HR: 1330h
AN: V22B-0582    [PDF]
TI: Fracture Toughness of Silicate Garnets: Applications and Preliminary Data
AU: * Broz, M E
EM: broz@cems.umn.edu
AF: University of Minnesota, Dept. of Chemical Engineering and Materials Science, Minneapolis, MN 55455 United States
AU: Cook, R F
EM: cookx062@umn.edu
AF: University of Minnesota, Dept. of Chemical Engineering and Materials Science, Minneapolis, MN 55455 United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics, Minneapolis, MN 55455 United States
AB: A novel technique for assessing and possibly quantifying the magnitude of high-temperature decompression and the unroofing pressure-temperature trajectory of formerly deep rocks involves interpreting microcrack patterns in garnet. Of particular interest are microcracks that radiate from mineral inclusions, as their formation is likely initiated by decompression. By documenting the conditions at which the difference between lithostatic pressure and the internal pressure on mineral inclusions (e.g., quartz, plagioclase) will fracture a host garnet in a radial pattern about the inclusion, it may be possible to calculate tectonic variables related to exhumation of mid- to lower-crustal rocks. Application of such techniques has been hindered by a lack of information about the fracture toughness of silicate garnets. Previous modeling of garnet fracture mechanics used data from YAG (e.g., Whitney et al., 2000). We have obtained contact deformation and fracture data on silicate garnet (almandine-pyrope solid solution) using microindentation and depth-sensing indentation (nanoindentation) techniques. As a preface to the garnet study, a systematic study of the minerals in the Mohs hardness scale was conducted in which hardness, modulus and fracture toughness data were obtained. The results indicated that neither of the above material properties vary monotonically with Mohs number. This is unsurprising in that scratch testing, the basis of the Mohs classification, is affected to different extents by hardness, modulus and fracture toughness values for each mineral. For example, while almandine-pyrope garnet has hardness and modulus similar to topaz (H = 21 GPa; E = 265 GPa), the garnet's fracture toughness is near that of calcite (T = 0.187 MPa*mE0.5). Here we report contact deformation and fracture data for a variety of end-member garnets.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3900 MINERAL PHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting