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Age of the eastern Kaapvaal mantle; Re-Os isotope data for peridotite xenoliths from the Monastery kimberlite

Richard W. Carlson and Rory O. Moore
Age of the eastern Kaapvaal mantle; Re-Os isotope data for peridotite xenoliths from the Monastery kimberlite (in Kaapvaal Craton, Maarten J. de Wit (editor), Stephen H. Richardson (editor) and Lewis D. Ashwal (editor))
South African Journal of Geology (June 2004) 107 (1-2): 81-90


New whole rock chemical and Re-Os isotopic analyses of peridotite xenoliths from the Monastery kimberlite show the majority of the samples to have the low Fe, Al and Ca, and high MgO contents typical of magmaphile-element depleted cratonic lithospheric mantle. Two out of nine samples have elevated Al and lower Mg contents. Re-Os results show these two samples to be mid-Proterozoic in age indicating that additions of compositionally distinct material to this mantle section took place well after its initial assembly in the Archean. The remainder of the samples have the low Re/Os and (super 187) Os/ (super 188) Os characteristic of old cratonic peridotites, overlapping extensively with most other peridotite xenoliths contained in kimberlites erupted through the Kaapvaal craton. Excluding the two Proterozoic samples and two samples with anomalously old Re-Os model ages, the Monastery results give a mean Re-depletion model age (T (sub RD) ) of 2.76+ or -0.41 Ga and mantle model age (T (sub MA) ) of 3.25+ or -0.28 Ga. The oldest T (sub RD) observed at Monastery is 3.02 Ga. Compared to two well-studied localities from west of the Colesberg lineament that may mark a 2.9 Ga suture between the western and eastern blocks of the Kaapvaal craton, the Monastery mean T (sub MA) is older, but within uncertainty of, the mean T (sub MA) observed for Kimberley (2.93 Ga) and Newlands (3.18 Ga) peridotites. Similarly, the maximum T (sub RD) obtained at Kimberley (2.95 Ga) and Newlands (3.22 Ga) span the maximum T (sub RD) observed at Monastery. These results show conclusively that the most significant geochemical differentiation event affecting the Kaapvaal mantle occurred in the Archean. Compared to the results for peridotite xenoliths in kimberlites that penetrate the western Kaapvaal, the older mean T (sub MA) observed for Monastery peridotites is suggestive of older mantle beneath the older crust of the eastern Kaapvaal. However, the combination of the scatter observed in the T (sub MA) ages for samples from each locality and the uncertainty in the interpretation of Re-Os mantle model ages caused by the multistage, e.g. metasomatic, history of the samples does not allow conclusive resolution of this possible age difference.

ISSN: 1012-0750
EISSN: 1996-8590
Coden: TGSAA3
Serial Title: South African Journal of Geology
Serial Volume: 107
Serial Issue: 1-2
Title: Age of the eastern Kaapvaal mantle; Re-Os isotope data for peridotite xenoliths from the Monastery kimberlite
Title: Kaapvaal Craton
Author(s): Carlson, Richard W.Moore, Rory O.
Author(s): de Wit, Maarten J.editor
Author(s): Richardson, Stephen H.editor
Author(s): Ashwal, Lewis D.editor
Affiliation: Carnegie Institute of Washington, Department of Terrestrial Magnetism, Washington, DC, United States
Affiliation: University of Cape Town, Department of Geosciences, Rondebosch, South Africa
Pages: 81-90
Published: 200406
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 46
Accession Number: 2005-073190
Categories: Geochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
S35°00'00" - S15°30'00", E12°00'00" - E33°40'00"
Secondary Affiliation: University of the Witwatersrand, ZAF, South AfricaMineral Service Canada, CAN, Canada
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200524
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