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The 24th September 2007 ash eruption of the carbonatite volcano Oldoinyo Lengai, Tanzania: mineralogy of the ash and implications for formation of a new hybrid magma type

¹ Department of Geology, Lakehead University, Thunder Bay, ON, Canada P7B 5E1
² School of Geosciences, Grant Institute, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK

^* E-mail: rmitchel{at}lakeheadu.ca

The September 2007 ash eruption at Oldoinyo Lengai terminated a period of >30 y of relatively quiet extrusion of natrocarbonatite lavas. Ash erupted on the 24^th September comprised nuclei of ijolitic phases, surrounded by finer peralkaline, silica-undersaturated material. The mantling material, as well as that forming nucleus-free particles, is dominated by nepheline, Ti-andradite, Na-melilite, combeite and a Na-Ca-phosphate-carbonate with minor amounts of K-Fe-sulphide and Mn-magnetite in a sparse matrix of alkali carbonate. The absence of clinopyroxene precludes this material from being termed either nephelinite or melilitite. We propose that the material represented by the mantles is an extremely undersaturated, peralkaline hybrid magma resulting from interaction between natrocarbonatite and nephelinite.

KEYWORDS: carbonatite, volcanic ash, lapilli, nepheline, combeite, melilite, Oldoinyo Lengai, Tanzania