HR: 0830h
AN: T41D-0247    [PDF]
TI: Dislocation Creep in Magnesium Calcite
AU: * Xu, L
EM: lilixu@mit.edu
AF: Dept. Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AU: Xiao, X
EM: xhxiao@mit.edu
AF: Dept. Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AU: Evans, B J
EM: brievans@mit.edu
AF: Dept. Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AB: To investigate the effect of dissolved Mg on plastic deformation of calcite, we performed triaxial deformation experiments on synthetic calcite with varying amount of Mg content. Mixtures of powders of calcite and dolomite were isostatically hot pressed (HIP) at $850\deg$C and $300$ MPa confining pressure for different intervals ($2$ to $20$hrs) resulting in homogeneous aggregates of high-magnesium calcite; Mg content varied from $0.07$ to $0.17$ mol$%$. Creep tests were performed at differential stresses from $20$ to $160$ MPa at $700$ to $800\deg$C. Grain sizes before and after deformation were determined from the images obtained from scanning electron microscope (SEM) and optical microscope. Grain sizes are in the range of $5$ to $20$ microns depending on the HIP time, and decrease with increasing magnesium content. Both BSE images and chemical analysis suggest that all dolomite are dissolved and the Mg distribution is homogeneous through the sample, after $2$ hrs HIP.\\At stresses below $40$ MPa, the samples deformed in diffusion region (Coble creep), as described previously by Herwegh. The strength decreases with increasing magnesium content, owing to the difference of grain size. At stresses above $80$ MPa, the stress exponent is greater than $3$, indicating an increased contribution of dislocation creep. The transition between diffusion to dislocation creep occurs at higher stresses for the samples with higher magnesium content and smaller grain size. Preliminary data suggests a slight increase in strength with increasing magnesium content, but more tests are needed to verify this effect. In a few samples, some strain weakening may have been evident. The activation energy in the transition region (at $80$ MPa) is $\sim200$ KJ/mol with no dependence on magnesium content, agreeing with previous measurements of diffusion creep in natural and synthetic marbles.
UR: http://www.agu.org\~lilixu
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2003 Fall Meeting