HR: 0800h
AN: V21A-0586    [Abstracts]
TI: The Role of Deformation on the Microstructural Evolution of Calcite
AU: * Austin, N J
EM: naustin@mit.edu
AF: Massachusetts Institute of Technology Department of Earth, Atmospheric, and Planetary Sciences, 54-710 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Evans, B
EM: brievans@mit.edu
AF: Massachusetts Institute of Technology Department of Earth, Atmospheric, and Planetary Sciences, 54-710 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: Ductile shear zones are frequently spatially associated with a reduced grain-size, in a progression from undeformed protomylonites to fine grained ultramylonites. The kinetics of the microstructural evolution towards this modified grain-size, and its relationship to the mechanical properties, however, remains unclear. This work presents a series of hydrostatic and constant force, conventional triaxial deformation experiments performed on reagent grade calcite, between 823 and 1023 K, at a confining pressure of 300 MPa, and differential stresses between 0 and 40 MPa. In deformation experiments, stresses were applied such that deformation should be accommodated purely by diffusion creep, or by a combination of diffusion and dislocation creep (± grain boundary sliding), based on pre-existing flow laws. Under static conditions, grain-growth kinetics agree with those observed by Freund et al. (2001) for >99.9% pure calcite, containing trace amounts of Mn. Under conditions of low differential stress, or for fine-grained samples, grain-growth is indistinguishable from that under static conditions. For high stresses, or for coarse-grained samples, grain-size reduction is observed, the kinetics of which are correlated with the strain-rate and grain-size. The onset of grain-size reduction correlates with an increased component of dislocation creep. All samples that exhibit grain-size reduction have other features indicative of dislocation creep, including irregular grain boundaries, and progressive flattening of grains, whereas all samples that exhibit grain-growth during deformation have polygonal grains that are indistinguishable from those produced under static conditions. Further, at the conditions of our experiments, samples that exhibit grain-size reduction plot in the transitional deformation regime of Walker et al. (1990), whereas all of those that exhibit grain-growth plot in the diffusion creep field. The observed grain size reduction kinetics are best explained by summing the calculated reduction rate (d_rate=ε_ratedε_c^-1), with the predicted parabolic growth rate from Freund et al. (2001), as formulated by Hall and Parmentier (2003). The transition between grain-growth and grain-size reduction observed in our experiments can be explained by equating growth and reduction kinetics. When these kinetics are extrapolated to temperatures (523-623K) and strain rates (10^-10 to 10^-11 s^-1) consistent with natural shear zones, the predicted grain size is similar to that measured in shear zones in the Swiss Helvetic Alps (Herwegh et al., 2005). These data suggest that it may be possible to obtain a robust relationship between grain-size and strain-rate that, when coupled with knowledge of metamorphic conditions, may allow natural strain rates to be estimated.
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8164 Stresses: crust and lithosphere
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005