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 (f(O2)) of the deformation environment. Under oxidizing conditions (ferrosilite-hematite-quartz), the presence of CO2 reduces the water fugacity (f(H2O)) and results in strengthening of the quartz. Under moderately reducing conditions (similar to Ni-NiO), CO2 reacts with H-2 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 f(H2O) at constant confining pressure, by changing fluid composition, has a similar effect as increasing f(H2O) by increasing confining pressure. The f(H2O) 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 f(O2) that effectively change the f(H2O) in the presence of CO2.