HR: 10:20h
AN: T41F-01 INVITED     [PDF]
TI: Measurement of Grain Boundary Partitioning of Ar and He
AU: * Baxter, E F
EM: efb@bu.edu
AF: Department of Earth Sciences, Boston University 685 Commonwealth Ave, Boston, MA 02215 United States
AU: * Baxter, E F
EM: efb@bu.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology MC 170-25, Paasadena, CA 91125 United States
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology MC 170-25, Paasadena, CA 91125 United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology MC 170-25, Paasadena, CA 91125 United States
AB: An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the grain boundary region between crystals (GB) in synthetic polycrystalline diopside aggregates. $^{37}$Ar and $^{4}$He are introduced into solid glass samples via neutron irradiation, eliminating issues of possible atmospheric gas contamination. Samples are then crystallized in sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus. During crystallization, noble gases simultaneously diffusively equilibrate between the evolving crystal and grain boundary reservoirs. After equilibration, GB Ar and He is differentiated from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e. constant equilibrium partitioning) is reached after about 20 hours of crystallization in our experiments. Data for longer durations show the predicted systematic trend of decreasing GB Ar and He with increasing grain size. These data yield values of effective grain boundary surface partitioning, K$_{(Ar)surf}$, in units of (mol Ar/m3 of crystal)/(mol Ar/m$^{2}$ of GB) of 6.8 x 10$^{3}$ to 2.4 x 10$^{4}$ m$^{-1}$. Using a nominal GB thickness of 2-3 nm (corroborated by TEM imaging of experimental samples) a true partition coefficient (units of (mol Ar/m$^{3}$ of crystal)/(mol Ar/m$^{3}$ in GB)), K$_{(Ar)}$, may be determined: 1.4 x 10$^{-5}$ to 7.1 x 10$^{-5}$. This value is about an order of magnitude lower than Ar partitioning between diopside and glass. He partitioning data provide a less robust constraint with K$_{(He)}$between 10$^{-4}$ and 10$^{-5}$. These data suggest that grain boundaries constitute a significant, but not infinite, reservoir, and therefore bulk transport pathway, for noble gases in the lower crust and mantle.
DE: 3630 Experimental mineralogy and petrology
DE: 3670 Minor and trace element composition
DE: 3947 Surfaces and interfaces
DE: 5112 Microstructure
DE: 5139 Transport properties
SC: Tectonophysics [T]
MN: 2003 Fall Meeting