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An Assessment of ^(40)Ar-^(39)Ar Dating of Incompletely Degassed Xenoliths

Gillespie, A. R. and Huneke, J. C. and Wasserburg, G. J. (1982) An Assessment of ^(40)Ar-^(39)Ar Dating of Incompletely Degassed Xenoliths. Journal of Geophysical Research B, 87 (B11). pp. 9247-9257. ISSN 0148-0227. doi:10.1029/JB087iB11p09247.

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The possibility of measuring the age of eruption of Pleistocene lavas by ^(40)Ar-^(39)Ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems. Upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic ^(40)Ar which has accumulated in them since their original crystallization. The residual ^(40)Ar will increase the apparent K/Ar age of the xenolith. However, if a xenolith is of Cretaceous age or younger, then a plateau in its ^(40)Ar-^(39)Ar age spectrum giving the age of eruption is expected to extend over 25–50% of the total 39Ar released if degassing of the xenolith in the magma exceeded 90% and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies. If diffusion was from a bimodal population of spheres, then the radii must differ by a factor of 10 or more (or the diffusion coefficients by a factor of 100 or more); or if the spheres were equal in size (and in diffusion coefficients), then the activation energies must differ by a factor of at least 1.5. That such requirements may be realized in real xenoliths containing K-feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early Pleistocene basalt flow. The experimental results appear to establish that old xenoliths may contain Ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption.

Item Type:Article
Related URLs:
URLURL TypeDescription
Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1982 American Geophysical Union. Received April 14, 1982; revised July 12, 1982; accepted July 28, 1982. This research was supported by NSF grants EAR 78-01787 and EAR 79-19997. F. Begemann improved the presentation through his detailed criticisms and suggestions. The presentation also benefitted from the reviews of G. Turner and two anonymous reviewers. Contribution 3649 (399) of the Division of Geological and Planetary Sciences, California Institute of Technology.
Funding AgencyGrant Number
NSFEAR 78-01787
NSFEAR 79-19997
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences3649
Lunatic Asylum Lab399
Issue or Number:B11
Record Number:CaltechAUTHORS:20140115-130828974
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Official Citation:Gillespie, A. R., J. C. Huneke, and G. J. Wasserburg (1982), An assessment of 40Ar-39Ar dating of incompletely degassed xenoliths, J. Geophys. Res., 87(B11), 9247–9257, doi:10.1029/JB087iB11p09247.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43387
Deposited By: Ruth Sustaita
Deposited On:15 Jan 2014 21:26
Last Modified:10 Nov 2021 16:36

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