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

Published online by Cambridge University Press:  05 July 2018

R. MacDonald ,
B. Baginíski ,
H. E. Belkin ,
P. Dzieržanowski  and
L. Jeżak
R. MacDonald*
Affiliation:
IGMiP Faculty of Geology, University of Warsaw, Al. , Żwirki i Wigury 93, 02-089 Warsaw , Poland Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK
B. Baginíski
Affiliation:
IGMiP Faculty of Geology, University of Warsaw, Al. , Żwirki i Wigury 93, 02-089 Warsaw , Poland
H. E. Belkin
Affiliation:
U.S. Geological Survey, 956 National Center, Reston, VA 20192, USA
P. Dzieržanowski
Affiliation:
IGMiP Faculty of Geology, University of Warsaw, Al. , Żwirki i Wigury 93, 02-089 Warsaw , Poland
L. Jeżak
Affiliation:
IGMiP Faculty of Geology, University of Warsaw, Al. , Żwirki i Wigury 93, 02-089 Warsaw , Poland
*
*E-mail: r.macdonald@lancaster.ac.uk
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Abstract

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 fO2conditions 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

fluorapatitetrachytesperalkaline rhyolitesmagma mixingbritholite
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 6 , December 2008 , pp. 1147 - 1161
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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