The water-related chemical parameter that affects dislocation creep in quartzite has been determined from variations in sample strength and microstructure with chemical environment in buffered deformation and hydrostatic annealing experiments. Samples were weld-sealed in double capsules; fu 2, fo 2, fu2o, and a.+ were buffered using solid oxygen buffers, AgCI or CO 2. Black Hills quartzite was deformed at 900øC and 1.5xl 0-5s -1. Two samples were deformed at ~ 1700 MPa confining pressure at constantfu2o and a.+, withfu 2 andfo 2 varying over 8 and 15 orders of magnitude, respectively. Both samples deformed by climb-accommodated dislocation creep with flow stresses of 300 MPa. Two additional samples were deformed at ~700 MPa at constantfu2o lower than for the 1700-MPa samples, with a.+ varying over 2 orders of magnitude. Both samples faulted with a peak strength of-800 MPa. These four experiments suggest no dependence of dislocation creep strength on fu 2, fo2, or a.+; instead, a strong dependence of strength on fu2o is inferred. Previously deformed samples of Heavitree quartzite were hydrostatically annealed for 4 days at 800øC and 1200 or 500 MPa confining pressure, varying a.+ and fu2o over 2.5 and 1 order of magnitude, respectively. The microstructures of these samples show increased rates of dislocation climb and grain boundary migration with increasing fu2o but no dependence on a.+. These buffered experiments indicate that dislocation creep is affected by f,2o alone and suggest that the exponent for the fu2o term in the power law creep flow law is >2. glide and/or recovery. Water fugacity (f. 2o), or fH20 in 22,143 22,144 POST ET AL.: CHEMICALLY CONTROLLED DEFORMATION OF QUARTZITE combination with oxygen fugacity OCo2 ), has been postulated to enhance dislocation glide and climb [Ord and Hobbs, 1986; Paterson, 1989] by controlling the concentration of some point defect or by causing hydrolized Si-O bonds [Griggs and Blacic, 1965; Paterson, 1989; Cordier et al., 1994]. The fH20 may affect the rate of dislocation climb since it enhances oxygen diffusion in quartz [Farver and Yund, 1991; McConnell, 1995]. Elphick and Graham [1988] predicted that proton
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