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Rb-Sr and Sm-Nd isotope geochemistry and chronology of cherts from the Onverwacht Group (3.5 AE), South Africa

Weis, D. and Wasserburg, G. J. (1987) Rb-Sr and Sm-Nd isotope geochemistry and chronology of cherts from the Onverwacht Group (3.5 AE), South Africa. Geochimica et Cosmochimica Acta, 51 (4). pp. 973-984. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20131029-151401863

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Abstract

We report Rb-Sr and Sm-Nd isotopic analyses of chert samples from the Geluk Subgroup (previously called the mafic-to-felsic unit (MFU)) of the Onverwacht Group in South Africa. These Archean cherts have the same characteristic Rb and Sr concentrations as Phanerozoic cherts with high Rb/Sr ratios. The dense, massive, essentially low-carbonate cherts from the Onverwacht Group show a very large range of ^(87)Rb/^(86)Sr ratios (from 0.859 to 7.99). One sample of chert leached in acid yielded ^(87)Rb/^(86)Sr = 7.04. The data define a very good correlation line in the ^(87)Rb-^(87)Sr evolution diagram which corresponds to an age of 2119 ± 44 m.y. with an initial ^(87)Rb/^(86)Sr ratio of 0.72246 ± 16. Exchange of Sr in the cherts with the associated carbonate appears very limited. The correlation line is interpreted as reflecting the age of rehomogenization of the Sr in the protolith and the recrystallization of these cherts due to circulating hydrothermal fluids during regional metamorphism about 1.4 AE after deposition of the Onverwacht Group. It may also correspond to the closure of the Rb-Sr system in the cherts when tectonic activity stopped in the region (~2200 m.y.). The initial ^(87)Rb/^(86)Sr ratio obtained for the cherts requires the protolith which provided the Sr to have Rb/Sr ~ 1. The Sm-Nd systematics do not provide any strict chronologic information but appear to reflect an ancient source age ~3.5 AE. The ϵ_(Nd) at 3.54 AE for the cherts are grouped around zero, i.e. close to the initial value observed for volcanics of the same age, and provide no evidence for the presence of abundant crustal material older than 3.5 AE.


Item Type:Article
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http://dx.doi.org/10.1016/0016-7037(87)90109-8DOIArticle
http://www.sciencedirect.com/science/article/pii/0016703787901098PublisherArticle
Additional Information:© 1987 Pergamon Journals Ltd. Received May 26, 1986; accepted in revised form January 19, 1987. This paper is dedicated to the late H. Leonard Allsopp of the Bernard Price Institute of Geophysical Research, University of the Witwatersrand, Johannesburg. South Africa. J. W. Schopf and S. Epstein are thanked for providing and discussing the samples. Drs. Y. Kolodny and S. L. Goldstein have made constructive reviews which have been very helpful. Maarten De Wit was kind enough to volunteer his expertise and aided us considerably, although the resultant manuscript will undoubtedly not achieve the levels he hoped for. The senior author thanks D. A. Papanastassiou for helping her learn some of the arts of the Lunatics I and III. H. H. Ngo and P. Knust-Graichen are thanked for technical assistance in the lab and for help with the mass spectrometry. J. Blum translated the "frenglish" into something more readable. C. Price typed the numerous drafts of this manuscript. The National Aeronautics and Space Administration Grant NAG 9-43 supported the functioning laboratory in which this work was carried out. Some support came from NSF Grant EAR-8312882. Division Contribution No. 4326 (527). Editorial handling: J. D. Macdougall
Funders:
Funding AgencyGrant Number
NASANAG 9-43
NSFEAR-8312882
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Caltech Division of Geological and Planetary Sciences4326
Lunatic Asylum Lab527
Record Number:CaltechAUTHORS:20131029-151401863
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20131029-151401863
Official Citation:D Weis, G.J Wasserburg, Rb-Sr and Sm-Nd isotope geochemistry and chronology of cherts from the Onverwacht Group (3.5 AE), South Africa, Geochimica et Cosmochimica Acta, Volume 51, Issue 4, April 1987, Pages 973-984, ISSN 0016-7037, http://dx.doi.org/10.1016/0016-7037(87)90109-8. (http://www.sciencedirect.com/science/article/pii/0016703787901098)
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ID Code:42118
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:29 Oct 2013 22:37
Last Modified:29 Oct 2013 22:37

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