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Journal of Petrology | Volume 39 | Number 11-12 | Pages 2137-2151 | 1998
© Oxford University Press 1998

Phase Relationships of a Silicate-bearing Natrocarbonatite from Oldoinyo Lengai at 20 and 100 MPa

C. M. Petibon1,*, B. A. Kjarsgaard2, G. A. Jenner1 and S. E. Jackson1

1 Department of Earth Sciences, Memorial University of Newfoundland St John's, NFLD, Canada, A1B 3X5
2 Geological Survey of Canada 601 Booth Street, Ottawa, Ont., Canada, K1A 0E8

Received September 30, 1997; Revised typescript accepted June 12, 1998


  Abstract

Silicate-bearing and silicate-free natrocarbonatites have been recently erupted at Oldoinyo Lengai. Low-pressure (20 and 100 MPa) experiments were undertaken to determine the phase relationships of a natural silicate-bearing natrocarbonatite (OL5) from Oldoinyo Lengai. Nepheline appears first on the liquidus. Over a protracted cooling interval (>200°C), only silicate and oxide phases crystallize, with nyerereite and gregoryite precipitating at temperatures <635°C. Results of this study, used in conjunction with the phase assemblage and mineral chemistry data available for natrocarbonatites from Oldoinyo Lengai, are used to test hypotheses relating silicate-bearing natrocarbonatite to the silicate-free variety. Our preferred model involves low-P (~20 MPa) in situ fractional crystallization of silicate minerals from a silicate-bearing natrocarbonatite parent magma to produce silicate-free natrocarbonatites. Subsequent evolution of low-density, silica-poor natrocarbonatite is dominantly controlled by carbonate (nyerereite and gregoryite) crystallization and fractionation. The low density and viscosity of silicate-free natrocarbonatite allow these magmas to erupt more easily than silicate-bearing natrocarbonatite.

KEY WORDS: magma evolution; fractionation; natrocarbonatite; Oldoinyo Lengai

* Corresponding author. Telephone: (709) 737-8142. Fax: (709) 737-2589. e-mail: g47cmp{at}morgan.ucs.mun.ca


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