The 2007-2008 Explosive Events of Oldoinyo Lengai: New Insights into Natrocarbonatite generation

Josephine Smit October 2011

Oldoinyo Lengai, a stratovolcano in the northwestern Tanzanian portion of the East African Rift Valley, is the only volcano in the world that demonstrates active natrocarbonatite volcanism. Carbonatite lava differs from more common lava types in that it has a carbonate content greater than 50% and an unusually low SiO2 content of less than 10% (Woolley and Church 2005). The composition of Oldoinyo Lengai carbonatite is also unique in its high content of sodium and potassium, hence the name natrocarbonatite. The two main minerals in natrocarbonite are nyerereite (Na2Ca(CO3)2) and gregoryite (Na2K2CaCO3) (Gittins 1980). Carbonatite lavas are rare, and extrusive carbonatites such as those produced at Oldoinyo Lengai are especially uncommon (Gittins 1980). Not surprisingly, the origin of Oldoinyo Lengai’s natrocarbonatite has puzzled volcanologists since the discovery of this unusual lava. Two main models of carbonatite genesis have previously been put forward, but neither has fully resolved the issue. This paper reports on a recent study by Keller et al. (2010) that describes fundamental changes in Oldoinyo Lengai’s magmatic composition during a phase of explosive eruption in 2007-2008, providing new insights into the volcano’s magmatic system and the origin of its natrocarbonatite lava. The preferred model for carbonatite genesis posits separation of carbonatite melt and silicate melt by liquid immiscibility (Woolley and Church 2005; Fischer et al. 2009). Liquid immiscibility is a temperature dependent process of magmatic differentiation by which a single fluid separates into two liquids of different compositions with no intervening phase of intermediate composition (Woolley and Church 2005). The liquid immiscibility model is supported by several observations. First, the isotopic signature of natrocarbonatites suggests an igneous origin as a primary magma (Teague et al. 2008). Second, the natrocarbonatites’ very high Na2O and very low MgO content suggest that they are highly evolved magmas (Keller et al. 2006). The association of Oldoinyo Lengai’s carbonatites with alkali-rich silicate rocks is further evidence for genesis by liquid immiscibility between silicate and carbonate magmas (Woolley and Church 2005). Although the liquid immiscibility hypothesis is relatively widely accepted, the precise details of the primary and secondary melts of the immiscibility process for natrocarbonite formation are still unknown. Attempts to replicate Oldoinyo Lengai’s natrocarbonatite by experimental investigation of natural natrocarbonatite and silicate rock systems have thus far been unsuccessful: experimentally equilibrated carbonatite melt was apparently too calcic to cross the thermal barrier to natrocarbonatite composition (Nielsen and Veksler 2002). Given that no natural silicate magma has been shown experimentally to produce natrocarbonatite compositions by liquid immiscibility, Nielsen and Veksler (2002) argue that natrocarbonatites are instead secondary expulsions evolved from alkaline and CO2-rich fluid condensates. Carbonatite melts with a 10-20% alkali content would produce fluid condensates with the composition of Oldoinyo Lengai’s natrocarbonatite (Nielsen and Veksler 2002). However, this alternative model remains to be tested empirically. A recent study by Keller et al. (2010) sheds new light on Oldoiyo Lengai’s magmatic system and natrocarbonatite genesis through compositional analyses of juvenile ejectile materials produced in the volcano’s latest eruptive activity in 2007-2008. Analysis of juvenile lapilli collected from the cinder cone several weeks after the 2007 eruption revealed a carbonate rich silicate magmatic composition that differed significantly from the natrocarbonatite magma that the volcano had produced in the previous 25 years.

