Effect of aqueous and carbonic fluids on the dislocation creep strength of quartz

Dislocation creep experiments conducted on quartzite indicate that the presence of CO₂ can cause strengthening or weakening depending on the oxygen fugacity ? of the deformation environment. Under oxidizing conditions (ferrosilite-hematite-quartz), the presence of CO₂ reduces the water fugacity ? and results in strengthening of the quartz. Under moderately reducing conditions (∼Ni-NiO), CO₂ reacts with H₂ from the sample assembly to form graphite; the water produced by this reaction causes weakening. Under extremely reducing conditions (quartz-fayalite-iron), CO₂ is reduced to methane, a reaction that consumes original water, thereby increasing the strength of quartz. Our results show that increasing ? at constant confining pressure, by changing fluid composition, has a similar effect as increasing ? by increasing confining pressure. The ? exponent suggested by our data for the dislocation creep flow law is 0.375 to 1 (assuming a stress exponent of 3 to 4), which is on the low side of previously reported values. Differences in deformation behavior over small length scales may thus be related to local differences in the ? that effectively change the ? in the presence of CO₂.