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 2x10(-6) and 1x10(-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 m for the dry material and 3.4 +/- 0.2 mum 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 mum(m) s(-1) for the dry material and 1.7 MPa-n mum(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.