What is Carbonatite?

Carbonatites are unusual igneous rocks that contain more than 50% carbonate minerals. There are about 330 known occurrences of carbonatites worldwide (Woolley and Kempe 1989). Most carbonatites are intrusive igneous rocks that occur as volcanic plugs, dykes and cone sheets. They often occur as minor members of larger intrusions of silicate igneous rocks such as nepheline syenite that are commonly referred to as ‘carbonatite complexes’. Most are calcite carbonatites.

The magma that is currently being erupted at Oldoinyo Lengai is very unusual for two main reasons. In the first place, it is an extrusive carbonatite. There are other sites where extrusive carbonatites (pyroclasts or lavas) occur, such as Kerimasi, Mosonik and Shombole nearby in the Eastern Rift Valley and Kaiserstuhl in the Rhine Rift. However over the Earth as a whole they are not common. In the second place, the rock at Oldoinyo Lengai contains a high proportion of alkalis, dominated by sodium carbonate, which is why it is called natrocarbonatite. Few other carbonatites contain significant quantities of sodium or of potassium, the other key element in alkaline igneous rocks. Oldoinyo Lengai’s uniqueness was first recognized as recently as 1960 (Dawson, 1962).

Natrocarbonatite is made up largely of two minerals, nyerereite (named after Julius Nyerere, the first president of independent Tanzania) and gregoryite (named after J.W. Gregory, one of the first geologists to study the East African Rift Valley and author of the book ‘The Great Rift Valley’). These minerals are both carbonates in which sodium and potassium are present in significant quantities. Both of these minerals are anhydrous (i.e they contain no water) and when they come into contact with the moisture of the atmosphere, they begin to react extremely quickly.

The black or dark brown lava and ash erupted at Oldoinyo Lengai begin to turn white within a few hours; see the photograph below, taken by Mary Graham-Morris in September 1994. Fresh black pahoehoe lava is flowing over pale brown and pale grey lava that is a few months old. The rapid change in colour takes place because new minerals are formed as water from the atmosphere reacts with the sodium and potassium carbonates.

Geologists have debated whether it is possible that all eruptive carbonatites are originally formed as natrocarbonatites. The reason that no natrocarbonatites are found at extinct volcanoes is because the rocks are extremely soluble in water, and are washed away by rain or circulating groundwater.

There is a voluminous literature on carbonatites in general, and quite a lot on Oldoinyo Lengai in particular. At the International Volcanological Congress held in Mainz, Germany in 1990 a symposium on carbonatite volcanism included a session dedicated solely to Oldoinyo Lengai, and the papers presented formed the basis of a volume of the IAVCEI Proceedings in Volcanology “Carbonatite Volcanism: Oldoinyo Lengai and the Petrogenesis of Natrocarbonatites”, edited by K. Bell and J. Keller and published by Springer-Verlag, Berlin in 1995. In the Introduction (page 3) it is suggested that on the basis of the evidence presented in that volume “it would be unreasonable to assume that natrocarbonatites play a pivotal role in understanding the genesis of all carbonatites. Although such carbonatites are intriguing, the findings in this volume suggest that natrocarbonatites reflect an extreme position in a protracted range of melts associated with nephelinite-carbonatite centres”. Thus the lavas from Oldoinyo Lengai probably represent a unique occurrence of volcanic activity not seen elsewhere.

For those of us who are not mineralogists or petrologists, perhaps the exciting thing about the lava of Oldoinyo Lengai is the low eruption temperature of the lavas. Natrocarbonatite is fluid at much lower temperatures than silicate lavas; temperature measurements have shown it to be fluid at between 540 and 593 degrees C (Pinkerton et al. 1995), compared with over 1100 degrees C for basalt. This means that it is possible to get close to carbonatite lava flows and lava lakes to sample, photograph or observe them without protective clothing or other apparatus. The photograph below, taken by Celia Nyamweru in November 1988, shows Harry Pinkerton of Lancaster University, U.K., measuring the viscosity of carbonatite in an open vent on the crater floor. The late Katja Krafft, who with her husband Maurice spent several days filming Oldoinyo Lengai’s activity in June 1988, described Oldoinyo Lengai as “a toy volcano”!

Although natrocarbonatite is fluid at such low temperatures, it is fluid without being incandescent. Flowing or bubbling natrocarbonatite looks much like black mud, and early visitors to the crater mistook it for such. The photograph below, taken by Jorg Keller on 27 June 1988, shows Celia Nyamweru jumping over a (very narrow!) carbonatite flow. At night an active vent or flow on Oldoinyo Lengai may glow a very deep red, but it is not hot enough to glow during the day like basalt.

Another unusual feature of some natrocarbonatite flows is their very low viscosity; Pinkerton’s measurements have shown it to be among the lowest of any terrestrial melts (Pinkerton et al. 1995). Like basalt, the liquid lava flows almost like water and creates intricate flow structures; see the photograph to the left, taken by Celia Nyamweru in June 1988. Radar images of Venus show volcanic landforms that look like fluvial landforms on earth, with meandering channels, braided bars and bird’s foot deltas. It has been suggested that these venusian landforms may have been formed by “unusually fluid, low-temperature lavas of an extraordinary composition, such as molten mixtures of carbonates, sulfates and other salts” (Kargel et al. 1994: 220). Thus the processes at work in the active crater of Oldoinyo Lengai may be of interest to scientists investigating the surface of Venus. Other natrocarbonatite magmas, rich in crystals at the time of eruption, are as viscous as rhyolite and form thick blocky flows; see the photograph on the right, taken by Burra Gadiye in September 1996. The thickness of the flow can be gauged by comparison with the person in the pale blue shirt in front of it.

As we mentioned earlier, natrocarbonatite is composed of minerals that react rapidly with the water and oxygen in the atmosphere. The dark brown or black lava changes, first to grey and pale brown and eventually to almost white. The colour change from black to white can occur within a few months. At the same time the texture of the rock changes from hard to soft and crumbly. New flows and cones are formed, overlapping and covering the older ones, and gradually the crater floor fills up with progressively younger lava.

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