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. ¿ 2001 American Geophysical Union