EFFECTS OF CHEMICAL ENVIRONMENT ON DISLOCATION CREEP OF QUARTZITE

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 h ydrostatic annealing experiments. Samples were weld-sealed in double c apsules; fH(2), fO(2), fH(2)O and a(H)+ were buffered using solid oxyg en buffers, AgCl or CO2. Black Hills quartzite was deformed at 900 deg rees C and 1.5x10(-5)s(-1). Two samples were deformed at similar to 17 00 MPa confining pressure at constant fH(2)O and a(H)+, with fH(2) and fO(2) varying over 8 and 15 orders of magnitude, respectively. Both s amples deformed by climb-accommodated dislocation creep with flow stre sses of 300 MPa. Two additional samples were deformed at similar to 70 0 MPa at constant fH(2)O lower than for the 1700-MPa samples, With a(H )+ varying over 2 orders of magnitude. Both samples faulted with a pea k strength of similar to 800 MPa. These four experiments Suggest no de pendence of dislocation creep strength on fH(2), fO(2) or a(H)+; inste ad, a strong dependence of strength on fH(2)O is inferred. Previously deformed samples of Heavitree quartzite were hydrostatically annealed for 4 days at 800 degrees C and 1200 or 500 MPa confining pressure, va rying a(H)+ and fH(2)O over 2.5 and 1 order of magnitude, respectively . The microstructures of these samples show increased rates of disloca tion climb and grain boundary migration with increasing fH(2)O but no dependence on a(H)+. These buffered experiments indicate that dislocat ion creep is affected by fH(2)O alone and suggest that the exponent fo r the fH(2)O term in the power law creep flow law is >2.