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Intercrystalline stable isotope diffusion: a fast grain boundary model

Eiler, John M. and Baumgartner, Lukas P. and Valley, John W. (1992) Intercrystalline stable isotope diffusion: a fast grain boundary model. Contributions to Mineralogy and Petrology, 112 (4). pp. 543-557. ISSN 0010-7999. http://resolver.caltech.edu/CaltechAUTHORS:20121024-165137834

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Abstract

We formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals, and the intragranular distribution of stable isotopes in each mineral, as functions of mineral grain sizes and shapes, diffusivities, modes, equilibrium isotopic fractionations, and the cooling rate of a rock. One of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes. This Fast Grain Boundary (FGB) model describes interdiffusion between any number of mineral grains, assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled. The model can be used for a rock containing any number of minerals, and number of grain sizes of each mineral, several grain shapes, and any thermal history or domain size desired. Previous models describing stable isotope interdiffusion upon cooling have been based on Dodson's equation or an equivalent numerical analogue. The closure temperature of Dodson is the average, bulk temperature recorded between a mineral and an infinite reservoir. By using Dodson's equation, these models have treated the closure temperature as an innate characteristic of a given mineral, independent of the amounts and diffusion rates of other minerals. Such models do not accurately describe the mass balance of many stable isotope interdiffusion problems. Existing models for cation interdiffusion could be applied to stable isotopes with some modifications, but only describe exchange between two minerals under specific conditions. The results of FGB calculations differ considerably from the predictions of Dodson's equation in many rock types of interest. Actual calculations using the FGB model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate. Closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient, and may differ from that predicted by Dodson's equation by hundreds of degrees C. Either or both of two minerals may preserve detectable zonation, which may in some instances be larger in the faster diffusing mineral. Rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone. The results of FGB diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations, concordant or discordant, recorded within any rock in which diffusion could have acted over observable scales. The predictions of this model are applicable to thermometry, evaluation of open or closed system retrogression, and determination of cooling rates or diffusion coefficients.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1007/BF00310783DOIUNSPECIFIED
http://www.springerlink.com/content/w8085563140q7516/PublisherUNSPECIFIED
Additional Information:© 1992 Springer-Verlag. Received February 10, 1992; Accepted May 11, 1992. This paper was substantially improved by reviews from J.M. Ferry, C. Steefel, and J.R. Farver. Thanks are due to H.F. Wang, M. Kohn, D. Elsenheimer and J. Bahr for helpful discussions and reviews. We wish to acknowledge the financial support of an NSF Graduate Fellowship (JME), and NSF grants EAR-8914477 (JWV) and EAR-9106313 (LPB). Editorial responsibility: J. Ferry.
Funders:
Funding AgencyGrant Number
NSF Graduate Research FellowshipUNSPECIFIED
NSFEAR-8914477
NSFEAR-9106313
Record Number:CaltechAUTHORS:20121024-165137834
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121024-165137834
Official Citation:Eiler, J. M., L. P. Baumgartner, et al. (1992). "Intercrystalline stable isotope diffusion: a fast grain boundary model." Contributions to Mineralogy and Petrology 112(4): 543-557.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35083
Collection:CaltechAUTHORS
Deposited By: Aucoeur Ngo
Deposited On:25 Oct 2012 14:47
Last Modified:25 Oct 2012 14:47

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