HR: 14:25h
AN: T42D-04    [PDF]
TI: Effect of Mineral Reaction on the Deformation of Plagioclase-Olivine Aggregates
AU: * de Ronde, A
EM: almar.deronde@unibas.ch
AF: Earth Sciences, Basel University, Bernoullistrasse 32, Basel, 4056 Switzerland
AU: St\"{u}nitz, H
EM: Holger.Stuenitz@unibas.ch
AF: Earth Sciences, Basel University, Bernoullistrasse 32, Basel, 4056 Switzerland
AU: Tullis, J
EM: jan_tullis@brown.edu
AF: Geological Sciences, Brown University, Box 1846, Providence, RI 02912 United States
AB: There have been many studies that have described the relative timing of deformation and metamorphism in natural rocks, but there are few studies that have analysed the interrelationship of the processes that produce simultaneous deformation and chemical reactions. We studied the physical and microstructural relationships between the processes of deformation and chemical reaction by performing simple shear deformation experiments on plagioclase-olivine aggregates. Our study shows that deformation and reaction processes are strongly related by a mutual positive feedback, and point out possible mechanisms for shear localization in the upper mantle lithosphere during subduction and extension. Shear deformation experiments were carried out on dry plagioclase-olivine mixtures (4-10 $\mu$m grain sizes, $\pm$50-50 vol.% olivine-plagioclase ratio) in a Griggs apparatus under a constant applied shear strain rate of $\sim$5*10$^{-5}$s$^{-1}$ at $900\deg$C and 1.0-1.6 GPa confining pressures. We have studied the plagioclase-olivine deformation in presence and absence of reaction by choosing plagioclase to be of labradorite (An60) and anorthite (An92) composition. At 1.0-1.6 GPa confining pressures, the anorthite-olivine samples react to spinel-pyroxene bearing assemblages, whereas the labradorite-olivine samples do not. Labradorite and anorthite are expected to have fairly similar deformation behaviour. {\it Effect of reaction on deformation:}\\ In the absence of reaction, labradorite-olivine mixtures are so strong at the chosen experimental conditions that they hardly deform plastically. In contrast, anorthite-olivine mixtures deformed at the same temperature and strain rate react during deformation, producing a stress maximum ($\sim$375 MPa) followed by a significant stress decrease. Regardless of the chosen confining pressure, reacting samples commonly weaken towards a shear stress of $\sim$275 MPa. The plagioclase-olivine reaction strongly reduces the grain size of the samples (down to 0.1 $\mu$m), and causes a transition from grain size insensitive (dislocation) creep of plagioclase and olivine to diffusion accommodated grain boundary sliding of the reaction products, even though the newly formed minerals (e.g. pyroxene, spinel) are mechanically stronger than the original minerals. {\it Effect of deformation on reaction:}\\ The plagioclase-olivine reaction progress is greatly enhanced by differential stress and/or crystal plastic strain. At 1.5 GPa, hydrostatic experiments of 168 hours show less than 30% reaction progress, whereas in the deformation experiments ($\sim$58 hours) the reaction goes to completion.
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 3660 Metamorphic petrology
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
SC: Tectonophysics [T]
MN: 2003 Fall Meeting