Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

Abstract

With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940-1941, and 1966-1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007-2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe-Ti oxides, and a Na-Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite-wollastonite-melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25-30 wt.% SiO2, with 7-11 wt.% CO2, high alkalies (Na2O 15-19%, K2O 4-5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite-wollastonite nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition. © 2010 Springer-Verlag.