HIGH-TEMPERATURE DEFORMATION OF CALCITE SINGLE-CRYSTALS

To investigate the high-temperature plasticity of calcite, we performe d creep experiments in a 0.1-MPa, dead-load creep rig at temperatures T = 690 degrees-880 degrees C, oxygen fugacities f(O2) = 10(-20) - 10( -2) MPa, and a constant CO2 fugacity f(CO2) approximate to 10(-1) MPa. single crystals from four different sources were compressed along [<4 0(4)over bar 1>]. Applied stress, sigma, was controlled at 5 to 75 MPa to yield strain rates, epsilon(.), between 10(-7) and 10(-4) s(-1). I nductive coupled plasma mass spectrometry analysis indicated that Mn i s the major impurity in the four crystals; concentrations varied betwe en 0.002 and 0.06 wt %. Two deformation regimes were revealed: a power law regime and a power law breakdown regime. At low stress (sigma les s than or equal to 30 MPa), steady state creep data yielded a stress e xponent of similar to 3.5 +/- 0.5. Optical microscopy showed few twins . Transmission electron microscopy revealed that f <<10(1)over bar 1>> and c <<2(11)over bar 0>> were the major slip systems. The majority o f dislocations in deformed samples are curved, suggesting that disloca tion climb is the rate-limiting step within the low stress regime. Thr ee deformation mechanisms with different flow laws operated. For the s amples 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 (greater than or equal to 600 pp m), creep depends on both,f(O2) and temperature. A deconvolution of th e f(O2) - epsilon(.) data and the epsilon(.) - 1/T data yielded that, at low f(O2) and low T, m = 0 and Q = 200 kJ/mol, while at high f(O2) and/or high T, m = 1/6 and Q = 400 kJ/mol. At similar temperature and stresses, samples with low Mn concentration deform substantially slowe r than those with high Mn concentration. Appararently, variations in M n concentration affect creep through the charge neutrality conditions governing point defect structure. Based on the mechanical data, publis hed diffusion data and point defect chemistry calculations, three rate -controlling mechanisms for creep are proposed. At high stress (sigma greater than or equal to 30 MPa), deformation rate is much more sensit ive to stress, indicating power law creep down break. The microstructu re showed increased activity of dislocation glide, cross slip, and twi nning. The value of the stress separating the two deformation regimes is about 10(-3) when normalized by the shear modulus, in agreement wit h those for some other geological materials.