Journal of Petrology Advance Access published online on January 22, 2009
Journal of Petrology, doi:10.1093/petrology/egp001
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Fractionation of Peralkaline Silicic Magmas: the Greater Olkaria Volcanic Complex, Kenya Rift Valley
1Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK
2Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, 02-089 Warsaw, Poland
3Department of Earth Sciences, Cespar, Open University, Milton Keynes MK7 6AA, UK
4Grant Institute of Geosciences, University of Edinburgh, Edinburgh EH9 3JW, UK
Received May 21, 2008; Revised typescript accepted December 31, 2008
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Abstract |
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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.
KEY WORDS: Kenya Rift; peralkaline rhyolites; mineral/glass partition coefficients; plumbing system; eruptive centres
*Corresponding author. Present address: Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK. E-mail: r.macdonald{at}lancaster.ac.uk
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