Effects of chemical environment on dislocation creep of quartzite

Abstract

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

Effect of aqueous and carbonic fluids on the dislocation creep strength of quartz

“…An additional 0.15 wt.% water (W1341, 0.2 wt.% total) caused a further decrease in strength of $115 MPa. The strength of very white BHQ decreased further with 0.5 wt.% water added (W1352), an amount we infer to result in water saturation of the quartzite sample (not the quartz itself) based on previous studies [e.g., Kronenberg and Tullis, 1984;Post et al, 1996]. 3.2.2.…”

“…This quartzite, which has been used in many previous studies [e.g., Hirth and Tullis, 1992;Gleason and Tullis, 1995;Stipp et al, 2006;Stipp and Kunze, 2008], contains <1% feldspar and iron oxides, and trace amounts of apatite, zircon, and rutile. BHQ has a grain size of approximately 100 mm and $1 -4% porosity as measured by Hirth and Tullis [1991] and Post et al [1996]; the grains contain no deformation microstructures and there is no lattice preferred orientation. Two different colors (white and very white) of the starting material were used; they are loosely correlated with different fluid inclusion populations, with progressively whiter samples containing a greater proportion of aqueous versus carbonic fluid inclusions.…”

“…The f H 2 O term was estimated by Kohlstedt et al [1995] to have an exponent (m) of 0.8 to 1.2 based on data from Kronenberg and Tullis [1984]. Post et al [1996] calculated m % 2 but Hirth et al [2001] found that an exponent of m % 1 gave the best agreement between experiments and naturally deformed quartzites with wellconstrained deformation conditions.…”

“…Although they reported that strength decreased with increasing f O 2 and f H 2 O , consistent with our results, a later study by Fitz Gerald et al [1991] determined that the samples had fractured during pretreatment, which influenced subsequent deformation. The effect of chemical environment on dislocation creep of quartzite was studied by Post et al [1996], who deformed quartzite with 0.4 wt.% water added at several different values of f O 2 . They found that the strength of quartzite deforming by dislocation creep depended on f H 2 O but not on f H 2 , f O 2 or a H +.…”

Coupled trace element mobilisation and strain softening in quartz during retrograde fluid infiltration in dry granulite protoliths

“…Griggs and Blacic 1965;Kronenberg 1994;Kronenberg et al 1986;Post and Tullis 1998). Of several parameters in the chemical environment (aH + , fH 2 O; aOH -), only fH 2 O has significant effects on creep strength as well as annealing rate (Post et al 1996). Strain induced grain boundary migration and dislocation climb is enhanced by H 2 O.…”