Rheology of fine-grained forsterite aggregate at deep upper mantle conditions

High-pressure and high-temperature deformation experiments on fine-grained synthetic dunite (forsterite aggregate) were conducted to determine the dominant deformation mechanism in the deep upper mantle. The sintered starting material has 90% forsterite, 10% enstatite, and an average grain size of ∼1 μm. Deformation experiments were performed using a deformation-DIA apparatus at pressures of 3.03-5.36 GPa, temperatures of 1473-1573 K, and uniaxial strain rates of 0.91 × 10^-5 to 18.6 × 10^-5 s ^-1 at dry circumstances <50 H/10⁶Si. The steady state flow stress was determined at each deformation condition. Derived stress-strain rate data is analyzed together with that reported from similar but low-pressure deformation experiments using flow law equations for diffusion creep (stress exponent of n = 1, grain-size exponent of p = 2) and for dislocation- accommodated grain-boundary sliding (GBS-disl, n = 3, p = 1). The activation volume for diffusion creep (V_dif) and for GBS-disl (V_GBS) of dunite is determined to be 8.2 ± 0.9 and 7.5 ± 1.0 cm ³/mol, respectively. Calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the Earth's asthenospheric upper mantle whereas the contribution of GBS-disl is very limited, and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith. Though these conclusions are not definitive, these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure, temperature, stress, and grain-size space.