointing to the fact that the assemblage does not behave as a simple mechanical mixture of the two end-member mineral phases.

In all samples, an important grain size reduction is noticeable for mica flakes. Grain size reduction is also present for quartz grains, especially with decreasing proportion of mica in the assembly. Most of the strain is accommodated by quartz as an interconnected network in Phl-poor samples, while in Phl-rich samples, phlogopite grains are interconnected. In quartz, a weak intragrain deformation is observed for most samples with no evidence for dynamic recrystallization Quartz is largely reworked as attested by trace element variations, with the reworked quartz proportion decreasing with increasing abundance of phlogopite Only the 10% vol. Phl sample is characterized by incipient polygonal subgrains between parent quartz grains (Qz1). Quartz is reworked mainly by dissolution-precipitation, with newly formed product (Qz2) surrounding original, inherited quartz grains. Qz2 is characterized by a large microporosity, and is enriched in aluminum compared to Qz1. The contribution of dissolution-precipitation seems to be maximal in the weakest sample (30% vol. Phl). Therefore, our study shows that dissolution precipitation processes are instrumental to weaken the two-phase material and that bulk strength strongly deviates from composite flow laws in case of a purely mechanical mixing.