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Single whole rock K—Ar isochrons

Published online by Cambridge University Press:  01 May 2009

F. J. Fitch ,
J. A. Miller  and
P. J. Hooker
F. J. Fitch
Affiliation:
Department of Geology, Birkbeck College, London, W1P 1PA
J. A. Miller
Affiliation:
Department of Geodesy & Geophysics, University of Cambridge, Cambridge, CB3 0EZ
P. J. Hooker
Affiliation:
Department of Geodesy & Geophysics, University of Cambridge, Cambridge, CB3 0EZ
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Summary

Conventional K—Ar age determinations are frequently discrepant. A major factor is the failure of rocks to conform to the basic simplifying assumptions made in the total fusion method. Many rocks incorporate initial argon with an isotopic composition different from the present atmosphere and others have not remained closed systems.By careful sampling and the application of isochron geochronometry more reliable and geologically acceptable K-Ar ages can be obtained. K-Ar step heating isochrons from Kenyan Miocene volcanic sanidines are used to illustrate the accumulating evidence for the presence in ancient rocks of initial argon with an isotope ratio unlike that of the modern atmosphere. Single whole rock total fusion K-Ar isochrons from the Olduvai basalt, Tanzania and the Upper basalts of Antrim are presented to illustrate a new version of the K-Ar isochron technique. Special circumstance multiple rock K-Ar isochrons from the Belfast section of the Antrim basalts and from an ignimbrite-ash flow unit in Bed I, Olduvai Gorge, are used in a further discussion of the general applicability and usefulness of K-Ar isochrons.

Type
Articles
Information
Geological Magazine , Volume 113 , Issue 1 , January 1976 , pp. 1 - 10
Copyright
Copyright © Cambridge University Press 1976

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