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Differentiation of natrocarbonatite magma at Oldoinyo Lengai volcano, Tanzania

Published online by Cambridge University Press:  05 July 2018

J. Gittins  and
B. C. Jago
J. Gittins
Affiliation:
Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1
B. C. Jago
Affiliation:
Lakefield Research Ltd., Postal Bag 4300, 185 Concession Street, Lakefield, Ontario, Canada K0L 2H0
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Abstract

Natrocarbonatite magma, erupted as lava flows in the Tanzanian volcano Oldoinyo Lengai in June and November of 1988, has evolved chemically since its formation. The June and November flows of 1988 display increasing Cl, F, Ba, K, Mg and Mn, concomitantly with Na, Ca and P depletion. Furthermore, the June magma, at the time of eruption, had higher Cl, F, Ba and K contents and lower Ca than the November magma and evolved to higher levels of Cl, F, Ba and K content and lower Ca, Na and P. The mineralogy of the lavas reflects these trends. Crystallization of fluorite and halite–sylvite solid solution, usually as a symplectic intergrowth, occurs when Cl and F concentrations reach the critical value necessary to stabilize both minerals and explains why neither occurs as a phenocryst phase. Natrocarbonatite magma has undergone considerable and rapid magmatic evolution, probably in small and separate magma chambers. Two minerals, nyerereite and gregoryite, have dominated the crystallization history of natrocarbonatite magma, and many lavas are phenocryst-rich. However, because most of the lavas are composed principally of these two minerals, crystal accumulation has not greatly changed their composition and, consequently, we suggest that the bulk composition of the lavas closely approximates that of the parental magma.

Keywords

natrocarbonatiteOldoinyo Lengaimagmatic evolution
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 6 , December 1998 , pp. 759 - 768
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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