Plasticity and diffusion creep of dolomite
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Natural and synthetic dolomites have been shortened in triaxial compression experiments at temperatures of 400-850C, equilibrium CO2 pore pressures, effective confining pressures of 50-400MPa, and strain rates of 10- 4 to 10- 7s- 1. At low temperatures (T < 700C) natural and synthetic dolomites exhibit high crystal-plastic strengths (> 600MPa), both for coarse-grained (240m) and fine-grained (2m and 12m) 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 (e{open} = e{open} o exp[(1 - 3)] between strain rate e{open} and differential stress (1 - 3) describes the crystal plasticity of dolomite at a fixed Pe and T, with = 0.079 ( 0.01) MPa- 1 and 0.023 ( 07.03) MPa- 1 for coarse- and fine-grained materials, respectively. However, measured values of (1 - 3) 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 800C for coarse, T 700C 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 e{open} = e{open} o[(1 - 3) / ]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 800C 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 ( 45kJ/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. 2008.
