Ol Doinyo Lengai
Oldoinyo Lengai is the only active carbonatite volcano in the world and is known for the unusually alkali-rich composition of its carbonatite lavas. Natrocarbonatite characterises the recent activity which is expressed by the current quiet effusions in the northern summit crater. It is inferred from historical reports that this type of activity goes back more than 100 years (Dawson et al., 1995a). Oldoinyo Lengai is located in the eastern branch of the East African Rift System, south of Lake Natron (Fig. 1) and close to the ca. 400 m high Natron Escarpment of the Gregory Rift, which formed 1.2 Ma ago. Volcanic activity in the Gregory Rift started about 10 Ma ago in its northern part and propagated to the SW to the site of recent volcanic activity of Oldoinyo Lengai on the southern edge of the Lake Natron basin.
Fig.1: Location of Oldoinyo Lengai and regional geology. Modified from Kervyn, M., et al. (2008).
The summit of the volcano contains two craters, north and south, separated by an east-west aligned ridge. Volcanic activity forming the Younger Extrusives of the Gregory Rift, is thought to have started after the generation of the major North-South fault approximately 1.2 Myrs ago, which now marks the western boundary of the rift valley (Dawson, 2010). The onset of activity has been poorly constrained with age measurements of the oldest pyroclastics (as determined by stratigraphic positioning) at 0.37-0.22 Ma (Bagdasaryan et al., 1973).
The eruptive style of Oldoinyo Lengai is characterized by alternating periods (often months to years) of effusive activity, primarily contained within the northern crater and upper flanks, and explosive phases of Vulcanian / Plinian type volcanism lasting for shorter time periods of weeks to months. The effusive activity is predominantly carbonatitic material whilst explosive episodes can see the release of both alkaline silicate material and soda ash (Keller and Krafft, 1990). During its effusive phases Oldoinyo Lengai generates natrocarbonatite via lava fountains up to a few metres in height (Norton and Pinkerton, 1997, Keller and Krafft, 1990) from active spatter cones generating droplet lapilli. Lava flows from active vents are generally small (<100 m3) with low effusion rates (0.3 m3s-1) (Keller and Krafft, 1990). Interconnected lava lakes within the crater regularly overflow generating further lava flows which can top the crater rim and extend a few hundred metres down the upper flanks.
Stratigraphy
Dawson (1962) first established a stratigraphy. As major structural units we define:• Lengai I: phonolite tuffs and phonolite lavas.
• Lengai II A: nephelinite tuffs and nephelinite lavas.
• Lengai II B: nephelinite tuffs, nephelinite lavas, gray melilite-bearing nephelinite tuffs and carbonatite tuffs and lavas of the active northern crater.
Phonolites are dominant in the early southern cone of Lengai I, while nephelinites of Lengai II characterize the pyroclastics and lavas of the northern cone. Primary deposits of Lengai II A occupy the middle part of the northern cone, whereas the Lengai II B unit forms the upper cone including the summit area. A buried sector collapse scarp separate Lengai I and Lengai II (Fig. 2). This means that Lengai II formed after a major sector collapse event, which affected approximately 18% of the recent cone volume at around 10 ka ago (Klaudius and Keller, 2004). Lengai II A is separated from Lengai II B by a crater-rim unconformity visible at the upper northern flank. Most of the summit area is covered by gray colored combeite-wollastonite nephelinites and melilite-bearing combeite-wollastonite nephelinites pyroclastics of recent explosive eruptions (1917, 1940 and 1966/67), which consist of agglomerates, spherical lapilli, crystal lapilli and ash. Lengai I forms approximately 60% of the volcano's volume, Lengai II ~35% and carbonatites are subordinate with less than 5%.
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Fig.2: Cross section of the Oldoinyo Lengai upper cone showing geometry and stratigraphic relations of units Lengai I and Lengai II. Modified from Klaudius, J. (2006).
