To investigate the high-temperature plasticity of calcite, we performed creep experiments in a 0.1-MPa, dead-load creep rig at temperatures T=690¿--880 ¿C, oxygen fugacities fO2=10-20-10-2 MPa, and a constant CO2 fugacity fCO2≈10-1 MPa. Single crystals from four different sources were compressed along <404¿1>. Applied stress, &sgr;, was controlled at 5 to 75 MPa to yield strain rates, &egr;*, between 10-7 and 10-4 s-1. Inductive coupled plasma mass spectrometry analysis indicated that Mn is the major impurity in the four crystals; concentrations varied between 0.002 and 0.06 wt%. Two deformation regimes were revealed: a power law regime and a power law breakdown regime. At low stress (&sgr;≤30 MPa), steady state creep data yielded a stress exponent of ~3.5¿0.5. Optical microscopy showed few twins. Transmission electron microscopy revealed that f⟨101¿1⟩ and c ⟨21¿1¿0⟩ were the major slip systems. The majority of dislocations in deformed samples are curved, suggesting that dislocation climb is the rate-limiting step within the low stress regime. Three deformation mechanisms with different flow laws operated. For the samples with Mn concentration <200 ppm, the oxygen fugacity exponent, m, is 0 and the activation energy for creep, Q, is 305 kJ/mol. For the samples with higher Mn concentration (≥600 ppm), creep depends on both, fO2 and temperature. A deconvolution of the fO2-&egr;* data and the &egr;*-1/T data yielded that, at low fO2 and low T, m=0 and Q=200 kJ/mol, while at high fO2 and/or high T, m=1/6 and Q=400 kJ/mol. At similar temperature and stresses, samples with low Mn concentration deform substantially slower than those with high Mn concentration. Apparently, variations in Mn concentration affect creep through the charge neutrality conditions governing point defect structure. Based on the mechanical data, published diffusion data and point defect chemistry calculations, three rate-controlling mechanisms for creep are proposed. At high stress (&sgr;≥30 MPa), deformation rate is much more sensitive to stress, indicating power law creep down break. The microstructure showed increased activity of dislocation glide, cross slip, and twinning. The value of the stress separating the two deformation regimes is about 10-3 when normalized by the shear modulus, in agreement with those for some other geological materials. ¿ American Geophysical Union 1996