HR: 09:15h
AN: MR31B-06    [Abstracts]
TI: Reaction Driven Shear Localization: the Example From Torsion Experiments on Dolomite
AU: * Delle Piane, C
EM: claudio.dellepiane@erdw.ethz.ch
AF: Geological Institute ETH, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AU: Burlini, L
EM: burlini@erdw.ethz.ch
AF: Geological Institute ETH, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AU: Grobety, B
EM: bernard.grobety@unifr.ch
AF: Geosciences Department Fribourg University, Chemin du Mus‚e 6 - P‚rolles , Fribourg, CH-1700 Switzerland
AB: We performed torsion experiments using a Paterson rig apparatus on cylindrical (diam = 1cm; H = 1cm) samples of natural specimen of pure dolomite (<0.3 wt % of impurities on XRF analysis), drilled on a block of the Dolomia from Mt. Frerone (Adamello, N-Italy). Experiments were run at 800°, 300 MPa confining pressure under vented and non-vented conditions, up to a bulk strain of about γ =1, at different strain rates (3*10^-5s^-1 up to 1*10^-4s^-1). Deformation was localized near both porous spacers placed at the extremities of the specimen, corresponding to the zone were dolomite broke down into a fine grained mixture of calcite and periclase according to the reaction: CaMg(CO₃)₂=CaCO₃ + MgO + CO₂. At these conditions the nominal equilibrium PCO₂ should be around 70 MPa (Graf and Goldsmith 1955), but at the spacer interface the CO₂ was free to escape, triggering the reaction. On the other hand, in the central part of the sample, the reaction occurred only along the grain boundaries of the dolomite grains. Due to the low permeability pore fluid could not escape building up CO₂ pressure which deactivated the reaction. Here deformation was accommodated by brittle shear bands in synthetic Riedel geometry, consistently oriented with the sense of shear; these bands do not extend up to the sample-spacers. We conclude that the decarbonatation reaction produced a weaker matrix in which deformation could localize. Moreover, the very fine grain size was probably produced during the decarbonatation reaction, and was kept small by the deformation due to pinning between the different phases. Graf, D. L. and J. R. Goldsmith (1955). ``Dolomite-magnesian calcite relations at elevated temperatures and CO₂ pressure." Geochimica et cosmochimica acta 7: 109-128.
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005