Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes |
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Authors: | Stuart Black Ray Macdonald Barbara A Barreiro Peter N Dunkley Martin Smith |
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Institution: | (1) Environmental Science Division, IEBS, Lancaster University, Lancaster, LA1 4YQ, UK, Tel.: 01524 594209; Fax: UK 01524 593985; E-mail: s.black@lancaster.ac.uk, GB;(2) NERC Isotope Geoscience Laboratory, Keyworth, Nottingham, NG12 5GG, UK, GB;(3) British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK, GB;(4) British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK, GB |
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Abstract: | U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young
basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and
are associated with small volumes of hawaiite. The mafic rocks are characterised by high (230Th/232Th) (≥1.06) with low (238U/230Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent
a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly
lower (230Th/232Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (238U/230Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity
must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar
to single crystal laser fusion 40Ar/39Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were
very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (230Th/232Th)0 ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse
magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano.
Received: 29 May 1996 / Accepted: 24 November 1997 |
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