The natrocarbonatite lavas
The natrocarbonatite lavas consist of the water-soluble phenocryst minerals nyerereite, (Na,K)2Ca(CO3)2, and gregoryite, (Na,Ca,K)2CO3, in a matrix of symplectically intergrown fluorite and (halite-sylvite). Apatite, magnetite, monticellite, cuspidine-niocalite, sellaite, alabandite, sphalerite and galena occur as trace minerals.Continuous lava effusion from Oldoinyo Lengai since 1988 has provided the opportunity for sampling and testing all of the stages of atmospheric alteration and hydration of natrocarbonatites. Identification of pre-1917 carbonatites in the former, pre-1917, crater platform and in the overflow fields of the steep north flank extended the observational base considerably. Most of these transformed older natrocarbonatites, form loose, sandy masses, but may contain more solid relict cores. Field observations indicate three major processes that lead to the formation of subsolidus (secondary) minerals at Oldoinyo Lengai (i) by sublimation from hot gases escaping during lava cooling, (ii) by atmospheric alteration and reaction with meteoric water and (iii) by reaction with fumarole gases. Subsolidus mineralogical re-equilibration of the lava starts immediately after solidification and cooling, transforming them, under atmospheric influences (air and meteoric water), in a very short time into white secondary products. Immediately after solidification, the natrocarbonatite lavas are cut by a network of thin cracks. The edges of the cracks become covered by white, brownish and yellowish encrustations up to several centimeters in thickness which are known as "efflorescence" or "salt fringes and tubes".
The origin of natrocarbonatite
Natrocarbonatite flows have emerged periodically over the last 4 decades in the summit crater of the Oldoinyo Lengai, accompanied by H2O-CO2 vapor. The currently favored explanation for the genesis of these carbonatites by liquid immiscibility between a silicate and a carbonatite melt is questioned based on the extremely low eruption temperatures of 544-593°C and compositional and mineralogical characteristics not in agreement with experimental constraints. Experimental investigations of the relationship between Oldoinyo Lengai natrocarbonatite and related silicate rock compositions do indicate that alkali-bearing peralkaline carbonatite with liquidus calcite can form by liquid immiscibility. At the same time, these experiments result in evidence which speaks against a liquid immiscibility origin for the highly alkaline and peralkaline Oldoinyo Lengai natrocarbonatite. No natural silicate magma is known to produce natrocarbonatite compositions by liquid immiscibility. The best interpretation of the Oldoinyo Lengai natrocarbonatite involves expulsion of a cognate, mobile, alkaline, and CO2-rich fluid condensate.The activity at Oldoinyo Lengai can be divided into three main types: (1) dormant stage; (2) carbonatite stage, and (3) Plinian stage. Dormant stages are periods with no activity or only minor degassing, carbonatite stages encompass periods of carbonatite extrusion and enhanced degassing, while Plinian stages refer to silicate dominated eruptions during which the silicate cone of the volcano is constructed. The dormant stage
During dormant periods (Fig.3) fluid is suggested to separate at depth from cooling low-alkali carbonatite, or carbonated peralkaline silicate melt in the subvolcanic magma chamber of the volcano. The fluid will react with previously crystallized phases (auto-metasomatism) and precipitate solids and/or leave droplets of carbonatite liquid behind in the fenitized zone above the magma chamber. This would be due to a decrease in solubility of the natrocarbonatite component with falling P and T, and equilibration with the mineral assemblages of the host silicate rocks (evolved nephelinite and phonolite). Volatiles dominated by CO2 and H2O will penetrate and react with the host rocks and be lost.
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Fig.3: Dormant stage. After Nielsen, T. F (2002).
Carbonate stage
During moderately active silicate magmatism e.g., replenished of the magma chamber, previously deposited natrocarbonatite liquid and solid components could be reactivated to form small melts pools. The natrocarbonatite melt may contain variable amounts of silicate phase as supported by the presence of silicate melt droplets in some natrocarbonatite lavas. The natrocarbonatite liquid accumulates in small pockets, less than 10 m in radius, in the upper part of the volcanic edifice (Fig.4); natrocarbonatite and accompanying vapor are extruded from these little chamber and continued loss of CO2 and H2O vapor transform the fluid into natrocarbonatite melt. The resulting natrocarbonatite liquid will be dry and have strongly enhanced alkali content.
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Fig.4: Carbonate stage. After Nielsen, T. F (2002).
Plinian stage
The magma chamber is replenished from the feeder system and the residing magma is expelled during violent Plinian eruption (Fig.5). Carbonatite melt in the small pockets or in the conduit may be entrained. After a Plinian eruption the volcano would return to a carbonatite or dormant stage.
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Fig.5: Plinian stage. After Nielsen, T. F (2002).
