Abstract

Electron microprobe analyses are presented of chevkinite-group minerals occurring as microphenocrysts 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 ⇌ Ce. The overall charge-balancing substitution seems to be (McDowell, 1979):

 
\[Ca_{\mathit{A}}^{2+}\ +\ Ti_{\mathit{C}}^{4+}\ {\rightleftharpoons}\ \mathit{REE}_{\mathit{A}}^{3+}\ +\ \mathit{M}_{\mathit{C}}^{3+,2+}\]

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.

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