Oldoinyo Lengai (Oldonyo L'Engai, Ol Doinyo Lengai)

The natrocarbonatite lavas of Oldoinyo Lengai have ensured that this volcano is now perhaps the most widely known carbonatite occurrence in the world. Although it is generally accepted that the natrocarbonatite is of primary igneous origin there is some dissent (Milton, 1989), and there is controversy over the petrogenetic significance of the lavas (e.g. Gittins, 1989). A view of the volcano is used as the frontispiece to the book Carbonatites (Tuttle and Gittins, 1966) and is on the fly cover of Bell (1989); good views of the crater will be found in the review paper by Dawson (1989). An essential reference is the book Carbonatite volcanism, edited by Bell and Keller (1995), which is focussed mainly on the natrocarbonatites but includes an initial chapter by Dawson et al. (1995a) reviewing the historic and recent eruptive activity.

Oldoinyo Lengai lies close to the rift escarpment against which ejecta on the western side of the volcano are piled. It forms a steep, symmetrical cone 12 km in diameter and standing nearly 2000 m above the rift valley floor. There are two summit craters; an older ash-filled one to the south and a northern active one. Six stratigraphic units are recognised (Dawson, 1962b, 1966 and 1989) the general distribution of which is shown on the Quarter Degree Sheet (Guest et al., 1961). From oldest to youngest these are: (1) yellow palagonitised tuffs and agglomerates of phonolitic and nephelinitic composition with some rare nephelinite and phonolite lavas. This unit comprises >90% of the volume of the volcano and outcrops on the higher slopes and in the lower parts of the more deeply eroded radial gullies. It is thought to have emanated principally from the southern crater. (2) Mica and pyroxene tuffs which form parasitic cones and craters on the lower western and eastern slopes. (3) Black tuffs and agglomerates of nephelinitic composition which clothe much of the volcano and reach 200 m in thickness on the western and northern slopes. (4) Minor flows of nephelinite in the summit area and forming parasitic cones on the lower northern slopes. (5) Carbonatite ashes cover the summit area, western and northern slopes and the active northern crater. (6) Recent natrocarbonatite lavas and carbonatite-silicate ashes. Units 3-6 were erupted from the northern crater (Dawson, 1989).

More than 99% of the volcano comprises tuffs, agglomerates and scarce lavas, the last defining a series from nephelinite through phonolitic nephelinite to phonolite (Donaldson et al., 1987). Most rocks contain 10-40% phenocrysts and microphenocrysts including nepheline, aegirine-augite, trending to salite in nephelinites, and in the nephelinitic rocks garnet varying from schorlomite to melanite, titanomagnetite, wollastonite, apatite, some sodalite, and in the phonolitic rocks titanite and sanidine; vishnevite phenocrysts also occur. The fine- to very fine-grained groundmass has up to 30% green glass, together with nepheline, aegirine-augite, pyrrhotite and titanomagnetite in all rocks with sanidine in the phonolites; wollastonite microlites have been identified, apatite is sparse and melilite altered. Abundant vesicles are partly filled with carbonate, zeolites, apophyllite and analcime. Three of a series of six samples described by Peterson (1989a and b) contain either wollastonite phenocrysts or combeite with abundant sodalite phenocrysts in both types. An exceptionally peralkaline nephelinite also contains the rare minerals lamprophyllite, delhayelite, magmatic (as opposed to metasomatic) combeite and a Na,K,Ba,Ca phosphate (Dawson and Hill, 1998). Electron microprobe analyses of nepheline, clinopyroxene, alkali feldspar, Ti-garnet, vishnevite and glass are given by Donaldson et al (1987) who also give analyses of 19 rocks with a range of trace elements including REE for six samples (see also Dawson, 1989). Peterson (1989a) includes analyses of nephelinite, pyroxene, nepheline and glasses and examines the origins and evolutionary series from melilitite or olivine-melilite nephelinite through to wollastonite and combeite-bearing nephelinite (Peterson, 1989b). Other rock analyses are collected in Dawson (1966), and for these samples data for U and Th and Sr isotopes are to be found in Dawson and Gale (1970) and Bell et al. (1973). The pyroclastic rocks vary from crystal, lithic and lapilli tuffs to agglomerates.

