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

Macdonald, R.; Baginski, B.; Belkin, H. E.; Dzierzanowski, P.; Jezak, L.

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 fo₂ 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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Article

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70036983

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Macdonald, R.; Baginski, B.; Belkin, H. E.; Dzierzanowski, P.; Jezak, L.

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1161

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English

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Mineralogical Magazine

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REE partitioning between apatite and melt in a peralkaline volcanic suite, Kenya Rift Valley; 2009; Article; Journal; Mineralogical Magazine; Macdonald, R.; Baginski, B.; Belkin, H. E.; Dzierzanowski, P.; Jezak, L.