Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Full description of the project: The objective of this project is to study experimentally the plastic deformation of quartz in the presence of water. Even if the weakening effect of water on plastic quartz deformation is known since the 1960s, the processes of water weakening controlling the deformation are poorly known. The project will study systematically the effect of two important variables on the strength of polycrystalline aggregates of quartz: (1) The water content and (2) The confining pressure. Their effect will be quantified by experiments and micro-structural, x-ray tomography, and infrared spectroscopy measurements. The problem It has been shown that the solubility and flux of OH in quartz are extremely low (e.g. Paterson 1989) and diffusion of H2O into the quartz structure has not been achieved in laboratory experiments so far, but all transport has involved microcracking and advective transport of molecular water (Kronenberg et al. 1986, Gerretsen et al. 1989). The water content and its distribution, affecting the mechanical strength, in deforming quartz assemblages are therefore dynamic parameters. They depend on the interaction between crystal deformation, moving dislocations, sweeping grain boundaries, and microfracturing. In naturally and experimentally deformed polycrystalline samples, most H2O is contained in the grain boundary region. As a high density of grain boundaries is formed during deformation by recrystallization, it is unclear to what extent the resulting strength of the aggregate is affected by the fraction of H2O in the boundary region or grain interiors. In order to quantify the weakening it is required to assess the effect of variable concentrations in H2O on the mechanical strength. One possible process accounting for the weakening associated with large concentrations in H2O is the pore pressure effect, i.e. the reduction of the strength by the presence of free water, with a fluid pressure, distributed in pores or cracks. In the presence of a very small pore volume, its mechanical effect will be negligible. Thus, the mechanical problem of pore fluid pressure in the upper to middle sections of the crust becomes a problem of pore fluid volume in the lower sections of the crust and in the upper mantle (the main part of the lithosphere). The data base for the volume limit of a pore pressure effect is very poor, and a systematic study of threshold values for weakening effects of H2O pore volume versus H2O weakening effects by plasticity is missing and urgently needed. Such a value will help to delineate seismic from aseismic deformation. Confining pressure is the second controlling parameter of crystal plastic weakening. The pressure dependence of water weakening demonstrated by Kronenberg and Tullis (1984) is incorporated into the flow law by introducing a fugacity term (Hirth et al. 2001). There have been attempts to determine the fugacity exponent more accurately (Chernak et al. 2009, Holyoke and Kronenberg 2013), but the data are not in agreement. The experimental data set, which has been used to establish the pressure dependence, has been obtained by Kronenberg and Tullis (1984). Given the importance of the topic, this data set is not state-of-the-art anymore by modern standards of experimentation and analytics, particularly because the Novaculite material used has a very high water content and very small grain size and shows an undesired coarsening during the deformation, so that the effective storage capacity of the grain boundary region varies with time of the experiment. There is a need for a new database for the pressure dependence of wet quartz deformation, using better controlled H2O-contents, microstructures, and grain sizes. The work package New experiments in shear deformation will be carried out with different H2O contents (pressure range of 0.5 to 1.5 GPa) in a solid medium Griggs apparatus at UiT according to routines already developed in previous projects. A higher-pressure range of experiments (between 1.5 and 3.0 GPa) will be carried out in the new solid medium deformation apparatus at Orleans University. The SEM work will be performed at the UiT-SEM lab. UiT has purchased two new SEM´s, one of which is a dedicated high-end FE-SEM for materials research, equipped with EBSD-, CL-, EDS-, and WDS-detectors. FTIR analysis shall be carried out to estimate the average water concentration and the speciation of water. These analyses will be carried out at Orleans University in collaboration with Lionel Mercury and Hugues Raimbourg. Both have extensive experience with analyses of H2O in quartz. Tomography to characterize porosity will be carried out for higher resolution at a synchrotron lab. Access to the synchrotron X-ray tomography will be facilitated through Florian Fusseis at Edinburgh University, who has asserted access to synchrotron facilities in Europe and the USA. The resulting water-parameters will be incorporated as concentration and fugacity expressions in the flow law of quartz. Such a new flow law has long-reaching consequences for the modeling of lithosphere deformation and our understanding of mountain building and plate tectonics. In addition, the measured parameters will help the determination of depth and strength estimates of crustal rocks for earthquake locations. Chernak, L. J., Hirth, G., Selverstone, J., & Tullis, J. (2009). Effect of aqueous and carbonic fluids on the dislocation creep strength of quartz. Journal of Geophysical Research: Solid Earth, 114(B4). Hirth, G., Teyssier, C., & Dunlap, J. W. (2001). An evaluation of quartzite flow laws based on comparisons between experimentally and naturally deformed rocks. Intern. Journal of Earth Sciences, 90(1), 77-87. Holyoke, C. W., & Kronenberg, A. K. (2013). Reversible water weakening of quartz. Earth and Planetary Science Letters, 374, 185-190. Gerretsen, J., Paterson, M. S., & McLaren, A. C. (1989). The uptake and solubility of water in quartz at elevated pressure and temperature. Physics and Chemistry of Minerals, 16(4), 334-342. Kronenberg, A. K., et al. "Solubility and diffusional uptake of hydrogen in quartz at high water pressures: implications for hydrolytic weakening." Journal of Geophysical Research B 91.B12 (1986): 12723-12741. Kronenberg, A. K., & Tullis, J. (1984). Flow strengths of quartz aggregates: grain size and pressure effects due to hydrolytic weakening. Journal of Geophysical Research: Solid Earth, 89(B6), 4281-4297. Paterson, M. S. (1989). The interaction of water with quartz and its influence in dislocation flow—an overview. Rheology of Solids and of the Earth, 107-142.