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Book Chapter

Trace-Element Composition of Cherts from Alkaline Lakes in the East African Rift: A Probe for Ancient Counterparts

By
Robert Kerrich
Robert Kerrich
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
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Robin W. Renaut
Robin W. Renaut
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
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Thomas Bonli
Thomas Bonli
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
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Published:
January 01, 2002

Abstract

Magadiite and cherts from the Magadi basin in southern Kenya and four other localities in the East African Rift all share distinctive compositional systematics: (1) negatively fractionated REE patterns; (2) high absolute concentrations of U, Nb, and Zr (up to 1,500 ppm versus a crustal average of 190 ppm); (3) normalized enrichments of U, Nb, and Zr relative to REE; (4) extreme fractionations of U-Th, Nb-Ta, and Zr-Hf; (5) positive Ce but negative Eu anomalies; and (6) normalized peaks at Mo, Ag, and Sb. Alkaline lake cherts composed of secondary quartz retain the trace-element patterns of the precursor, albeit at lower absolute element contents. Sublacustrine sinters at Lake Bogoria share some of the compositional features of the cherts from Magadi but lack the Ce and Eu anomalies and U-Th, Nb-Ta, and Zr-Hf fractionations. Cherts from ocean-ridge, pelagic, and continental-shelf settings are characterized by progressively larger Zr contents (≤ 170 ppm), but all are distinctly lower than in the alkaline-lake cherts, where micron-scale Zr-rich phases were identified as possible authigenic zircons. Both marine and alkaline-lake cherts share positive Ce anomalies. These result from scavenging of Ce by Fe-Mn oxyhydroxides in seawater, but from the solubility of Ce (IV) in oxidized alkaline brines for the lacustrine cherts. Europium anomalies range from small negative to positive in marine cherts, whereas alkaline-lake cherts feature large negative anomalies. These compositional systematics, which reflect the aqueous environment in which the chert precursor formed, confer a tool for interpreting the paleoenvironment of cherts in the geological record.

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SEPM Special Publication

Sedimentation in Continental Rifts

Robin W. Renaut
Robin W. Renaut
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
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Gail M. Ashley
Gail M. Ashley
Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08854-8066, U.S.A.
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SEPM Society for Sedimentary Geology
Volume
73
ISBN electronic:
9781565761957
Publication date:
January 01, 2002

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