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Mineral chemistry of a peralkaline combeite-lamprophyllite nephelinite from Oldoinyo Lengai, Tanzania

Published online by Cambridge University Press:  05 July 2018

J. B. Dawson  and
P. G. Hill
J. B. Dawson
Affiliation:
Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
P. G. Hill
Affiliation:
Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
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Abstract

A peralkaline nephelinite lava ([Na+K]/Al 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to alumina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.

Keywords

Oldoinyo LengaiTanzanianephelinitenatrocarbonatitecombeiteBa lamprophyllitedelhayeliteCaNa phosphateperalkaline glasses
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 2 , April 1998 , pp. 179 - 196
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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