Rb-Sr and Sm-Nd isotope geochemistry and chronology of cherts from the Onverwacht Group (3. 5 AE), South Africa

Journal Article · Wed Apr 01 04:00:00 EST 1987 · Geochim. Cosmochim. Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(87)90109-8· OSTI ID:7188777

Weis, D; Wasserburg, G J

The authors report Rb-Sr and Sm-Nd isotopic analyses of chert samples from the Geluk Subgroup 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 /sup 87/Rb//sup 86/Sr ratios. One sample of chert leached in acid yielded /sup 87/Sr//sup 86/Sr = 7.04. The data define a very good correlation line in the /sup 87/Rb-/sup 87/Sr evolution diagram which corresponds to an age of 2119 +/- 44 m.y. with an initial /sup 87/Sr//sup 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 (approx. 2200 m.y.). The initial /sup 87/Sr//sup 86/Sr ratio obtained for the cherts requires the protolith which provided the Sr to have Rb/Sr approx. 1. The Sm-Nd systematics do not provide any strict chronologic information but appear to reflect an ancient source age approx. 3.5 AE. The epsilon/sub 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.

• Research Organization: California Institute of Technology, Pasadena (USA)
• OSTI ID: 7188777
• Journal Information: Geochim. Cosmochim. Acta; (United States), Journal Name: Geochim. Cosmochim. Acta; (United States) Vol. 51:4; ISSN GCACA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580100 -- Geology & Hydrology-- (-1989)
580400* -- Geochemistry-- (-1989)
AFRICA
AGE ESTIMATION
ALKALI METALS
ALKALINE EARTH METALS
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
CHERT
DIAGENESIS
ELEMENTS
FLUIDS
GEOCHEMISTRY
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GEOTHERMAL FLUIDS
HYDROTHERMAL ALTERATION
ISOTOPE RATIO
METALS
MINERALS
NEODYMIUM
ORIGIN
RARE EARTHS
RECRYSTALLIZATION
ROCKS
RUBIDIUM
SAMARIUM
SEDIMENTARY ROCKS
SILICATE MINERALS
SOUTH AFRICA
STRONTIUM