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[1] Dislocation creep experiments conducted on quartzite indicate that the presence of CO 2 can cause strengthening or weakening depending on the oxygen fugacity ( f O 2 ) of the deformation environment. Under oxidizing conditions (ferrosilite-hematite-quartz), the presence of CO 2 reduces the water fugacity ( f H 2 O ) and results in strengthening of the quartz. Under moderately reducing conditions ($Ni-NiO), CO 2 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), CO 2 is reduced to methane, a reaction that consumes original water, thereby increasing the strength of quartz. Our results show that increasing f H 2 O at constant confining pressure, by changing fluid composition, has a similar effect as increasing f H 2 O by increasing confining pressure. The f H 2 O 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 O 2 that effectively change the f H 2 O in the presence of CO 2 .

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