HR: 09:30h
AN: T41B-06    [PDF]
TI: Grain Boundary Microstructures of Wet and Dry Recrystallizing Marble
AU: * de Bresser, H
EM: j.h.p.debresser@geo.uu.nl
AF: HPT-laboratory, Faculty of Geosciences, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: Urai, J
AF: Geologie-Endogene Dynamik, RWTH, Lochnerstr. 4-20, Aachen, D-52056 Germany
AU: Olgaard, D
AF: ExxonMobil Upstream Research Company, P.O.Box 2189, Houston, TX 77252-2 United States
AB: We analyzed 2D grain boundary maps of samples of marble that were deformed at high temperature with and without added water. Our aim was to relate the grain boundary geometry of wet and dry marble to the observed mechanical behavior, and to obtain criteria that can help interpretation of natural calcite rocks in terms of the influence of water on their deformation.\\ We made use of cylindrical samples of pure white, microporous Carrara marble that were axially compressed in a gas medium deformation apparatus at temperatures (T) ranging 600-1000$\deg$C, a constant confining pressure of 300 MPa and strain rates around 10$^{-5}$ s$^{-1}$. Samples were jacketed in sealed Pt-capsules with or without the addition of 0.4-2.1 wt% water. Microstructural analysis was carried out using Scanning Electron Microscopy (SEM) and Light Optical Microscopy. Traced grain boundary maps were made from ultra thin sections of samples, and were quantitatively analyzed using Image Analysis techniques.\\ The strength of water-added samples was found to be slightly less than of dry samples at all temperatures investigated (weakening ~40% at T=600$\deg$C, decreasing to $\sim$10% at higher T), with one exception at T=800$\deg$C. Microstructurally, the samples showed grain flattening and twinning at T=600$\deg$C and development of new grains by dynamic recrystallization at higher T, dominated by grain boundary migration. Grain boundaries in wet samples showed isolated or locally continuous remnants of fluid pockets in SEM. Quantitatively, the mean grain size and grain size distribution were found to only marginally vary between dry and wet samples. Average roundness of grains in wet recrystallized samples is systematically better than in dry samples. The fractal dimension D for the relationship between grain diameter d and grain perimeter P (expressed P$\sim$d$^{D}$) for wet samples is systematically lower than for dry samples. Thus, grain boundaries in wet-deformed samples have less irregular shapes than in dry samples. Average grain aspect ratios of deformed samples were compared to strain ratios calculated on the basis of the imposed bulk shortening. The results demonstrated that wet samples did not track the bulk strain to the same extend as dry-deformed samples, indicating higher grain boundary mobility in the wet samples. We suggest that the small difference in strength between the wet and dry marble was caused by an enlarged contribution of grain boundary sliding mechanisms to the overall creep, governed by limited grain shape modification by fluid-enhanced dynamic recrystallization. Analysis of naturally deformed calcite rocks might benefit from quantification of the grain diameter-perimeter fractal dimension, but only if relative changes from the same geological setting are used.
DE: 3902 Creep and deformation
DE: 3947 Surfaces and interfaces
DE: 5112 Microstructure
DE: 8030 Microstructures
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2003 Fall Meeting