HR: 1340h
AN: T53B-1433    [Abstracts]
TI: Permeability evolution during non-linear viscous creep of porous calcite rocks
AU: * Xiao, X
EM: xhxiao@mit.edu
AF: EAPS, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Evans, B
EM: brievans@MIT.EDU
AF: EAPS, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Bernab‚, Y
EM: ybernabe@eost.u-strasbg.fr
AF: Ecole et Observatoire des Sciences de la Terre de Strasbourg Universit‚ Louis Pasteur , 5, rue Ren‚ Descartes, Strasbourg Cedex, 67084 France
AB: Below the brittle-ductile transition, permeability might be exceedingly small, due to compaction facilitated by intracrystalline plasticity or viscous creep. The ductile lower crust may consist of depth intervals or isolated domains of relatively high permeability, where the fluid pressures are at or near lithostatic values. Fluid escape from metamorphic rocks likely involves episodic hydrofracturing or porosity-wave propagation driven by the difference between the gradients of fluid and rock pressure. Although it is generally agreed that fluid flow in ductile porous rocks is critically dependent on the interplay between the fluid properties and the rheology of the rock matrix, more experimental work is needed to elucidate the ways that permeability and porosity change during deformation at elevated temperature and pressures. Triaxial tests of synthetic calcite marbles containing 10, 20, or 30 wt% quartz and up to 9% residual porosity done at temperature up to 873K, reported earlier (Xiao and Evans, 2003), indicate that shear-enhanced compaction occurs under triaxial conditions, roughly consistent with a model of void collapse by viscous creep (Budiansky et al., 1982). In this study, we report the effect of viscous creep on the permeability of those porous rocks during both isostatic and conventional triaxial loading. The tests were performed at confining pressure of 300 MPa, pore pressures between 50 to 290 MPa, temperatures from 673 to 873K and strain rates of 3.0× 10^-5 s^-1. Argon gas was used as the pore fluid. Under isostatic loading conditions, permeability, k, is nonlinearly related to porosity, Φ. Over small changes in porosity, the two parameters are approximately related as k~Φⁿ. The exponent n progressively increases as the porosity decreases to a finite value, suggesting a percolation porosity. When subjected to triaxial deformation, the calcite-quartz aggregates exhibit a shear-enhanced compaction, but permeability does not decrease as rapidly as it does during isostatic conditions; the exponent n varies between 2 and 3. Non-isostatic deformation seems to reduce the percolation threshold, and, in fact, enhances the permeability relative to that at the same porosity during isostatic compaction. Our data provide constraints on the governing parameters of the compaction theory, and may have far-reaching implications for melt extraction from partially molten rocks, for the expulsion of sedimentary fluids, and for fluid flow during deformation and metamorphism.
DE: 3653 Fluid flow
DE: 3902 Creep and deformation
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005