HR: 1340h
AN: T33C-1507    [Abstracts]
TI: Dehydration Kinetics of Volterra Gypsum: Experiments and Overview
AU: * Llana-Funez, S
EM: slf@liverpool.ac.uk
AF: University of Liverpool, Dept of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AU: Wheeler, J
EM: johnwh@liverpool.ac.uk
AF: University of Liverpool, Dept of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AU: Faulkner, D
EM: faulkner@liverpool.ac.uk
AF: University of Liverpool, Dept of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom
AB: Dehydration reactions are often envisaged as a triggering mechanism for seismicity in rocks under tectonic loads due to the reduction in effective pressure during the release of fluids that may eventually produce mechanical embrittlement. Understanding of metamorphic transformation in deforming rocks is even more important in fault zones where periods of seismic slip are reported. Dehydration of gypsum under controlled conditions, in laboratory experiments and in numerical models, provides information on deformation processes operating in seismically active regions and may be of help in understanding their cyclicity and their evolution. Two series of simple heating experiments of Volterra gypsum samples at room pressure, using intact and powdered specimens, provide reference data for further experiments under confining and differential stress during dehydration. Heating experiments were run at constant temperature between 80 degC and 140 °C in intact specimens and at 86 °C and 97 °C using powders with five different grain size fractions: <0.063, 0.063-0.125, 0.125-0.25, 0.25-0.5 and >0.5 mm. The complete dehydration of 1 mol of gypsum produces 1 mol of anhydrite and two moles of water generating a porosity of about 38% and implying a weight loss of 21% upon removal of water. The progressive loss of weight during dehydration was used as the method to estimate the progress of the reaction. The reaction is characterized by an initial stage under 10% reaction were reaction rate accelerates, which is followed by a linear stage for about 50 to 70% of the reaction and a final third stage with decelerating reaction rates. All tests run above 85 °C reached about 90% reaction. Those below 85 °C seem to converge to a lower final fraction (75%) suggesting partial dehydration, very likely to bassanite. The temperature dependence of the linear rates indicates in an Arrhenius plot that the full dehydration of gypsum has an activation enthalpy of 96 kJ/mol. The two temperatures tested with powdered specimens are consistent with this activation enthalpy, although the higher intercepts with the y-axis indicate that reaction rates are about one order of magnitude faster. This is interpreted in relation to the very large initial porosity of the un-compacted specimens (>45% porosity). There is an additional increase in rates in powdered specimens as the grain size decreases, however, the difference is marginal despite the eight-times difference in grain size. At laboratory deformation rates, gypsum behaves in a brittle-ductile mode by a mixture of plasticity (twinning) and cataclasis in discrete and short-lived bands. A first series of deformation tests on dry gypsum have also been run to set a reference for mechanical behaviour to be compared in the future with behaviour during syn-tectonic dehydration. The deformation tests were run at room temperature, at strain rates of 2x10^-5s^-1 and confining pressures of 13, 50, 100, 146, 190 MPa. Tests were run in a triaxial rig using 20 mm diameter by 45 mm length specimens. Stress-strain curves show well-defined yield points and an almost straight plastic behaviour with a slight strain hardening component, similar to previous work. Stress-strain curves have minor, episodic and short-lived stress drops that have been related with the development of cataclastic bands (Milsch and Scholz, 2005). The grain fracturing associated with the generation of these cataclastic bands during experimental deformation of gypsum will have an effect in the dehydration kinetics by providing fined-grained gypsum and thus high surface area to speed up the reaction.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 5114 Permeability and porosity
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting