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Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite

Rajan, R. S. and Huneke, J. C. and Smith, S. P. and Wasserburg, G. J. (1979) Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite. Geochimica et Cosmochimica Acta, 43 (7). pp. 957-971. ISSN 0016-7037. doi:10.1016/0016-7037(79)90086-3.

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^(40)Ar-^(39)Ar age spectra have been measured on plagioclase separates from three basaltic clasts (A, B, C), a pyroxene separate from clast B, and a total sample of a fourth basaltic clast (ρ) from the Kapoeta achondritic meteorite. The Ar data show that three of the four clasts crystallized ≥4.5 AE ago. Xe measurements indicate all four formed within a 0.1 AE period (Huneke, et al., 1977, Lunar Science VIII, pp. 484–486). Three clasts have suffered various degrees of ^(40)Ar loss since that time. The times of ^(40)Ar degassing do not cluster about a single time analogous to the lunar cataclysm. The survival of ≥4.5 AE ages contrasts with the general absence of ages ≥4.0 AE on the moon. The Ar retention age of clast B of ≥4.57 AE is atypically older than the Rb-Sr age of 3.6 AE (Papanastassiou et al., 1974, Lunar Science V, p. 583). The 3.5 AE Ar age of clast A is distinctly younger than the Rb-Sr age of 3.9 AE (Papanastassiou et al., 1974). The K-Ar and Rb-Sr systems are clearly not equivalent dating techniques in these instances. The combined evidence of Ar, Xe and Rb-Sr studies suggests the period of volcanism on the Kapoeta parent planet was restricted to the first ~0.2 AE of solar system history. The subsequent thermal metamorphic histories recorded in each of the four clasts after formation are distinctly different. The clasts must have existed as independent fragments at least as recently as 3.5 AE ago. The cosmic ray exposure ages of all the four clasts are similar (~ 3 Myr), and are not significantly different from that of the bulk meteorite. The clasts spent essentially all of the time prior to the formation of Kapoeta at depths greater than a few meters.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1979 Pergamon Press Ltd. Received 23 May 1978; accepted in revised form 20 February 1979. We wish to thank Dr. A. H. ISHAG and Dr. A. WIDATALLA of the geological survey of the Republic of Sudan for their kind and generous cooperation in allowing access to the proper samples without which this work would not have been possible. We are especially grateful to Prof. M. A. ROWE of the Texas A & M University for generously giving us the remainings of the important sample of clast p. We have benefited from discussions with Drs. D. A. PAPANASTASSIOU and R. F. DYMEK. We would like to thank J. G. BROWN for assistance in mineral separations. This research was supported by NASA NGL 05-002-188 and NSF PHY 76-02724.
Funding AgencyGrant Number
NASANGL 05-002-188
NSFPHY 76-02724
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Caltech Division of Geological and Planetary Sciences2997
Lunatic Asylum Lab261
Issue or Number:7
Record Number:CaltechAUTHORS:20131023-094108190
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Official Citation:R.S. Rajan, J.C. Huneke, S.P. Smith, G.J. Wasserburg, Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite, Geochimica et Cosmochimica Acta, Volume 43, Issue 7, July 1979, Pages 957-971, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42020
Deposited On:23 Oct 2013 17:18
Last Modified:10 Nov 2021 04:37

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