Geochemistry of late Archaean stromatolites from Zimbabwe: evidence for microbial life in restricted epicontinental seas

Kamber, B. S., Bolhar, R. and Webb, G. E. (2004) Geochemistry of late Archaean stromatolites from Zimbabwe: evidence for microbial life in restricted epicontinental seas. Precambrian Research, 132 4: 379-399. doi:10.1016/j.precamres.2004.03.006

Author Kamber, B. S.
Bolhar, R.
Webb, G. E.
Title Geochemistry of late Archaean stromatolites from Zimbabwe: evidence for microbial life in restricted epicontinental seas
Journal name Precambrian Research   Check publisher's open access policy
ISSN 0301-9268
Publication date 2004-07
Sub-type Article (original research)
DOI 10.1016/j.precamres.2004.03.006
Volume 132
Issue 4
Start page 379
End page 399
Total pages 21
Place of publication Netherlands
Publisher Elsevier Science BV
Collection year 2004
Language eng
Subject C1
260304 Inorganic Geochemistry not elsewhere classified
0499 Other Earth Sciences
Abstract We report comprehensive trace element and Sr-isotope data for microbial carbonates from the Archaean Mushandike limestone, Masvingo Greenstone Belt, Zimbabwe. The stromatolites have very coherent REE + Y patterns and share the essential shale-normalised characteristics of other Archaean marine precipitates (positive La and Gd anomalies, absence of a negative Cc anomaly and a strongly superchondritic Y/Ho ratio). Mixing models constrain the maximum amount of shale contamination to 0.25-1% and calculated detritus-free carbonate REE + Y systematics require precipitation from seawater. In terms of light-REE over heavy-REE depletion, however, the studied samples are very different from all other known Archaean marine precipitates. In shale-normalised plots, the Mushandike samples yield a negative slope. A very restricted, regional input source of the dissolved load is indicated because normalisation with locally occurring tonalite gneiss REE + Y data yields a pattern closely resembling typical shale-normalised Archaean marine chemical sediments. The disappearance of a negative Eu anomaly when patterns are normalised with local tonalite gneiss strengthens this interpretation. Sr-isotope ratios are strongly correlated with trace element contents and ratios, which explains the modest scatter in Sr-isotope ratios as representing (minor) clastic contamination. Importantly, even the least contaminated samples have very radiogenic initial Sr-87/Sr-86 ratios (0.7184) implying Sr input from an ancient high Rb/Sr source, such as the early Archaean gneisses of south-central Zimbabwe. A local ancient (3.5-3.8 Ga) source is also indicated by previously published Pb-isotope datasets for the Mushandike stromatolites. This is entirely compatible with the occurrence of 3.7-3.8 Ga zircons in quartzites and metapelites from comparably old greenstone belts within less than 150 km of the studied locality. Comparison of the Pb-isotope ratios of the Mushandike stromatolites with 2.7 and 2.6 Ga old stromatolites from the neighbouring, Belingwe Greenstone Belt demonstrates differences in initial isotope composition that relate to the extent of exchange with the open ocean. The development of numerous basins on old continental crust, with water masses variably restricted from the open ocean. suggests a lack of strong vertical topography on this late Archaean craton. (C) 2004 Elsevier B.V. All rights reserved.
Keyword Geosciences, Multidisciplinary
Microbial Carbonate
Ultra Trace Elements
Radiogenic Isotopes
Restricted Basin
Early Archaean Crust
Belingwe Greenstone-belt
Northern Marginal Zone
Banded Iron-formations
Limpopo Belt
Q-Index Code C1

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Earth Sciences Publications
2005 Higher Education Research Data Collection
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Citation counts: TR Web of Science Citation Count  Cited 67 times in Thomson Reuters Web of Science Article | Citations
Scopus Citation Count Cited 70 times in Scopus Article | Citations
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Created: Wed, 15 Aug 2007, 04:44:11 EST