Peralkaline nephelinite–natrocarbonatite immiscibility and carbonatite assimilation at Oldoinyo Lengai, Tanzania
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This study presents petrographic and compositional data for coexisting peralkaline silicate glass and quenched natrocarbonatite melt in nepheline phenocrysts from the 24 September 2007 and July 2008 eruptions of the natrocarbonatite volcano Oldoinyo Lengai (Tanzania). Data are also given for peralkaline residual glass in combeite nephelinite ash clasts occurring in the March–April 2006 large volume natrocarbonatite flow. These data are considered to demonstrate the occurrence of liquid immiscibility between strongly peralkaline Fe-rich nephelinite melt and natrocarbonatite at Oldoinyo Lengai. Compositional data for coexisting silicate–carbonate pairs in conjunction with previous experimental studies suggest that the size of the field of liquid immiscibility for carbonated nephelinitic magmas is a function of their peralkalinity. It is shown that peralkaline combeite wollastonite nephelinite was present at Oldoinyo Lengai prior to, and during, the 24 September 2007 ash eruption. It is postulated that the driving force for this major eruption was assimilation and decomposition of previously emplaced solid natrocarbonatite. Assimilation resulted in the formation of the unusual hybrid nepheline–andradite–melilite–combeite–phosphate magma represented by the 24 September 2007 ash.
KeywordsNatrocarbonatite Peralkaline nephelinite Silicate glass Liquid immiscibility Assimilation Oldoinyo Lengai Tanzania
This work is supported by the Natural Sciences and Engineering Council of Canada and Lakehead University. Dorobo Safaris of Arusha, Tanzania, are thanked for logistical assistance at Oldoinyo Lengai. Fred Belton is thanked for providing the sample of the July 2008 nephelinite. Barry Dawson and Jörg Keller are thanked for numerous discussions concerning the petrology of this unique volcano. Don Baker, John Gittins and an anonymous reviewer are thanked for comments on an initial draft of this paper.
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