The mineralogy of juvenile lapilli was characterized by igneous nepheline ((Na,K)AlSiO4), wollastonite (CaSiO3) and combeite (Na2Ca2Si3O9). This mineralogy is consistent with the combeite-wollastonite nephelinite (CWN) lavas and pyroclasts that compose Oldoinyo Lengai’s youngest cone (Keller et al. 2006), and provide strong evidence of a nephelinite dominated magma system. Nephelinite is a peralkaline (low in aluminium but high in sodium and potassium) silica undersaturated rock that forms in the process of partial melting at low pressure. Further geochemical analysis of the juvenile material revealed a composition of 25-30 wt.% SiO2, 7-11 wt.% CO2, 15-19 wt.% Na2O and 4-5% K2O (Keller et al. 2010). This highly alkaline, undersaturated carbonate rich composition is one that is intermediate between Oldoinyo Lengai’s common natrocarbonatite and CWN lavas. These observations are consistent with analyses of ashes from earlier eruptions in 1940-1941 and 1966-1967, which were similarly described to be of mixed silicate-natrocarbonatite composition. This suggests that the change in magmatic composition observed in the 2007-2008 eruption also occurred in earlier eruptive phases in 1940-41 and 1966-67. Keller et al. (2010) thus identify a cyclicity in the magma types generated by Oldoinyo Lengai during and between its eruptive phases: the volcano produces lava effusions of pure natrocarbonite in periods of quiescence and magma of intermediate nephelinite-natrocarbonatite composition during explosive eruptions. The composition of Oldoinyo Lengai’s eruptive magma is surprising because it falls in the ‘forbidden field’ of the Freestone and Hamilton (1980) pseudoternary triangle (see Figure 1). The Freestone and Hamilton pseudoternary triangle is a visual representation of the compositional stages of the liquid immiscibility process in silicate-carbonatite systems. The fact that the juvenile lapilli occupy a forbidden area in the triangle suggests that Oldoinyo Lengai’s eruptive magma is at compositional stage ready for immiscible separation (Keller et al. 2010). One explanation for this puzzling magmatic composition is that two independent natrocarbonatite and nephelinite ash or melt compositions were physically mixed during eruption (Mitchell and Dawson 2007). This implies that Oldoinyo Lengai has two separate plumbing systems, containing independent natrocarbonatite and nephelinite magmas, and that eruptive events are caused by the evolution of CO2 during the reaction of these two magmas. A random mixing event of this sort is discredited by Keller et al.’s (2010) analyses of juvenile lapilli, which demonstrate a homogenous distribution of the carbonate phase in a bulk silicate groundmass. Moreover, the observed constant composition of juvenile lapilli and ashes throughout the explosive period argue against mechanical mixing of two independent magmas. Trace element characteristics of the juvenile lapilli also show no evidence of a liquid- liquid partitioning between silicate and carbonatite melts, thereby discrediting a possible mixing of independent natrocarbonatite and nephelinite magmas (Keller et al. 2010). Mitchell and Dawson’s (2007) hypothesis also fails to explain what triggers the reaction of magmas from the proposed separate plumbing systems, nor does it account for the formation of natrocarbonatites in the first place. Keller et al. (2010) provide an alternative explanation for the intermediate nephelinite- natrocarbonatite composition of the explosive magma. The authors argue that the juvenile lapilli composition represents the composition of Oldoinyo Lengai’s magma just prior to liquid immiscibility. Based on this assumption, Keller et al. (2010) formulate a model of magma genesis at Oldoinyo Lengai that accounts for the cyclicity in magmatic composition during and between the volcano’s eruptive phases. In Keller et al.’s (2010) model, Oldoinyo Lengai has a plumbing system in which the major reservoir consists of peralkaline nephelinite. This nephelinite then evolves into natrocarbonatite magma by liquid immiscibility. Several important processes precede the actual formation of natrocarbonatite to alter the original nephelinite to a composition that is ready to undergo liquid immiscibility. These processes are fractional crystallization of the nephelinite involving primarily pyroxenes, garnet and Ti- magnetite, followed by enrichment with volatiles (in particular CO2) and alkalies (primarily sodium and potassium). The subsequent formation of natrocarbonatite by liquid immiscibility has been described by two different models (Pyle et al. 1991). The first model is one of instantaneous natrocarbonatite generation, in which a finite mass of natrocarbonatite magma forms immediately after the termination of an explosive phase (Pyle et al. 1991). In the alternative steady-state model, natrocarbonatite magma is continuously extracted by immiscible separation from a steady influx of carbonate-rich magma from Oldoinyo Lengai’s magmatic system (Pyle et al. 1991). While it remains to be seen which model of natrocarbonatite genesis is valid, there is convincing evidence that natrocarbonatite is generated from a parent nephelinite by liquid immiscibility. For instance, the presence of rounded silicate blebs in natrocarbonatite lavas possibly indicate intrusion and quenching of nephelinite melt in a natrocarbonatite magma (Keller et al. 2010). Fischer et al.’s (2009) analysis of Oldoinyo Lengai’s gas chemistry provides further support for Keller et al.’s (2010) model. Quantification of Oldoinyo Lengai’s volatile budget suggests that carbonatite magma is contained in a shallow reservoir extending at most several hundred meters below the crater floor (Fischer et al. 2009). In addition, natrocarbonatite degassing contributes minimally to the overall volatile budget, which is dominated by degassing of the deeper and presumably much larger nephelinite magma reservoir (Fischer et al. 2009). Keller et al. (2010) favor the instantaneous model of natrocarbonatite genesis, because it is able to describe the evolution of the carbonate-rich silicate magma that characterizes eruptive phases. An important consequence of the instantaneous model is that available mass of natrocarbonatite magma is limited (until a new cycle develops). Keller et al. (2010) argue that eruptive phases follow exhaustion of the natrocarbonatite magma chamber, as explosive eruptions have historically been observed to follow large pre-eruption effusions of natrocarbonatite lavas. Upon exhaustion of the upper level natrocarbonatite magma chamber, the volcanic system is dominated by silicate magmas of nephelinitic composition. A continuous upward flow of this nephelinite magma into the volcano’s upper column is accompanied by enrichment with CO2 and other volatiles, thus resulting in a composition that is intermediate between nephelinite and natrocarbonatite as observed in the juvenile lapilli of the 2007-2008 eruption (Keller et al. 2010). Keller et al.’s (2010) model of Oldoinyo Lengai’s magmatic system provide new and convincing insights into natrocarbonatite genesis. Questions remain, however, about why carbonatite volcanism occurs here in the first place and when natrocarbonatite magmas became a feature of Oldoinyo Lengai’s volcanism. The evolution of natrocarbonatite lava represents only a minor volume. The oldest part of the stratocone is primarily composed of phonolite and nephelinite. The volcano’s younger cone is dominated by CWN lavas similar in mineralogy to the juvenile lapilli from the 2007 eruption; this suggests that CWN magmas dominate the younger evolution of Oldoinyo Lengai. In addition, Keller et al.’s (2010) model does not fully resolve whether the instantaneous or steady-state model of natrocarbonatite formation is more valid, nor does it fully explain why explosive eruptions occur.

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Figure 1 Freestone and Hamilton (1980) type diagram showing the composition by weight percent of the 2007 juvenile lapilli and the averages for Oldoinyo Lengai natrocarbonatite and CWN. The 2007 lapilli and ashes occupy the “forbidden” area in the two-liquid-field, suggesting a compositional stage ready for immiscible separation into conjugate liquids.