Mechanisms of Phase Mixing and the Origin of Mylonites

Mylonites are ubiquitous structural features of dynamic plate boundaries, and are widely assumed to represent the product of localized deformation at high pressure and temperature. There are two features of mylonites that distinguish them from typical host rocks: grain-sizes that may be reduced by orders of magnitude and mineral phases that generally well-mixed. Together, these microstructural characteristics are thought to promote rheological weakening over long geologic intervals, an essential feature of Earth-like plate tectonics.

While the physical and/or chemical bases of grain-size reduction are well-understood, the underlying mechanisms of phase mixing are not. Ongoing experiments by several groups seek to clarify how different mineral phases mix together at various deformation conditions. In this contribution we describe experiments on composites of calcite-anhydrite and calcite-fluorite conducted at high pressure and temperature in the Large Volume Torsion (LVT) apparatus at Washington University. The objective of this particular series of experiments is to test the behavior of relatively weak materials at high confining pressure. These conditions are selected to reproduce as closely as possible the high ratio of isostatic pressure to deviatoric stress that is expected in the deep crust and mantle lithosphere.

Experiments up to shear strains of γ > 50 show several trends. With increasing strain the density of phase boundaries (quantified as a total length per unit area) increases. The thickness of monomineralic domains decreases as they are stretched and thinned. Grain-size is reduced, in some cases well below the grain-size predicted by single phase piezometers. Roughness along the phase boundary interface is observed, corresponding to the locations of newly formed triple junctions. Some mixing occurs by grain switching/migration across these interfaces, however most mixing appears to result from the geometric thinning and necking of monomineralic domains. Phase mixing is determined to be the product of several independent mechanisms, the relative importance of which depends on pressure, stress, strain, composition, and the ratio of the initial grain-size to the recrystallized grain size.