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Fractionation of peralkaline silicic magmas: the Greater Olkaria Volcanic Complex, Kenya Rift Valley

Marshall, A. S.; Macdonald, R.; Rogers, N. W.; Fitton, J. G.; Tindle, A. G.; Nejbert, K. and Hinton, R. W. (2009). Fractionation of peralkaline silicic magmas: the Greater Olkaria Volcanic Complex, Kenya Rift Valley. Journal of Petrology, 50(2) pp. 323–359.

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DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1093/petrology/egp001
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Abstract

The Greater Olkaria Volcanic Complex is a young (20 ka) multi-centred system in the central Kenya Rift Valley, mainly represented at outcrop by peralkaline rhyolites. The rhyolites show significant compositional variation; peralkalinity [mol. (Na2O + K2O)/Al2O3] varies from 1·01 to 1·55, Zr contents from 442 to 3640 ppm and Rb contents from 262 to 1056 ppm. More peralkaline rhyolites were generated along multiple, but generally closely similar, liquid lines of descent by 75% fractional crystallization of alkali feldspar–quartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. Crystal fractionation took place at more than one upper crustal level. The rhyolite magmas were erupted from 13 centres, each having an eruptive history and geochemical evolution broadly similar to, but distinct from, those of the other centres. For most of the life span of the Olkaria system, almost the whole spectrum of peralkaline rhyolite compositions was erupted from vents in the complex at any one time. Apparent partition coefficients are presented for 34 trace elements in sanidine, fayalite, ferrohedenbergite, amphibole, biotite, ilmenite and chevkinite-(Ce). With the exception of certain values for Ba, Rb and Sr in sanidine, most coefficients vary systematically with whole-rock composition.

Item Type: Journal Article
Copyright Holders: 2009 Oxford University Press
ISSN: 0022-3530
Keywords: Kenya Rift; peralkaline rhyolites; mineral/glass partition coefficients; plumbing system; eruptive centres
Academic Unit/Department: Science > Environment, Earth and Ecosystems
Science
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 14964
Depositing User: Elizabeth Lomas
Date Deposited: 10 Mar 2009 21:04
Last Modified: 24 Oct 2012 03:53
URI: http://oro.open.ac.uk/id/eprint/14964
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