Petrogenesis of the Oldoinyo Lengai lavas
More than 95 vol.% of the volcano is composed of silicate lavas and pyroclastics. Silicate lavas at Oldoinyo Lengai are dominantly nephelinites and phonolites. These lavas are highly alkaline and reach high levels of peralkalinity ((Na+K)/Al>1), which finds its expression in the unusual mineralogy with combeite (ideal formula Ca2Na2(Si3O9)) in the most recent combeite-wollastonite nephelinites. An important feature of all silicate lava samples from Oldoinyo Lengai is their evolved composition.Maximum MgO contents of all published and available analyses do not exceed about 2 wt.% MgO and all Mg# (100 Mg/(Mg + Fe2+)), calculated with Fe2O3/FeO=0.15, are below 30. This raises the question of primary magmas at Oldoinyo Lengai. The highly fractionated nature is underlined by Ni and Cr contents that are consistently < 10 ppm, and the generally enriched incompatible element patterns. Combeite-wollastonite nephelinites are mineralogically and geochemically unusual and unique at Oldoinyo Lengai, where they represent the dominant magma composition of Lengai II. They were erupted during a major cone building stage following a north-directed sector collapse of a phonolitic. Lengai I phonolite and Lengai II combeite-wollastonite nephelinites form chemical distinct groups that are not linked genetically to each other in a simple manner. Petrography of lava samples from the northern cone shows that nephelinite is combeite and wollastonite-bearing. Within the volcano's temporal evolution, phonolite and nephelinite show a distinct trend to more silica undersaturated compositions and increasing peralkalinity, especially within the combeite-wollastonite nephelinites of Lengai II. This trend is manifested in several ways including the appearance of combeite, a Na-Ca phase without Al in its structure, an indicator for high peralkalinity.
Models for the evolution of the Lengai suite from parental mantle melts invoke derivation from high magnesian olivine nephelinites and olivine Melilitites. Peterson (1989b) argues that olivine nephelinites evolve by fractionation towards mildly peralkaline nephelinites and associated calciocarbonatites, as at Shombole volcano, Kenya, whereas at Lengai the parental magma is thought to be melilititic, and the fractionation is directed towards highly peralkaline combeite-wollastonite nephelinites. From these, natrocarbonatite exsolves in the final stage of the evolution.
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View of Lengai from Engaro Sero village. From Photovolcanica.
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Oldoinyo Lengai Crater, July 2004. From Photovolcanica.
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Hornitos on crater floor seen from E rim, July 2004. Dark lava flow is only hours old. From Photovolcanica.
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Hornitos on crater floor seen from E rim, July 2004. Dark lava flow is only hours old. From Photovolcanica.
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Nighttime lava flow from hornito in July 2004. From Photovolcanica.
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Lava flow from vent in flank of hornito, July 2004. Dark lava flow is only hours old. From Photovolcanica.
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Lava cascading down flank of hornito, July 2004. Dark lava flow is only hours old. From Photovolcanica.
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Fumarolic deposits formed on fresh lava within 24 hours. Dark lava flow is only hours old. From Photovolcanica.
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Fumarolic deposits formed on fresh lava within 24 hours. Dark lava flow is only hours old. From Photovolcanica.
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Fumarolic deposits formed on fresh lava within 24 hours. Dark lava flow is only hours old. From Photovolcanica.
Bibliography
• A. N. Zaitsev; J. Keller (2006): Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania. Lithos 91 (2006) 191-207.
• A. Simonetti; K.Bell; C. Shrady (1997): Trace and rare earth element geochemistry of the June 1993 natrocarbonatite la vas, Oldoinyo Lengai (Tanzania): Implications for the origin of carbonatite magmas. Journal of Volcanology and Geothermal Research 75 (1997) 89-106.
• J. Klaudius; J. Keller (2006): Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania. Lithos 91 (2006) 173-190.
• Keller, J., Zaitsev, A. N., & Wiedenmann, D. (2006). Primary magmas at Oldoinyo Lengai: the role of olivine melilitites. Lithos, 91(1), 150-172.
• Kervyn, M., Ernst, G. G. J., Klaudius, J., Keller, J., Mbede, E., & Jacobs, P. (2008). Remote sensing study of sector collapses and debris avalanche deposits at Oldoinyo Lengai and Kerimasi volcanoes, Tanzania. International Journal of Remote Sensing, 29(22), 6565-6595.
• Nielsen, T. F., & Veksler, I. V. (2002). Is natrocarbonatite a cognate fluid condensate? Contributions to Mineralogy and Petrology, 142(4), 425-435.
• Roger H. Mitchell (2009): Peralkaline nephelinite–natrocarbonatite immiscibility and carbonatite assimilation at Oldoinyo Lengai, Tanzania. Contrib Mineral Petrol (2009) 158:589-598.
• Vaughan, R. G., Kervyn, M., Realmuto, V., Abrams, M., & Hook, S. J. (2008). Satellite measurements of recent volcanic activity at Oldoinyo Lengai, Tanzania. Journal of Volcanology and Geothermal Research, 173(3), 196-206.