Abstract
The alkalic pyroxenite nodule consists of megacrysts of diopside, apatite, perovskite and titanomagnetite in a groundmass consisting of diopside, apatite, titanomagnetite, nepheline, melilite, garnet and vishnevite crystals of various shapes, including previously undescribed skeletal and dendritic shapes, together with vesicles and residual glass. The residual glass is poor in SiO2 (38–40 wt%), and extraordinarily rich in Na2O (12.8–15 wt%), SO3 (1–1.5 wt%), and Cl (0.25–0.7 wt%), as a result of rapid, non-equilibrium crystallization of groundmass phases from a CO2-rich nephelinite melt.
The Oldoinyo Lengai alkalic carbonatite lavas do not represent extreme products of the fractional crystallization of pyroxene, wollastonite, nepheline and alkali feldspar from the carbonated nephelinite melt. The most likely connection between the carbonatite and silicate magma types is one of liquid immiscibility, probably involving phonolite melt.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Anderson, A.T.: Oxidation of the La Blache Lake titaniferous magnetite deposit, Quebec. J. Geol.76, 528–552 (1968)
Bell, K., Dawson, J.B., Farquhar, R.M.: Strontium isotope studies of alkalic rocks: the active carbonatite volcano Oldoinyo Lengai, Tanzania. Geol. Soc. Am. Bull.84, 1019–1030 (1973)
Brown, F.H.: Zoning in some volcanic nephelines. Am. Mineralogist55, 1670–1680 (1970)
Cooper, A.F., Gittins, G., Tuttle, O.F.: The system Na2CO3-K2CO3-CaCO3 at 1 kilobar and its significance in carbonatite petrogenesis. Am. J. Sci.275, 534–560 (1975)
Dawson, J.B.: The geology of Oldoinyo Lengai. Bull. Volcanol.24, 349–387 (1962a)
Dawson, J.B.: Sodium carbonate lavas from Oldoinyo Lengai, Tanganyika. Nature195, 1065–1066 (1962 b)
Dawson, J.B.: Oldoinyo Lengai: an active volcano with sodium carbonate lava flows. In: Carbonatites (O.F. Tuttle and J. Gittins, eds.), pp. 155–168. New York: Wiley
Dawson, J.B., Bowden, P., Clarke, C.: Volcanic activity of Oldoinyo Lengai, 1966. Geol. Rundschau57, 865–879 (1968)
Dawson, J.B., Powell, D.G.: The Natron-Engaruka explosion crater area, northern Tanzania. Bull. Volcanol.33, 791–817 (1969)
Deer, W.A., Howie, R.A., Zussman, J.: Rock-Forming Minerals. Vol. 5. (Longmans) (1962)
Donaldson, C.H.: An experimental investigation of olivine morphology. Contrib. Mineral. Petrol.57, 187–213 (1976)
Eggler, D.H.: Effect of CO2 on the melting of peridotites. Carnegie Inst. Washington Yearbook73, 215–224 (1974)
El Goresy, A., Yoder, H.S.: Natural and synthetic melilite compositions Ann. Rpt. Dir. Geophys. Lab.73, 359–371 (1974)
Freestone, I.C.: Some aspects of liquid immiscibility. Unpubl. Ph.D. thesis, Univ. of Leeds (1978)
Heiken, G.: An Atlas of Volcanic Ash. Smithsonian Contribs. to Earth Sci.12 (1974)
Heinrich, E.W.: The Geology of Carbonatites. Chicago: Rand McNally and Co. 1966
Huggins, F.E., Virgo, D., Huckenholz, H.G.: The crystal chemistry of melanites and schorlomites. Ann. Rpt. Dir. Geophys. Lab.75, 705–711 (1976)
Hutton, C.O.: The igneous rocks of the Brocken range-Ngahayse area, eastern Wellington. Trans. Roy. Soc. New Zealand72, 353–363 (1943)
Koster van Groos, A.F.: The effect of high CO2 pressures on alkalic rocks and its bearing on the formation of alkalic ultrabasic rocks and the associated Carbonatites. Am. J. Sci.275, 163–185 (1975 a)
Koster van Groos, A.F.: The distribution of strontium between coexisting silicate and carbonate liquids at elevated pressures and temperatures. Geochim. Cosmochim. Acta39, 27–34 (1975b)
Koster van Groos, A.F., Wyllie, P.J.: Liquid immiscibility in the join NaAlSi3O8-CaAl2Si2O8-Na2CO3-H2O. Am. J. Sci.273, 465–487 (1973)
Kushiro, I.: Clinopyroxene solid solutions. Part 1. The CaAl2 SiO6 component. Japanese J. Geol. Geog.33, 213–220 (1962)
Kushiro, I.: On the nature of silicate melt and its significance in magma genesis: regularities in the shift of the liquidus boundaries involving olivine, pyroxene, and silica minerals. Am. J. Sci.275, 411–431 (1975)
Le Bas, M.J.: A norm for feldspathoidal and melilite rocks. J. Geol.81, 89–96 (1973)
Le Bas, M.J.: Carbonatite-Nephelinite Volcanism. London: Wiley 1977
Saether, E.: The alkaline rock province of the Fen area in southern Norway. Del. Kgl. Norske Videnskabers Selskabs Skrifter1, 1–8 (1957)
Sahama, Th.G.: Iron content of melilite. Compt. Rend. Soc. Geol. Finlande39, 17–28 (1967)
Sahama, Th.G.: Composition of clinopyroxene and melilite in the Nyiragongo rocks. Ann. Rpt. Dir. Geophys. Lab75, 585–593 (1976)
Thugutt, St.J.: Sur la sodalite et ses dérivés. Anh. Min. Soc. Sci. Lett. Varsovie16, p. 14 (1946) (Min. Abst. 10-293)
Velde, D., Yoder, H.S.: Melilite and melilite-bearing igneous rocks. Carnegie Inst. Washington Yearbook76, 478–485 (1977)
Virgo, D., Mysen, B.O.: Influence of structural changes in NaAl1−x Fex Si2O6 melts on Fe3+-σFe ratios. Carnegie Inst. Washington Yearbook76, 400–407 (1977)
Vukalovich, M.P., Altunin, V.V.: Thermophysical Properties of Carbon Dioxide. London: Collet's 1968
Watkinson, D.H., Wyllie, P.J.: Experimental study of the composition join NaAlSiO4-CaCO3-H2O and the genesis of alkalic rock complexes. J. Petrol.12, 357–378 (1971)
Wilkinson, J.F.G.: The clinopyroxenes of a differentiated teschenite sill near Gunnedah, New South Wales. Geol. Mag.94, 123–134 (1957)
Wyllie, P.J., Cox, K.G., Biggar, G.M.: The habit of apatite in synthetic melts and natural rock systems. J. Petrol.3, 238–243 (1962)
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Donaldson, C.H., Dawson, J.B. Skeletal crystallization and residual glass compositions in a cellular alkalic pyroxenite nodule from Oldoinyo Lengai. Contr. Mineral. and Petrol. 67, 139–149 (1978). https://doi.org/10.1007/BF01046572
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF01046572