HR: 0800h
AN: V41E-1519    [Abstracts]
TI: Equilibrium interface segregation in diopside-forsterite system
AU: * Hiraga, T
EM: hirag001@umn.edu
AF: Dept Geol & Geophys, U Minnesota, Pillsbury Hall, Minneapolis, MN 55455 United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: Dept Geol & Geophys, U Minnesota, Pillsbury Hall, Minneapolis, MN 55455 United States
AB: We are investigating the chemistry of diopside grain boundaries and diopside/forsterite phase boundaries doped with Sr, Mn, Ni, Al, Y, Nd and/or Ti. Polycrystalline samples were annealed at 1573 K and 1 atm for >150 h. The chemical compositions of crystals and boundaries were measured by energy dispersive X-ray spectrometry using a scanning transmission electron microscope with a probe size of <2 nm. The measurement area was 20*50 nm for all analyses. We assume that the occupation of solutes is limited to a single layer at the boundaries in order to convert characteristic X-ray intensities from the boundary area (i.e., scanned box) into that from a boundary monolayer. Direct comparison of X-ray intensities from dopant elements normalized by that from Si in the crystals and in the monolayer yields boundary/crystal partition coefficients for the dopants. Divalent elements partition into the boundaries as predicted by misfit lattice strain energy. For example, the concentration of Sr at grain and phase boundaries is ~15 times larger than in diopside crystals. Segregation of Mn and Ni, which have ionic sizes similar to those of Ca and Mg, is weak and/or not detected. Sr partitioning is not affected by co-segregation of Ti. Partition coefficients of these elements between crystals and grain/phase boundaries are comparable to that between crystals and melt. Segregation of heterovalent elements varies significantly from one boundary to the next. Overall, they partition more strongly into boundaries than into melt. For example, the concentration of Y, which has approximately the same ionic size as Ca, is ~40 times larger in boundaries than in diopside crystals, compared to ~10 times in the case of melt vs crystal. Segregation of Y has a positive correlation with Al concentration at the boundaries with the same Al:Y ratio as in diopside crystals. These observations suggest that not only electrostatic work in the crystal lattice, but also electrostatic attraction of heterovalent elements at boundaries is important and also that charge balance at the boundaries is likely to be maintained by the same reaction as in the crystal lattice. All of the above observations also apply to the phase boundaries. This research will enable us to predict the storage capacities of grain and phase boundaries for highly incompatible elements in Earth's mantle.
DE: 1025 Composition of the mantle
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3904 Defects
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005