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 <0.7 (0.2) vol % glass, associated with fluid inclusions or contained at triple junctions. The arithmetic mean grain size of the specimens varied between 2.7±0.1 μm for the dry material and 3.4±0.2 μm for wet samples. Two different creep regimes were identified for dry and wet anorthite aggregates. The data could be fitted to a power law. At stresses >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 μm m s −1 for the dry material and 1.7 MPa −n μm m 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.