Intercrystalline stable isotope diffusion; a fast grain boundary model

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Authors:Eiler, John M.; Baumgartner, Lukas P.; Valley, John W.
Author Affiliations:Primary:
University of Wisconsin-Madison, Department of Geology and Geophysics, Madison, WI, United States
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 112(4), p.543-557. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1992
Note:In English. 59 refs.; illus.
Summary:A numerical model has been formulated for stable isotope interdiffusion which predicts the T 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 modelled. The model can be used for a rock containing any number of minerals, any 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 were based on an earlier worker's equation or an equivalent numerical analogue. The closure T in this analogue is the average, bulk T recorded between a mineral and an infinite reservoir. By using this equation, these models have treated the closure T 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 inter-diffusion 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 an earlier equation in many rock types of interest. Actual calculations using the FGB model indicate that closure T and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are of functions of grain size and cooling rate. Closure T recorded by two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from prediction by 100's of degrees C. [P.Br.]
Subjects:Amphibole group; Chain silicates; Clinoamphibole; Closed systems; Cooling; Diffusion; Feldspar group; Framework silicates; Geologic thermometry; Grain boundaries; Hornblende; Isotope ratios; Isotopes; Numerical models; O-18/O-16; Open systems; Oxygen; Quartz; Silica minerals; Silicates; Stable isotopes; Temperature
Abstract Numbers:93M/2781
Record ID:1993019861
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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