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Niocalite-cuspidine solid solution and manganoan monticellite from natrocarbonatite, Oldoinyo Lengai, Tanzania

Published online by Cambridge University Press:  05 July 2018

R. H. Mitchell  and
F. Belton
R. H. Mitchell*
Affiliation:
Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1
F. Belton
Affiliation:
825 Fitzpatrick Road, Nashville, TN 37214, USA
*
*E-mail: [email protected]
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Abstract

Lapilli and spatter of natrocarbonatite extruded from the T56B cone of Oldoinyo Lengai (Tanzania) in August 2003 are unusually rich in apatite, silicate, oxide and sulphide minerals. These minerals occur primarily within inclusions of quenched natrocarbonatite in gregoryite and fractures in nyerereite-gregoryite intergrowths. Silicates include members of the sodian cuspidine–niobian cuspidine–niocalite solid-solution series and manganoan monticellite. Oxides are represented by members of the magnesioferrite–jacobsite–magnetite solid-solution series. Sulphides occurring in decreasing order of abundance include: ferroan alabandite, manganoan ferroan sphalerite, galena and pyrrhotite. Petrographic and compositional data for these minerals are interpreted to indicate that all are high-temperature (<900–650°C), early-crystallizing phases from relatively-unevolved natrocarbonatite magma, with the sulphides forming prior to the silicates, and the latter before gregoryite. Sulphur fugacityand oxygen fugacity of natrocarbonatite magma are considered to decrease and increase, respectively, during the formation of the sulphide-oxide assemblage. Crystallization of cuspidine-niocalite, monticellite and Si-bearing apatite rapidly depletes natrocarbonatite magma of its initial low silica content, preventing crystallization of late-stage groundmass silicates.

Keywords

natrocarbonatitecarbonatitecuspidineniocalitemonticellitesulphidesOldoinyo LengaiTanzania
Type
Research Article
Information
Mineralogical Magazine , Volume 68 , Issue 5 , October 2004 , pp. 787 - 799
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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