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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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References

Brereton, N. R. 1972. A reappraisal of the 40Ar/39Ar stepwise degassing technique. Geophys. J. R. astr. Soc. 27, 449–78.CrossRefGoogle Scholar
Bryhni, I., Fitch, F. J. & Miller, J. A. 1971. 40Ar/39Ar dates from recycled PreCambrian rocks in the gneiss region of the Norwegian Caledonides. Norsk geol. Tiddskr. 51, 391–406.Google Scholar
Cherdyntsev, V. V. & Shitov, Yu. V. 1967. Excess 36Ar in volcanic and post-volcanic gases. Geokhimiya 5, 618.Google Scholar
Curtis, G. H. & Hay, R. L. 1972. Further geological studies and potassium-argon dating at Olduvai Gorge and Ngorongora Crater. In Bishop, W. W. & Miller, J. A. (Eds), Calibration of Hominoid Evolution, 289–301. Scottish Academic Press, Edinburgh.Google Scholar
Dalrymple, G. B. 1969. 40A/36A analyses of historic lava flows. Earth Planet Sci Lett. 6, 47–55.CrossRefGoogle Scholar
Evans, A. LI., Fitch, F. J. & Miller, J. A. 1973. Potassium—argon age determinations on some British Tertiary igneous rocks. J. geol. Soc. Lond. 129, 419–43.CrossRefGoogle Scholar
Evernden, J. F. & Curtis, G. H. 1965. The potassium—argon dating of late Cenozoic rocks in East Africa and Italy. Current Anth. 6, 343.CrossRefGoogle Scholar
Fitch, F. J. 1972. Selection of suitable material for dating and the assessment of geological error in potassium-argon age determination. In Bishop, W. W. and Miller, J. A. (Eds), Calibration of Hominoid Evolution, 77–91. Scottish Academic Press, Edinburgh.Google Scholar
Fitch, F. J. & Miller, J. A. 1971. Atmospheric argon correction in the K—Ar dating of young volcanic rocks. J. geol. Soc. Lond. 127, 277–80.CrossRefGoogle Scholar
Fitch, F. J. & Miller, J. A. 1973. Dating granites by the potassium—argon method. In Lister, L. A. (Ed), Symposium on Granites, Gneisses and Related Rocks Spec. Publ. No. 3, geol. Soc. S Africa, 219–55.Google Scholar
Fitch, F. J., Miller, J. A. & Mitchell, J. G. 1969. A new approach to isotopic dating in orogenic belts. In Kent, P. E. et al. (Eds), Time and Place in Orogeny, 157–96. London. Geological Society.Google Scholar
Fitch, F. J., Watkins, R. T. & Miller, J. A. 1975. Age of a new carbonatite locality in northern Kenya. Nature Lond. 254, 581–3.CrossRefGoogle Scholar
Flett Brown, J., Harper, C. T. & Odom, A. L. 1974. Petrogenetic implications of argon isotopic evolution in the upper mantle. Nature Lond. 250, 130–3.CrossRefGoogle Scholar
Hayatsu, A. 1972. On the basic assumptions in K—Ar dating method. Comments on Earth Sciences, Geophysics 3, no. 3, 69–75.Google Scholar
Hayatsu, A. & Carmichael, C. M. 1970. K—Ar isochron method and initial argon ratios. Earth Planet. Sci. Lett. 8, 71–6.CrossRefGoogle Scholar
Krummenacher, D. 1970. Isotopic composition of argon in modern surface volcanic rocks. Earth Planet. Sci. Lett. 8, 109–17.CrossRefGoogle Scholar
Mellor, D. W. & Musett, A. E. 1975. Evidence for initial 36Ar in volcanic rocks, and some implications. Earth Plan. Sci. Lett. 26, 312–18.CrossRefGoogle Scholar
Miller, J. A. 1972. Dating Pliocene and Pleistocene strata using the potassium—argon and argon—40/argon—39 methods. In Bishop, W. W. & Miller, J. A. (Eds), Calibration of Hominoid Evolution, 63–76. Scottish Academic Press, Edinburgh.Google Scholar
Miller, J. A., Mitchell, J. G. & Evans, A. Ll. 1970. The argon—40/argon—39 dating method applied to basic rocks. In Runcorn, S. K. (Ed), Palaeogeophysics, 481–9. Academic Press.Google Scholar
Mussett, A. E. & Dalrymple, G. B. 1968. An investigation of the source or air argon contamination in K—Ar dating. Earth Planet. Sci. Lett. 4, 422.CrossRefGoogle Scholar
Shafiqullah, M. & Damon, P. E. 1974. Evaluation of K—Ar isochron methods. Geochim. cosmochim. Acta 38, 1341–58.CrossRefGoogle Scholar
York, D. 1966. Least-squares fitting of a straight line. Can. J. Phys. 44, 1079.CrossRefGoogle Scholar