High‐Temperature Deformation of Enstatite‐Olivine Aggregates

Synthesized polycrystalline samples composed of enstatite and olivine with different volumetric ratios were deformed in compression under anhydrous conditions in a Paterson gas‐medium apparatus at 1150–1300°C, an oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 or 450 MPa (protoenstatite or orthoenstatite fields). Mechanical data suggest a transition from diffusion to dislocation creep with increasing differential stress for all compositions. Microstructural analyses by optical and scanning electron microscopy reveal well‐mixed aggregates and homogeneous deformation. Crystallographic preferred orientations measured by electron backscatter diffraction are consistent with activation of the slip systems (010)[100] and (010)[001] for olivine and (100)[001] and (010)[001] for enstatite, as expected at these conditions. Nonlinear least‐squares fitting to the full data set from each experiment allowed the determination of dislocation creep flow laws for the different mixtures. The stress exponent is 3.5 for all compositions, and the apparent activation energies increase slightly as a function of enstatite volume fraction. Within the limits of experimental uncertainties, all two‐phase aggregates have strengths that lie between the uniform strain rate (Taylor) and the uniform stress (Sachs) bounds calculated using the dislocation creep flow laws for olivine and enstatite. Calculation of the Taylor and Sachs bounds at strain rate and temperature conditions expected in nature (but not extrapolating in pressure) indicates that using the dislocation creep flow law for monomineralic olivine aggregates provides a good estimate of the viscosity of olivine‐orthopyroxene rocks deforming by dislocation creep in the deeper lithosphere and asthenosphere. Plain Language Summary The rheology of Earth's upper mantle is generally modeled using mechanical flow laws determined for aggregates composed only of olivine minerals, in spite of the polyphase nature of mantle rocks. In this study, we investigated the effect of phase volume proportions on the high‐temperature deformation properties of aggregates composed of the two most abundant minerals in the upper mantle, olivine and enstatite. The samples were deformed under dry conditions in triaxial compression at 1150–1300°C, under oxygen fugacity fixed at the Ni/NiO solid buffer, and confining pressures of 300 or 450 MPa, at conditions where enstatite has two different crystallographic structures. At both pressures,