Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain-size-sensitive creep: 2. Deformation experiments

In order to understand the effects of secondary minerals on the flow properties of rocks, we have conducted uniaxial compression experiments on polycrystalline forsterite (Fo) + enstatite (En) samples. At constant temperature and strain rate, the flow stress of the samples decreases with increasing enstatite volume fraction (f_En) for samples with 0 < f_En < 0.5 and increases with increasing f_En for samples with 0.5 < f_En < 1. The values of the preexponential term, stress and grain size exponents, and activation energy in the constitutive equation for a wide range of f_En were determined. Samples with a low f_En(≤0.03) deformed at strain rates of 2 × 10^-5 to 2 × 10^-4/s exhibit creep characteristics that correspond to dislocation-accommodated grain boundary sliding creep (i.e., stress exponent, n = 3), whereas diffusion-accommodated grain boundary sliding creep is typical of high f_En samples (i.e., stress exponent, n = 1). The change of flow strength as a function of f_En during grain-size-sensitive creep is primarily due to changes in grain size of both phases and secondarily due to changes in the volume fraction of phases with different flow strengths. Viscosities of all samples can be reproduced in a viscosity model that takes into account (1) the grain sizes estimated by the grain growth laws established in our part 1 paper and (2) flow laws determined for the individual phases, in this case, forsterite and enstatite. Furthermore, we demonstrate that our model can be extended to make predictions of viscosity in other mineral assemblages.