An experimental investigation into the effect of water on the flow of quartzite

Abstract

In this thesis, attention is focussed on the water-weakening effects seen in experiments on natural polycrystalline quartz rocks. When deformed, these polycrystals commonly show optical deformation microstructures and old-grain CPO's, believed to be indicative of crystal-plastic deformation (Tullis et al. 1973, Jaoul et al. 1984, Dell'Angelo & Tullis 1986). These experiments are important since they provide perhaps the strongest experimental support for crystal-plastic deformation as the dominant mechanism of crustal deformation. Yet, some of these experiments have also yielded results that seem to be inconsistent with predominance of crystalplastic mechanisms, the two principal consistencies of which are summarized below: (i) Kronenberg et al. (1986; but also Rovetta et al. 1986, and Gerretsen et al. 1989) have demonstrated that water cannot diffuse fast enough into quartz to explain the rapid water-weakening effect seen in experiments such as those of Tullis et al. (1973) in terms of an intracrystalline effect. Kronenberg et al. (1986) measured diffusive penetration velocities of less than 1 to 10 J.Ul1 per day. Yet, quartzites with grain sizes as large as 100 to 200 J.Ul1 became water-weakened in less than 2 hours in the experiments of Tullis & Yund (1985). Tullis & Yund (1985) suggested that micro-eracks could have been responsible for the relatively rapid uptake of the water, but these never have been observed. This raises the question of how is quartzite made ductile and easily deformable in the presence of added water, but without conventional diffusion of the added water into the grains? (ii) Jaoul et al. (1981, 1984) determined the flow behaviour of natural quartzites deformed with added water. The quartzites showed a very low stress exponent for power law creep (n=1.2-1.4). This is indicative for a diffusional deformation mechanism, and cannot be explained by dislocation glide and climb, i.e., by crystal-plastic deformation. But the optical and TEM deformation microstructures were reported to be indicative of dislocation creep. How can we explain this? It is the aim of this thesis to investigate this enigmatic behaviour, in order to find out how water weakens quartzite.