Geochemistry and petrogenetic significance of natrocarbonatites at Oldoinyo Lengai, Tanzania: Composition of lavas from 1988 to 2007

Abstract

The natrocarbonatites of Oldoinyo Lengai, Tanzania, are unique in magmatic petrology. The historical activity of Oldoinyo Lengai has seen changes from nephelinitic to natrocarbonatitic character of the emitted magmas. Since 1983 the activity was characterized by the effusion of fluid natrocarbonatite lava from which we have collected and analyzed fresh samples in the summit crater from 1988 to 2007. The available compositional data set forms the basis for presenting and discussing the typical composition and variation of natrocarbonatites and their relationship to the silicate magmas of Oldoinyo Lengai. The “type” natrocarbonatite major and trace element composition is derived for an average of 25 samples with low standard deviation. Oldoinyo Lengai carbonatites are unique in almost all aspects of their petrological and geochemical characteristics and are characterized as extremely alkali-rich, with Na2O+K2O generally about 40wt.%, and with high CaO contents of 14–18wt.%. This composition results from the presence of phenocrysts of nyerereite (Na,K)2Ca(CO3)2 and gregoryite (Na,K,Cax)2−x(CO3) dominating the highly porphyritic natrocarbonatite lavas, with sylvite and fluorite as main groundmass minerals. The significance of particular trace element concentrations and ratios of equally incompatible elements (REE, Ba, Sr, Th/U, Nb/Ta, Zr/Hf) is evaluated for models of liquid–liquid separation. In defining a “type” natrocarbonatite composition, we also distinguish special variations in chemical and/or mineralogical compositions as follows: (1) silicate-bearing natrocarbonatites, characterized by the occurrence of nephelinite spheroids, as in the 1993 and 2006 lavas; (2) residual melt compositions as described from the 1988 eruptive period as represented by the aphyric, filter-pressed interstitial melt of solidifying porphyritic lavas; (3) an interlude during 2000 when natrocarbonatites with sylvite and fluorite microcrysts were emitted. After 25years of mostly mild activity characterized by effusion and spattering of fluid natrocarbonatite lava, the paroxysmal ash eruptions of September 4, 2007, changed dramatically the crater morphology, eruptive dynamics and magma composition of Oldoinyo Lengai. Fresh natrocarbonatites — if present at the bottom of the deep crater pit formed during the 2007–2008 explosive activity — will possibly remain inaccessible for decades.