Blocks of a wide range of composition are found in the agglomerates (Dawson, 1966 and 1989) and define magmatic and metasomatic series. The magmatic rocks include jacupirangite, pyroxenite with perovskite, melteigite, ijolite, nepheline syenite, some with eucolite, nepheline-wollastonite rocks and sovite (Dawson et al., 1995b). Some of these rock types are not represented amongst the extrusive rocks. Analyses of four blocks are given by Dawson (1989). Fenitized blocks of granite gneiss and gabbro are described by Morogan and Martin (1985), who document the development of sanidine, nepheline and aegirine-augite in the highest grades of fenitization and the development of some glass. Carbonatite lavas containing approximately 40% total alkalis were extruded onto the floor of the northern crater in 1960 (Dawson, 1962a and 1966), but there have undoubtedly been earlier eruptions of such material (Dawson et al., 1995a). The 1960 flows were both highly mobile pahoehoe and blocky aa types and emanated from a 20 m-wide vent containing a lava pool which erupted sporadically throwing lava 15-20 m into the air. When fresh the lava is black in colour (see Dawson, 1962b, 1989, Fig.11.4 and Krafft and Keller, 1989), but is hygroscopic absorbing moisture from the atmosphere and whitening within a few days. Mixed natrocarbonatite/silicate lavas erupted in 1993 (Church and Jones, 1995; Dawson et al., 1994 and 1996). Explosive eruptions in 1966 were of mixed silicate-carbonate ashes, containing combeite, Na phosphates, natrite and Sr rankinite (Dawson et al., 1989, 1992, 1996), but emission of natrocarbonatite lavas certainly occurred between 1983 and 1986 (Nyamweru, 1988). In 1988 continuous lava-lake and lava flow activity was observed by Krafft and Keller (1989) who made temperature measurements in the highly fluid flows and lava lake, the highest recorded being 544° in the lake, but in the many flows between 491° and 506°. Somewhat higher temperatures (580°-590°) were recorded by Dawson et al. (1990), who also measured viscosities ranging from 0.3 to 100 Pas. More recent activity has been described by Nyamweru (1997). In 1999 the northern crater continued to be filled by further natrocarbonatite flows which started to overflow onto the outer slopes of the volcano (pers. comms M. Genge and J.B. Dawson, 1999), thus the volcano is now very similar to its state in the early years of this century (Dawson et al., 1995a).. The natrocarbonatite lavas of 1988 comprise phenocrysts of the alkali carbonates nyerereite (McKie and Frankis, 1977) and gregoryite (Gittins and McKie, 1980) in a matrix of aligned prisms of nyerereite, gregoryite and an opaque phase rich in Mn and S (Dawson, 1989). Analyses of gregoryite and nyereite, from 1963 lavas, will be found in Peterson (1990) and analyses of other mineral phases will be found in Dawson et al. (1995c). Rock analyses give Na2O 29.5-32.7%, K2O 6.6-8.6%, SiO2 trace, SO3 2.0-3.1%, F 1.8-3.6, and Cl 2.6-4.4 (Dawson, 1962a, 1966, 1989 and Dawson et al., 1995). Sr and Nd isotope data will be found in Bell and Dawson (1995), Nd, Sr and Pb isotope data on silicate rocks and carbonatites in Bell and Simonetti (1996), C and O isotope data in Keller and Hoefs (1995), U and Th isotope values in Pyle (1995), Dawson and Gale (1970) and Williams et al. (1986), and REE data in Dawson (1989). Williams et al. (1986) have suggested, based on Ra-Th disequilibrium systematics of 1960 and 1963 lava samples, and a 1966 ash, that the magmas were formed and erupted within 7-18 years; this subject is considered further by Pyle (1995).

The origin of the natrocarbonatite lavas and their relationship to the associated silicate lavas is discussed by Dawson (1989, 1998) and Gittins (1989) considers the 'Oldoinyo Lengai trend' (alkalic trend) of carbonatite evolution. Morogan and Martin (1985) proposed that two immiscible liquids were produced by partial melting of regionally fenitized basement beneath Lengai, one melt corresponding to the natrocarbonatite lavas and the other to the silicate lavas, a view opposed by Gittins (1988b). See also reply of Martin and Morogan (1988). Peterson (1990) favours an origin for the natrocarbonatites by immiscible separation from nephelinite, a point of view that is endorsed by Kjarsgaard et al. (1995), based on experimental evidence. During recent and historic eruptions of Oldoinyo Lengai carbonate ash was widely distributed (Dawson, 1962b; Dawson et al., 1968) and carbonate-rich horizons in the Olduvai Gorge succession, for instance, probably originated in Lengai (Hay and Roeder, 1978) - but see Kerimasi (No. 7) as a further potential source of carbonate ash.