Silicon diffusion in a natural quartz aggregate: constraints on solution-transfer diffusion creep

摘要

Silicon bulk diffusion rates were determined in a fine-grained (1.2 mum diameter) natural quartz aggregate (novaculite) from Hot Springs, AR. The diffusion experiments were done at 600-800 degreesC and 150 MPa confining pressure (hydrothermal) and at 800-1100 degreesC and 1 atm (in a stream of dry N-2). The technique employed an isotopic tracer (Si-30) and standard step-scan analysis using an ion microprobe (SIMS). The temperature dependence of silicon bulk diffusion in the novaculite is described by the Arrhenius parameters: D-0,D- (bulk) = 3 7 x 10(-10) m(2)/s and Q = 137 +/-18 kJ/mol, and D-0,D- (bulk) = 6.2 x 10(-9) m(2)/s and Q = 178 +/-38 kJ/mol for the hydrothermal and dry experiments, respectively. At 800 degreesC, the silicon bulk diffusion rate at 150 MPa, hydrothermal is about a factor of 10 greater than at 1 atm, dry. For an assumed effective grain boundary width of 2 nm, the silicon grain boundary diffusion rates in novaculite are a factor of two to three less than oxygen diffusion rates for similar hydrothermal conditions, and more than 12 orders of magnitude greater than anhydrous silicon volume diffusion rates extrapolated to the temperature of these experiments. The results of this study provide valuable constraints on the role of diffusion-accommodated deformation in quartz-rich rocks. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 55]