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Grain boundary partitioning of Ar and He

Baxter, Ethan F. and Farley, Kenneth A. and Asimow, Paul D. (2007) Grain boundary partitioning of Ar and He. Geochimica et Cosmochimica Acta, 71 (2). pp. 434-451. ISSN 0016-7037. doi:10.1016/j.gca.2006.09.011.

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An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, K¯^(Ar)_(ITM), with units of (Ar/m^3 of solid)/(Ar/m^2 of GB) of 6.8 x 10^3 – 2.4 x 104 m^(-1). Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient (K^(Ar)_(GB) = X^(Ar)_(crystals)/X^(Ar)_(GB) may be determined. For reasonable values of δ, K^(Ar)_(GB) is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with K¯^(He)_(ITM) between 4 x 10^3 and 4 x 10^4 cm^(-1), similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content.

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Farley, Kenneth A.0000-0002-7846-7546
Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2006 Elsevier Inc. Received 5 July 2005; accepted in revised form 12 September 2006. This study was conducted at Caltech. Manuscript preparation was conducted while EFB was at Boston University. We Jed Mosenfelder, John Beckett and Mike Baker for help with the experiments. We thank Pete Burnard for assistance with the noble gas analysis. We thank Carol Garland for TEM preparation and analysis. Steve Reese of the OSU Reactor Facility was helpful in accommodating our irradiations. We thank Lauren Cooper and Jennifer Wade for assistance in thin section observation. Ed Stolper provided the starting material and access to his labs. We thank Bruce Watson, Takeo Hiraga, and Terry Plank for valuable discussions at various stages of the project. Support from a Caltech Postdoctoral Fellowship to EFB and NSF Grants EAR-0125784 (to PDA and KAF) and EAR-0337527 (to EFB) is gratefully acknowledged. Associate Editor: Jun-ichi Matsuda Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.gca.2006.09.011.
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Official Citation:Ethan F. Baxter, Paul D. Asimow, Kenneth A. Farley, Grain boundary partitioning of Ar and He, Geochimica et Cosmochimica Acta, Volume 71, Issue 2, 15 January 2007, Pages 434-451, ISSN 0016-7037, 10.1016/j.gca.2006.09.011. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:33774
Deposited By: Aucoeur Ngo
Deposited On:31 Aug 2012 19:39
Last Modified:09 Nov 2021 21:37

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