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Chevkinite-group minerals from salic volcanic rocks of the East African Rift.

MacDonald, Raymond and Marshall, A. S. and Dawson, J. B. and Hinton, R. W. and Hill, P. G. (2002) Chevkinite-group minerals from salic volcanic rocks of the East African Rift. Mineralogical Magazine, 66 (2). pp. 287-299.

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Abstract

Electron microprobe analyses are presented of chevkinite-group minerals occurring as microphen-cryst in peralkaline rhyolites of the Greater Olkaria Volcanic Complex (Kenya) and as a groundmass phase in a peralkaline quartz trachyte lava from the Tarosero volcano (Tanzania), both in the East African Rift Valley. Their compositions conform closely to the formula: (REE , Ca, Th)4 Fe2+ (Fe2+, Al, Ti, Zr, Nb)2 Ti2 (Si4O22). Compared to published analyses of chevkinite-group minerals, the Olkaria phases are relatively enriched in Nb and the LREE; the Tarosero phase is more calcic and relatively Zr and Nb-rich. The main substitution in the A site at Olkaria is Ca &rlhar2; Ce. The overall charge-balancing substitution seems to be (McDowell, 1979): Ca2+A + Ti4+C &rlhar2; REE3+A + M3+,2+C Phenocryst/glass ratios are presented for Nb, REE , Sr, Th, U and Y in two, and Ba, Zr and Hf in one, Kenyan samples. Partition coefficients are lower in the more peralkaline rock, with the exception of Sr, which is higher. The lower values are consistent with a lower degree of polymerization of more peralkaline melts. The higher Sr value may be a function of Sr partitioning into phenocryst phases coexisting with chevkinite.

Item Type: Article
Journal or Publication Title: Mineralogical Magazine
Uncontrolled Keywords: CHEVKINITE-GROUP ; PERALKALINE RHYOLITES AND TRACHYTES ; KENYA ; TANZANIA
Subjects: G Geography. Anthropology. Recreation > GE Environmental Sciences
Departments: Faculty of Science and Technology > Lancaster Environment Centre
ID Code: 21480
Deposited By: ep_ss_importer
Deposited On: 15 Jan 2009 10:18
Refereed?: Yes
Published?: Published
Last Modified: 26 Jul 2012 15:47
Identification Number:
URI: http://eprints.lancs.ac.uk/id/eprint/21480

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