HR: 13:40h
AN: MR33A-01 INVITED    [Abstracts]
TI: Grain-Grain Interfaces in Diffusion and Deformation
AU: * Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Pillsbury Hall, 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AU: Dillman, A M
EM: dillm004@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Pillsbury Hall, 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AU: Scott, T J
AF: University of Minnesota, Department of Geology and Geophysics, Pillsbury Hall, 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AB: Grain-grain interfaces provide important paths for rapid diffusion of ions through rocks and critical sites for storage of incompatible elements within rocks. For an incompatible element for which the concentration in grain- grain interfaces far exceeds that in grain interiors, grain (homophase) boundaries and interphase (heterophase) boundaries provide the primary routes for mass transfer since flux is the product of concentration times diffusivity. For major elements, the rate of diffusion along boundaries depends on the composition of the interfaces. We investigate two kinetic properties, diffusion and deformation, to better understand the role of boundaries and their composition on kinetic properties of rocks. Mg-Co interdiffusion experiments were carried out using diffusion couples formed from fine-grained discs of Mg-olivine and Co-olivine. Results from undoped, Ca-doped, and Sc- doped samples permit us to quantify of the roles of ionic size and ionic charge of impurity ions on grain boundary diffusion. Addition of bi-valent Ca ions results in a small decrease in Mg-Co grain boundary interdiffusivity, possibly because these relatively large cations clog the boundaries by occupying sites that could have been used by diffusing bi-valent Mg and Co ions. In contrast, addition of tri-valent Sc ions leads to an increase in rate of diffusion of bi-valent Mg and Co ions along grain boundaries, suggesting that negatively charged defects such as metal vacancies are formed to charge balance excess positive charge introduced by tri-valent Sc ions occupying sites normally filled by Mg and Co. High-temperature creep experiments were performed on undoped and Sc- doped samples of forsterite, again to explore the effect of ionic charge on grain boundary diffusion. Deformation of our fine-grained samples in the diffusion creep regime was dominated by grain boundary diffusion (Coble creep). Samples enriched in Sc flowed a factor of at least two faster than their higher purity counterparts, again indicating that charge-compensating point defects form in response to the introduction of heterovalent cations. These results demonstrate that kinetic properties of rocks are sensitive not only to major element composition of grain interiors but also to trace element content of grain boundaries. Therefore, trace amounts of impurities along grain-grain interfaces will significantly influence properties such as viscosity and electrical conductivity in Earth's interior. Such impurities might be introduced through metasomatic processes or result from elemental segregation to grain boundaries during phase transitions where the impurity element is more incompatible in the new phase than in the original grains.
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3904 Defects
DE: 5120 Plasticity, diffusion, and creep
DE: 5139 Transport properties
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting