High-temperature creep of synthetic calcite aggregates: influence of Mn-content

Abstract

To investigate the role of impurities on the deformation of calcite rocks, we performed creep experiments on fine-grained synthetic calcite aggregates with varying Mn-content. The tests were performed in a uniaxial creep apparatus at temperatures of 700–830 °C and stresses between 15 and 65 MPa, resulting in strain rates of 10 −7 to 10 −4 s −1. Varying the oxygen fugacity, f O 2 , between 10 −18 and 10 −4 MPa did not affect the creep strength. Applying a power law constitutive relationship on the creep data, two distinct creep regimes were observed, a low stress regime (<15 MPa) and a high stress regime (>20–25 MPa). In the low stress regime, a stress exponent n of 1.7±0.3 and an activation energy of Q=300±45 kJ/mol were found. The mechanical data and microstructural observations suggest that grain-boundary sliding accommodated by grain-boundary diffusion is the dominant creep mechanism. At high stresses, we obtained a stress exponent of n=3.1±0.6 and an activation energy of Q=490±50 kJ/mol, indicating dislocation creep. Transmission-electron microscopy (TEM) observations show a dominance of recovery-related dislocation structures suggesting that dislocation climb is important. Increasing Mn-concentration from 10 to 670 ppm leads to an enhancement in creep rate in both creep regimes, but the effect is more substantial in the diffusion creep regime (one order of magnitude). In the diffusion creep regime, the enhancement of creep rate by Mn is attributed to the substitution of Ca by Mn, which may increase grain-boundary diffusivity of calcite. In the dislocation regime, it is interpreted as a combination of dislocation climb and glide with the exchange of Ca by aliovalent Mn, which may affect the point defect concentration.