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Distribution of germanium between phenocrysts and melt in peralkaline rhyolites from the Kenya Rift Valley

¹ Faculty of Geology, University of Warsaw, Al. wirki i Wigury 93, 02-089 Warsaw, Poland
² Department of Earth Sciences, CESPAR, Open University, Milton Keynes MK7 6AA, UK

^* E-mail: r.macdonald{at}lancaster.ac.uk

Germanium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glass matrices from a metaluminous trachyte and four peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley, Africa. Abundances (in ppm) are: sanidine 0.45–0.61; fayalite 4.8–11.7; hedenbergite 5.1–9.0; titanomagnetite 2.7; ilmenite 0.48; amphibole 8.3–8.9; biotite 7.0; chevkinite-(Ce) 309; trachyte glass 3.0; rhyolitic glasses 2.3–3.9. These values are generally greater than those recorded for silicic rocks in the literature, whilst the chevkinite-(Ce) value is the largest yet found in a magmatic mineral. Apparent partition coefficients range from 0.15–0.26 in sanidine to 124 in chevkinite-(Ce). Those for fayalite and hedenbergite increase with whole-rock peralkalinity and Fe content. The possibility of a role for accessory phases in influencing Ge distribution in rock-forming minerals is also raised.

KEYWORDS: peralkaline rhyolites, apparent partition coefficients, sanidine, fayalite, hedenbergite, FeTi-oxides, amphibole, biotite, chevkinite-(Ce), LA-ICP-MS