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Journal of Petrology | Volume 43 | Number 3 | Pages 557-577 | 2002
© Oxford University Press 2002
U–Th Disequilibrium and Rb–Sr Age Constraints on the Magmatic Evolution of Peralkaline Rhyolites from Kenya
1FACULTEIT DER AARDWETENSCHAPPEN, VRIJE UNIVERSITEIT AMSTERDAM, DE BOELELAAN 1085, 1081 HV AMSTERDAM, THE NETHERLANDS
2GEOCHEMISCHES INSTITUT, GEORG-AUGUST-UNIVERSITÄT, GOLDSCHMIDTSTR. 1, 37077 GÖTTINGEN, GERMANY
Mildly peralkaline rhyolites of the Olkaria Volcanic Complex, located in the Kenyan sector of the East African rift valley, have low Sr concentrations and elevated Rb/Sr ratios (Sr 1·3–2 ppm; 87Rb/86Sr = 748–1769) that potentially allow the resolution of time differences on the order of 1 ka by conventional Sr isotope determination. Because of their young eruption ages (
20 ka), a chemically independent assessment of the Sr isotope results has been obtained by U-series dating. Rb–Sr isochron ages of pristine glasses and phenocrysts from the most chemically evolved rhyolites pre-date the eruption ages and are best defined by a mineral isochron of 24 ± 1 ka. The glasses are in secular U–Th equilibrium so that no age information can be obtained. In contrast, glasses and minerals yield U–Th isochrons of 25 ± 10 ka and are probably controlled by the Th-enriched accessory phase chevkinite. We therefore ascribe the pre-eruptive age information to crystallization of the observed phenocryst phases. Inferred high magma fractionation rates of up to 2·5 x 10–3 km3/yr are comparable with those for much larger metaluminous silicic magma systems. Magma storage times (
22 ky), however, are much shorter and may best be accounted for by the specific size, longevity and thermal gradient of the silicic magma system.
KEY WORDS: silicic magmatism; isotope geochemistry; time scales; fractional crystallization; magma chambers; residence times
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