HR: 10:50h
AN: V42A-03 INVITED    [Abstracts]
TI: Kinetic Isotope Fractionation by Transport Prosesses in Geological Materails
AU: * RIchter, F M
EM: richter@geosci.uchicago.edu
AF: The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, United States
AB: Recent experimental results on kinetic isotope fractionations associated with mass transport within and between phases are showing that these fractionations can be very large compared to the analytical precision of modern isotope measurements. Five examples of kinetic isotope fractionations will be presented: 1. Isotope fractionation of Si, Mg and Fe by evaporation from a molten silicate liquid. 2. Isotope fractionation of Li, Ca, Mg and Fe by chemical diffusion between molten basalt and rhyolite. 3. Isotope fractionation of Li and Mg, and Cl by diffusion of dissolved salts in water. 4. Li isotopic fractionation by grain boundary diffusion. 5. Steady state fractionation of Ca, Mg, Fe, and Si by a 100 ?C temperature difference across molten basalt. The common theme that emerges from these studies is that isotopes can be used as fingerprints of diffusive transport processes and distinguish these from transport by advection or physical mixing processes. Another common theme is that while the experimental evidence for kinetic isotope fractionation is rapidly expanding, the same is not true of the theoretical understanding of these fractionations. For example, why is the observed isotopic fractionation of isotopes by evaporation significantly less than the often-assumed result that the relative evaporation rates should be proportional to the inverse square root of the mass of the evaporating species? Why are the kinetic isotope fractionations in molten silicates so very much larger than what is found for diffusion in water? Why is the thermal isotopic sensitivity factor (i.e., per mil fractionation per ?C) so much larger in molten basalt than in gases? Hopefully, experimental results of the sort presented here will be sufficiently interesting to stimulate molecular modeling that will begin to answer some of these questions.
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting