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High-Ti magnetite in some fine-grained carbonatites and the magmatic implications

Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

^* E-mail: ken.bailey{at}bris.ac.uk

ABSTRACT

Magnetite is present in most carbonatites, and in the most abundant and best-known form of carbonatite, coarse-grained intrusions, it typically falls in a narrow composition range close to Fe₃O₄. A fine-grained carbonatite from Zambia contains magnetites with an extraordinary array of compositions (from 18–1% TiO₂, 10–2% Al₂O₃, and 16–4% MgO) outranging previously-reported examples. Zoning trends are from high TiO₂ to high Al₂O₃ and MgO. No signs of exsolution are seen. Checks on similar rocks from Germany, Uganda and Tanzania reveal magnetites with comparable compositions, ranges, and zoning. Magnetites from alkaline and alkaline ultramafic silicate volcanic rocks cover only parts of this array. Magnetite analyses from some other fine-grained carbonatites, reported in the literature, fall in the same composition field, suggesting that this form of carbonatite may be distinctive. The chemistry and zoning would be consonant with rapid high-temperature crystallization in the carbonatite melts, with the lack of exsolution pointing to fast quenching: this contrasts with coarse-grained intrusive carbonatites, in which the magnetite compositions are attributed to slow cooling, with final equilibration at low temperature. In some complexes, both forms of carbonatite, with their different magnetite compositions, are represented.

KEYWORDS: high-ti magnetite, carbonatite, Zambia

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