HR: 10:40h
AN: T41F-02 INVITED     [PDF]
TI: Equilibrium Grain Boundary Segregation in Mantle Rocks: Grain Boundaries as Reservoirs of Incompatible Elements
AU: * Hiraga, T
EM: hirag001@umn.edu
AF: Tohoku University, Department of Machine Intelligence and Systems Engineering, Aramaki-Aza, Aoba 01,, Sendai, 980-8579 Japan
AU: Anderson, I M
EM: andersonim@ornl.gov
AF: Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN 37831-6064 United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota, Department of Geology & Geophysics, 310 Pillusbury Dr S.E, Minneapolis, MN 55455 United States
AB: The classical model developed by McLean describes segregation of solute or impurity atoms to grain boundaries in binary alloy systems using the absorption analogue. This thermodynamic model has not been applied to grain boundaries in Earth materials; grain boundaries have simply been treated as possible sinks for impurities in rocks. To test if the model describes segregation to grain boundaries in mantle rocks, the chemistry of olivine grain boundaries in many types of olivine aggregates annealed at 1373-1523 K and in natural peridotites was analyzed using energy dispersive X-ray (EDX) profiling obtained with a scanning transmission electron microscope (STEM) with a probe size of $<1.4$ nm. Profiles across grain boundaries reveal a substantial segregation of Ca, Al and Ti. High-resolution transmission electron microscopy (HREM) observations reveal that the boundaries do not contain a grain boundary phase. Moreover, an equilibrium thermodynamic model, assuming that the solute is confined entirely to the grain boundary plane, is quantitatively consistent with the level of Ca grain boundary segregation and its temperature dependence. Strain energy minimization and space charge compensation are two important thermodynamic driving forces for segregation to grain boundaries. A survey of all elements measured in the grain boundary profiles indicates that differences in ionic radius and valence state from that of host elements (i.e., Mg and Si) control the enrichment of elements at grain boundaries. The magnitude of the partitioning is explained well by regarding the segregation energy as the misfit strain energy calculated for substitution of Ca for Mg in olivine grains. Therefore, we can predict the value of the partition coefficient for different elements and different minerals with parameters such as ionic radius and Young's modulus for the crystals. Based on this calculation, we conclude that grain boundaries can be primary storage sites for the elements with large ionic radii such as Sr, Ba, K and Rb in mantle rocks.
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
DE: 1212 Earth's interior--composition and state (8105)
DE: 3947 Surfaces and interfaces
SC: Tectonophysics [T]
MN: 2003 Fall Meeting