EXPERIMENTAL-EVIDENCE FOR WATER WEAKENING OF QUARTZITE BY MICROCRACKING PLUS SOLUTION PRECIPITATION CREEP

Natural quartzites have been experimentally deformed at a temperature of 1073 K, a confining pressure of 1.2 +/- 0.1 GPa, strain rates of 10(-5), 10(-6) and 10(-7) s-1 and in the presence of 0.4 wt% of added water. Microstructures after deformation indicate that at strain rates of 10(-5)-10(-6) s-1 crystal plastic deformation is predominant. This is evident from abundant deformation lamellae in the original grains, and the fact that these grains show a (weak) crystallographic preferred orientation. New grains are absent or of relatively minor volumetric importance. In contrast, samples deformed at 10(-7) s-1 show numerous arrays of fine, new polygonal to euhedral quartz grains developed in axially aligned transgranular and grain boundary microcracks. These new-grain aggregates are characterized by abundant microscale voids, fine intergranular channels and fluid inclusion trails. Syntaxial overgrowth structures on crack walls are also widespread. The aggregates of new grains are thus interpreted to have formed by precipitation from solution. In addition, old grain boundaries oriented perpendicular to the shortening direction show evidence for dissolution. The old grains show almost no deformation lamellae, subgrains or c-axis preferred orientation. It therefore appears that at strain rates of 10(-7) s-1 microcracking and solution-precipitation creep are the dominant deformation mechanisms.