EXPERIMENTAL CONSTRAINTS ON DOLOMITE DEFORMATION MECHANISMS

Coarse- and fine-grained dolomites have been deformed experimentally in compression at temperatures T of 400 – 850 ^oC, 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 ^oC) coarse, natural dolomite (240 µm) and fine-grained dolomites (both natural and synthetic, 10 and 2 µm) exhibit high crystal plastic strength (differential stresses > 600 MPa) with little dependence on temperature or strain rate. Coarse-grained samples show evidence of mechanical twinning, dislocation slip and minor microcracking while fine-grained samples show no evidence of twinning and deform by slip until fracture. At high temperatures (T ≥ 800 ^oC for coarse, T ≥ 700 ^oC for fine) dolomite strengths are reduced with increasing temperature and decreasing strain rate, but the mechanisms of deformation differ depending on grain size. Coarse-grained dolomite samples exhibit evidence of dislocation creep and recrystallization at grain boundaries with only few mechanical twins. The temperature dependence of strength for dislocation creep of coarse-grained dolomite is characterized by a ratio of parameters H*/n = 60 kJ/mol where H* is the activation enthalpy and n is the power law exponent. The strain rate dependence is not well determined but the value of n is much greater than 3. Microstructures of fine-grained dolomite samples are subtle but their mechanical properties suggest diffusion creep. Based on temperature- and strain rate-stepping experiments, the activation enthalpy for diffusion creep of fine-grained dolomite is H* = 280 (± 45 kJ/mol) and the value of n is 1.28 (± 0.15).

Taken together, the experimental results for coarse- and fine-grained dolomites define a deformation mechanism map with fields where crystal plasticity, dislocation creep, and diffusion creep dominate. 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 at all geologic conditions. However, strengths of fine-grained dolomite deformed by diffusion creep are more comparable to those of fine-grained calcite, suggesting little contrast in rheology.