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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.