Brittle and semi-brittle fractures of antigorite serpentinite in triaxial compression

Deformation experiments on antigorite serpentinites at confining pressures of 100 similar to 400MPa, temperatures of 25 similar to 700 degrees C, and strain rates of 10(-5) similar to 1. 5 x 10(-6)s(-1) indicate that semi-brittle and brittle fractures take place, respectively, at high pressures (or/and dehydration conditions) and low pressures. Gradual and dramatic decreases in strength of samples are observed in the absence and presence of dehydration, respectively, with the increase of temperature. The preheating time has a more important effect on the strength under high temperatures (e.g., 600 degrees C) where antigorite may dehydrate during the experiment than under low temperatures (e.g., 550 degrees C) which are too low to enable dehydration in our laboratory experiments. Combined and compared with previous studies, we found that increasing confining pressure leads to increase in the strength at room temperature but to the overall trend of decreasing strength in the presence of dehydration, probably due to the partial loss of cohesive strength and the increase of ductility. The ductility of fault surface can increase with temperature, confining pressure, and fluid content on fault surface increasing, resulting in friction coefficient decreasing. Antigoiite, which is not yielded even after differential stress exceeded ca. 600MPa at high temperatures (ca. 600 degrees C) and ca. 1000MPa at low-moderate temperatures (<= 400 degrees C), has a strength much stronger than that of low temperature serpentines (e.g., lizardite and chrysotile). Surprisingly, dehydration embrittlement, a classic hypothesis proposed for generating intermediate-depth earthquakes, is not observed for dehydrating antigorite in our laboratory. Dehydration of antigorite in subduction zones may therefore induce seismogenic failure in the brittle wall rocks rather than in the dehydrating serpentinite itself.