Effect of aqueous and carbonic fluids on the dislocation creepstrength of quartz

摘要

Dislocation creep experiments conducted on quartzite indicate that the presence ofCO2 can cause strengthening or weakening depending on the oxygen fugacity (f02) of thedeformation environment. Under oxidizing conditions (ferrosilite-hematite-quartz), thepresence of CO2 reduces the water fugacity (fH2o) and results in strengthening ofthe quartz. Under moderately reducing conditions (—Ni-Ni0), CO2 reacts with H2 fromthe sample assembly to form graphite; the water produced by this reaction causesweakening. Under extremely reducing conditions (quartz-fayalite-iron), CO2 is reduced tomethane, a reaction that consumes original water, thereby increasing the strength of quartz.Our results show that increasing fH2o at constant confining pressure, by changing fluidcomposition, has a similar effect as increasing fH2o by increasing confining pressure. The.fH2o 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 reportedvalues. Differences in deformation behavior over small length scales may thus be relatedto local differences in the fogthat effectively change the fH,0 in the presence of CO_2.