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

R. Macdonald·B. Baginski·H. E. Belkin·P. Dzierzanowski·L. Jezak
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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 REE 3+ + Si 4+ ??? Ca 2+ + P 5+ . The substitution REE 3+ + Na + ??? 2Ca 2+ has been of minor importance. The relatively large Fe contents were perhaps facilitated by the low fo 2 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). ?? 2008 The Mineralogical Society.

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R. Macdonald, B. Baginski, H. E. Belkin, P. Dzierzanowski, L. Jezak. 2018-07-05. REE partitioning between apatite and melt in a peralkaline volcanic suite, Kenya Rift Valley. https://doi.org/10.1180/minmag.2008.072.6.1147

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