Mineral chemistry of a peralkaline combeitelamprophyllite nephelinite from Oldoinyo Lengai, Tanzania

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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: BA LAMPROPHYLLITE; CANA PHOSPHATE; COMBEITE; DELHAYELITE; NATROCARBONATITE; NEPHELINITE; OLDOINYO LENGAI; PERALKALINE GLASSES; TANZANIA

Document Type: Research Article

DOI: http://dx.doi.org/10.1180/002646198547567

Affiliations: Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK

Publication date: April 1, 1998

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