Crustal origin for peralkaline rhyolites from Kenya; evidence from U-series disequilibria and Th-isotopes

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doi: 10.1093/petrology/38.2.277
Authors:Black, S.; Macdonald, R.; Kelly, M. R.
Author Affiliations:Primary:
Lancaster University, Environmental Science Division, Lancaster, United Kingdom
Volume Title:Journal of Petrology
Source:Journal of Petrology, 38(2), p.277-297. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1997
Note:In English. 45 refs.; illus., incl. 5 tables, sketch maps
Summary:The Olkaria (or Naivasha) recent peralkaline rhyolites in the Kenya Rift Valley have U-Th disequilibria with variable Th/U ratios and 79% of the rhyolites showing U excess, which is related to major and trace element variations. Internal isochrons from the Broad Acres and Gorge Farm centres give U-Th ages from 14 000 to 50 000 yr., the latter being close to the date of formation of the rhyolites. The youngest centre has 226Ra/230Th >><[dollar]I> 1 indicating Ra-Th fractionation <<<[dollar]I> 8000 yr ago. Closed system fractionation cannot explain the U-series disequilibria. Correlation between 238U/230Th and pre-eruptive F (but not H2O) suggests rhyolite formation by halogen-fluxed melting of the crust. Major and trace element analyses of rhyolites given. [B.E.L.]
Subjects:Absolute age; Cenozoic; Continental crust; Crust; Dates; Geochronology; Igneous rocks; Peralkalic composition; Pleistocene; Quaternary; Rhyolites; Th/U; Uranium disequilibrium; Volcanic rocks; Africa; East Africa; Kenya; Kenya Rift valley; Olkaria Complex
Coordinates:S010000 S010000 E0360500 E0360500
Abstract Numbers:97M/2730
Record ID:1997034520
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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