Dislocation and diffusion creep of synthetic anorthite aggregates

Synthetic fine‐grained anorthite aggregates were deformed at 300 MPa confining pressure in a Paterson‐type gas deformation apparatus. Creep tests were performed at temperatures ranging from 1140 to 1480 K, stresses from 30 to 600 MPa, and strain rates between 2×10−6 and 1×10−3 s−1. We prepared samples with water total contents of 0.004 wt % (dry) and 0.07 wt % (wet), respectively. The wet (dry) material contained 120 MPa we found a stress exponent of n = 3 irrespective of the water content, indicating dislocation creep. However, the activation energy of wet samples is 356±9 kJ mol−1, substantially lower than for dry specimens with 648±20 kJ mol−1. The preexponential factor is log A = 2.6 (12.7) MPa−n s−1 for wet (dry) samples. Microstructural observations suggest that grain boundary migration recrystallization is important in accommodating dislocation creep. In the low‐stress regime we observed a stress exponent of n = 1, suggesting diffusion creep. The activation energies for dry and wet samples are 467 ±16 and 170 ± 6 kJ mol−1, respectively. Log A is 12.1 MPa−n μmm s−1 for the dry material and 1.7 MPa−n μmm s−1 for wet anorthite. The data show that the strengths of anorthite aggregates decrease with increasing water content in both the dislocation and diffusion creep regimes. A comparison of the creep data of synthetic plagioclase from this study with published data for feldspar, olivine, and quartz indicates a linear relationship between activation energy and log A similar to the suggested compensation law for diffusion in silicates.