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Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

Published online by Cambridge University Press:  14 December 2020

Anatoly N. Zaitsev
Affiliation:
Department of Mineralogy, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russia Imaging and Analysis Centre, Department of Earth Sciences, The Natural History Museum, Cromwell Road, SW7 5BD, UK
John Spratt
Affiliation:
Imaging and Analysis Centre, Department of Earth Sciences, The Natural History Museum, Cromwell Road, SW7 5BD, UK
Alexander G. Shtukenberg
Affiliation:
Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003, USA
Andrei A. Zolotarev
Affiliation:
Department of Crystallography, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russia
Sergey N. Britvin
Affiliation:
Department of Crystallography, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russia
Sergey V. Petrov
Affiliation:
Department of Mineral Deposits, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russia
Alina V. Kuptsova
Affiliation:
Department of Regional Geology, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russia
Anton V. Antonov
Affiliation:
A.P. Karpinsky Russian Geological Research Institute, Sredny Pr. 74, St. Petersburg, 199106, Russia
Corresponding
E-mail address:

Abstract

The Quaternary carbonatite–nephelinite Kerimasi volcano is located within the Gregory rift in northern Tanzania. It is composed of nephelinitic and carbonatitic pyroclastic rocks, tuffs, tuff breccias and pyroclastic breccias, which contain blocks of different plutonic (predominantly ijolite) and volcanic (predominantly nephelinite) rocks including carbonatites. The plutonic and volcanic carbonatites both contain calcite as the major mineral with variable amounts of magnetite or magnesioferrite, apatite and forsterite. Carbonatites also contain accessory baddeleyite, kerimasite, pyrochlore and calzirtite. Zr and Nb minerals are rarely observed in rock samples, though they are abundant in eluvial deposits of carbonatite tuff/pyroclastic breccias in the Loluni and Kisete craters. Pyrochlore, ideally (CaNa)Nb2O6F, occurs as octahedral and cubo-octahedral crystals up to 300 μm in size. Compositionally, pyrochlore from Loluni and Kisete differs. The former is enriched in U (up to 19.4 wt.% UO2), light rare earth elements (up to 8.3 wt.% LREE2O3) and Zr (up to 14.4 wt.% ZrO2), and the latter contains elevated Ti (up to 7.3 wt.% TiO2). All the crystals investigated were crystalline, including those with high U content (a = 10.4152(1) Å for Loluni and a = 10.3763(1) Å for Kisete crystals). They have little or no subsolidus alteration nor low-temperature cation exchange (A-site vacancy up to 1.5% of the site), and are suitable for single-crystal X-ray diffraction analysis (R1 = 0.0206 and 0.0290; for all independent reflections for Loluni and Kisete crystals, respectively). Observed variations in the pyrochlore composition, particularly Zr content, from the Loluni and Kisete craters suggest crystallisation from compositionally different carbonatitic melts. The majority of pyrochlore crystals studied exhibit exceptionally well-preserved oscillatory- and sometimes sector-type zoning. The preferential incorporation of smaller and higher charged elements into more geometrically constrained sites on the growing surfaces explains the formation of the sector zoning. The oscillatory zoning can be rationalised by considering convectional instabilities of carbonatite magmas during their emplacement.

Type
Article – Gregory Yu. Ivanyuk memorial issue
Copyright
Copyright © The Author(s), 2020. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland

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Footnotes

This paper is part of a thematic set ‘Alkaline Rocks’ in memory of Dr Gregory Yu. Ivanyuk

Associate Editor: Daniel Atencio

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Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania
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