Low-Temperature Plasticity and Dislocation Creep of Fangshan Dolomite

摘要

To explore the rheology of dolomite and investigate recent findings regarding the so-called inversion of activation energy between dislocation and diffusion creep, we compressed medium-grained Fangshan dolomite (113 +/- 42 mu m) at effective confining pressures of 50-300 MPa, temperatures of 27 degrees C-900 degrees C, and strain rates of 10(-6) to 2 x 10(-4) s(-1) using a Paterson gas-medium apparatus. Two end-member deformation regimes with corresponding diagnostic flow laws and microstructures were identified. At temperatures <= 500 degrees C, low-temperature plasticity (LTP), which is characterized by microstructures of predominant abrupt undulatory extinctions and f-twins, was determined to dominate the deformation of Fangshan dolomite. The corresponding flow behavior can be described by an (epsilon) over dot = (epsilon) over dot(0) x exp(alpha x sigma) with alpha =0.0806 +/- 0.0078 and ln (epsilon) over dot(0)=-76.66 +/- 6.24 (Regime 1). At temperatures >= 800 degrees C, dislocation creep, which shows characteristic microstructures of smooth undulating extinction and new recrystallized grains, dominated the deformation of Fangshan dolomite. The corresponding flow behavior can be expressed by a power law equation, (epsilon) over dot = A sigma(n) exp(-Q/RT) with n = 4.75 +/- 0.58, Q=436 +/- 54kJ/mol, and logA=3.48 +/- 1.41(Regime 2). At temperatures between similar to 500 and 800 degrees C, a transition regime between LTP and dislocation creep was identified (Regime 3) with the dependence of flow stress on strain rate increasing gradually with increasing temperature. When extrapolated to natural conditions, our flow law of dislocation creep for dolomite in combination with that of diffusion creep reported by Davis et al. (2008) suggests that the dislocation creep regime of dolomite is limited to a relatively narrow region of high temperature and relatively high stress, whereas the diffusion creep regime dominates the deformation of dolomite in tectonic settings with low stress levels.