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^(40)Ar-^(39)Ar chronology of isolated phases from Bununu and Malvern howardites

Rajan, R. S. and Huneke, J. C. and Smith, S. P. and Wasserburg, G. J. (1975) ^(40)Ar-^(39)Ar chronology of isolated phases from Bununu and Malvern howardites. Earth and Planetary Science Letters, 27 (2). pp. 181-190. ISSN 0012-821X.

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High resolution ^(40)Ar-^(39)Ar age spectra have been measured on plagioclase and glass from two howardites. Both the plagioclase and glass from the gas-rich Bununu howardite show well-defined age plateaux, yielding distinct ages of 4.42 ± 0.04 and 4.24 ± 0.05 AE, respectively. These age patterns are rather well behaved and are interpreted as representing the distinct times of formation of plagioclase from igneous processes and of glass fragments produced by impact on the meteorite body. The release pattern for the glass from the heavily shocked Malvern howardite is undulating at low and intermediate temperatures but does have a high-temperature plateau. Its age spectrum indicates little apparent diffusion loss, but rather an extensive redistribution of either ^(40)Ar during the shock event or of ^(39)Ar during the neutron irradiation or both. The total K-Ar age of Malvern glass is 3.64 ± 0.04 AE and the high-temperature plateau is 3.73 ± 0.05 AE. The age spectrum of the Malvern plagioclase has an intermediate temperature “plateau” at 3.80 AE that represents 20% of the total40Ar content and increases towards a high-temperature plateau at 4.29 ± 0.04 AE containing 26% of the total gas release. It seems likely that the event which formed the Malvern glass also reset part of the plagioclase. The distinct histories observed for the different phases of these howardites are consistent with their formation from a regolith. The present results along with similar young ages for igneous clasts from Kapoeta clearly show that the regoliths were extant on the parent bodies of howardites and that they were subjected to violent impact events at least as recently as 3.7 AE ago.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1975 Elsevier Scientific Publishing Company, Amsterdam. Received May 7, 1975; Revised version received June 13, 1975. The Bununu sample was obtained from the Smithsonian collection through the courtesy of Roy Clarke, Jr., A.F. Noonan and K. Fredriksson. The Malvern sample was obtained from South Africa through the courtesy of Prof. L.H. Ahrens. We would like to thank J.G. Brown for mineral separations and T.J. Gay for flux-wire counting. This work was supported by NSF under grant GP-28027.
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Caltech Division of Geological and Planetary Sciences2604
Issue or Number:2
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Official Citation:R.S. Rajan, J.C. Huneke, S.P. Smith, G.J. Wasserburg, 40Ar-39Ar chronology of isolated phases from Bununu and Malvern howardites, Earth and Planetary Science Letters, Volume 27, Issue 2, September 1975, Pages 181-190, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41090
Deposited By: Tony Diaz
Deposited On:20 Sep 2013 21:15
Last Modified:09 Mar 2020 13:19

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