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

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doi: 10.1180/0026461026630035
Authors:Bailey, D. K.; Kearns, S.
Author Affiliations:Primary:
University of Bristol, Department of Earth Sciences, Bristol, United Kingdom
Volume Title:Mineralogical Magazine
Source:Mineralogical Magazine, 66(3), p.379-384. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Publication Date:2002
Note:In English. 23 refs.; illus., incl. 2 tables
Summary: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.
Sections:Mineral data
Subjects:Accessory minerals; Carbonatites; Igneous rocks; Magmas; Magnetite; Metals; Mineral composition; Oxides; Titanium; Titanium oxides; Africa; East Africa; Zambia; Rufunsa Valley
Abstract Numbers:02M/3304
Record ID:2002073302
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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