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REE partitioning between apatite and melt in a peralkaline volcanic suite, Kenya Rift Valley

R. Macdonald^1,2,*, B. Bagiski¹, H.E. Belkin³, P. Dzieranowski¹ and L. Jeak¹

¹ IGMiP Faculty of Geology, University of Warsaw, Al. wirki i Wigury 93, 02-089 Warsaw, Poland
² Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK
³ U.S. Geological Survey, 956 National Center, Reston, VA 20192, USA

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

Electron microprobe analyses are presented for fluorapatite phenocrysts from a benmoreite-peralkaline rhyolite volcanic suite from the Kenya Rift Valley. The rocks have previously been well characterized petrographically and their crystallization conditions are reasonably well known. The REE contents in the M site increase towards the rhyolites, with a maximum britholite component of 35 mol.%. Chondrite-normalized REE patterns are rather flat between La and Sm and then decrease towards Yb. Sodium and Fe occupy up to 1% and 4%, respectively, of the M site. The major coupled substitution is REE³⁺ + Si⁴⁺ Ca²⁺ + P⁵⁺. The substitution REE³⁺ + Na⁺ 2Ca²⁺ has been of minor importance. The relatively large Fe contents were perhaps facilitated by the low f_O2 conditions of crystallization. Zoning is ubiquitous and resulted from both fractional crystallization and magma mixing. Apatites in some rhyolites are relatively Y-depleted, perhaps reflecting crystallization from melts which had precipitated zircon. Mineral/glass (melt) ratios for two rhyolites are unusually high, with maxima at Sm (762, 1123).

KEYWORDS: fluorapatite, trachytes, peralkaline rhyolites, magma mixing, britholite

This article has been cited by other articles:

				R. Macdonald and B. Baginski The central Kenya peralkaline province: a unique assemblage of magmatic systems Mineralogical Magazine, June 15, 2009; 73(1): 1 - 16. [Abstract] [Full Text] [PDF]