Influence of Mineral Fraction on the Rheological Properties of Forsterite + Enstatite during Grain Size Sensitive Creep

Abstract:

Since the majority of crustal and mantle rocks are polymineralic, it is important to consider the effects of secondary mineral phases on their rheological properties. To examine these effects, we have conducted grain growth and deformation experiments on samples composed of different volumetric fractions of forsterite (Fo) and enstatite (En) at 1 atmosphere and temperatures from 1260 to 1360°C.

The results of our grain growth experiments indicate that the grain size ratios of Fo and En in annealed (reference) and deformed samples follow a Zener relationship with d_I/d_II = b/f_II^z, where d_I/d_II is the grain size of the primary or secondary phase, b and z are the Zener parameters relating grain boundary energies and location of secondary phase, and f_II is the volume fraction of the associated phase. Grain growth in the reference samples conforms to the relationship d_s ⁴-d₀⁴ = kt, where d_s is the grain size under static conditions, d₀ is the initial grain size, k is the grain growth coefficient, and t is time. The growth coefficient of Fo decrease with increasing En volume fraction (f_En), and is consistent with theoretical predictions of Ardell's grain growth model that incorporates physical parameters such as diffusivity and interfacial energy of the mineral phases.

The results of our deformation experiments at constant temperature and strain rate indicate that the flow stress decreases with increasing 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 pre-exponential term, stress and grain size exponents, and activation energy in the constitutive equation for a wide range of f_En were determined. The majority of samples exhibited diffusion accommodated grain boundary sliding creep (i.e., stress exponent = 1). The viscosity measured for all samples is fit well by a model that takes into account (1) grain size calculated from grain growth laws established in our experiments and (2) the flow laws for monomineralic systems of forsterite and enstatite determined here. Our results have important implications for the lower crust and upper mantle where lithologies ranging from dunite to pyroxenite are prevalent. Furthermore, we demonstrate that our model can be extended to make predictions of viscosity in other mineral assemblages.