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 hydrostatic annealing experiments. Samples were weld-sealed in double capsules; fH(2), fO(2), fH(2)O and a(H)+ were buffered using solid oxygen buffers, AgCl or CO2. Black Hills quartzite was deformed at 900 degrees C and 1.5x10(-5)s(-1). Two samples were deformed at similar to 1700 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 samples deformed by climb-accommodated dislocation creep with flow stresses of 300 MPa. Two additional samples were deformed at similar to 700 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 peak strength of similar to 800 MPa. These four experiments Suggest no dependence of dislocation creep strength on fH(2), fO(2) or a(H)+; instead, 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, varying a(H)+ and fH(2)O 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 fH(2)O but no dependence on a(H)+. These buffered experiments indicate that dislocation creep is affected by fH(2)O alone and suggest that the exponent for the fH(2)O term in the power law creep flow law is >2.