Plasticity and diffusion creep of dolomite

Natural and synthetic dolomites have been shortened in triaxial compression experiments at temperatures of 400–850 °C, equilibrium CO2 pore pressures, effective confining pressures of 50–400 MPa, and strain rates of 10− 4 to 10− 7 s− 1. At low temperatures (T < 700 °C) natural and synthetic dolomites exhibit high crystal-plastic strengths (> 600 MPa), both for coarse-grained (240 μm) and fine-grained (2 μm and 12 μm) samples; differential stresses vary little with strain rate or temperature and microstructures of coarse-grained samples are dominated by f-twins and undulatory extinction. An exponential relation (ɛ˙ = ɛ˙o exp[α(σ1 − σ3)] between strain rate ɛ˙ and differential stress (σ1 − σ3) describes the crystal plasticity of dolomite at a fixed Pe and T, with α = 0.079 (± 0.01) MPa− 1 and 0.023 (± 07.03) MPa− 1 for coarse- and fine-grained materials, respectively. However, measured values of (σ1 − σ3) increase with increasing temperature, a trend that has been observed for dolomite single crystals but cannot be described by an Arrhenius relation.At high temperatures (T ≥ 800 °C for coarse, T ≥ 700 °C for fine), dolomite strengths are reduced with increasing temperature and decreasing strain rate, but the mechanisms of deformation differ depending on grain size. High temperature flow strengths of coarse-grained dolomite can be described by a power law ɛ˙ = ɛ˙o[(σ1 − σ3) / μ]nexp(− H⁎ / RT) with a large value of n (> 5) and a ratio of parameters H⁎ / n = 60 (± 6) kJ/mol. Microstructures of coarse-grained samples deformed at T ≥ 800 °C show evidence of dislocation creep with little mechanical twinning. High temperature flow strengths of fine-grained synthetic dolomite fit a thermally activated Newtonian law, where the effective n = 1.28 (± 0.15) and H⁎ = 280 (± 45 kJ/mol), consistent with diffusion creep.The change in mechanical response of coarse-grained natural dolomite with increasing temperature represents a transition from twinning and slip with little or no recovery to dislocation creep, while the change in response of fine-grained synthetic dolomite represents a transition from crystal plasticity to diffusion creep. The combined results for coarse- and fine-grained dolomites define a deformation mechanism map with fields of crystal plasticity, dislocation creep, and diffusion creep. Strengths of coarse-grained dolomite in the crystal plastic and dislocation creep fields are much larger than strengths of calcite rocks deformed by similar mechanisms. In contrast, strengths of fine-grained dolomite deformed by diffusion creep are more comparable to those of fine-grained calcite, suggesting little contrast in rheology.