Abstract
The phase relations of a “primary” sodic dolomitic carbonatite (CM1), determined by Wallace and Green (1988) to be in equilibrium with an amphibole lherzolite assemblage, have been investigated from 5 to 27 kb over a temperature range of 650–1225 °C with 2 and 4wt% added water in order to test the model, first proposed by Wallace and Green (1988), that natrocarbonatite may result from the closed system crystal fractionation of such a “primary” composition. Our experiments suggest that if Oldoinyo Lengai magmas evolve from a primary carbona-titic liquid at depth by crystallization of dolomite, olivine and spinel, then this liquid must be more silicic, less FeO-rich and have a greater K/Na ratio than the Wallace and Green (1988) “primary” carbonatite composition. These suggested differences may be consistent with differences in the mantle source region from which the Oldoinyo Lengai composition was derived and the peridotite composition (Hawaiian Pyrolite) with which the Wallace and Green (1988) composition was equilibrated.
The phase relationships of CM1 also demonstrate the existence of a decarbonation reaction to lower pressures and higher temperatures which results in the formation of a CO2-rich fluid phase. This fluid phase contains alkali elements and chlorine and may be responsible for the fenitization of wall-rocks associated with many carbonatite complexes. Additionally, a late-stage fractionate would be extremely sodic (approximately 40wt% Na2O) and may also fenitize wall-rocks.
A twofold increase in the added water content (from 2 to 4wt%) depressed the liquidus temperature by <30°C and had little effect on the observed liquidus phase relationships. No evidence for liquid immiscibility was observed in the P-T region studied.
Keywords
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Preview
Unable to display preview. Download preview PDF.
References
Baker MB, Wyllie PJ (1990) Liquid immiscibility in a nephelinite-carbonate system at 25 kb and implications for carbonatite origin. Nature 346:168–170.
Cooper AF, Reid DL (1991) Textural evidence for calcite carbonatite magmas, Dicker Willem, south-west Namibia. Geology 19:1193–1196.
Cooper AF, Gittins J, Tuttle OF (1975) The system Na2CO3-K2CO3-CaCO3 at lkb and its significance in carbonatite petrogenesis. Am J Sci 275:534–560.
Fanelli MF, Cava N, Wyllie PJ (1986) Calcite and dolomite without portlandite at a new eutectic in CaO-MgO-CO2-H2O, with applications to carbonatites. In: Morphology and phase equilibria of minerals. Proc 13th General Meeting Int Mineral Assoc Sofia, pp 313-322.
Green DH, Wallace ME (1988) Mantle metasomatism by ephemeral carbonatite melts. Nature 336:459–462.
Hogarth DD (1989) Pyrochlore, apatite and amphibole: distinctive minerals in carbonatite. In: Bell K (ed) Carbonatites — genesis and evolution. Unwin Hyman, London, pp 105–148.
Irving AJ, Wyllie PJ (1975) Subsolidus and melting relationships for calcite, magnesite and the join CaCO3-MaCO3 to 36 kb. Geochim Cosmochim Acta 39:35–53.
Jago BB, Gittins J (1991) The role of fluorine in carbonatite magma evolution. Nature 349:56–58.
Keller J (1981) Carbonatite volcanism in the Kaiserstuhl alkaline complex: evidence for highly fluid carbonatitic melts at the earth’s surface. J Volcanol Geotherm Res 9:423–431.
Keller J (1989) Extrusive carbonatites and their significance. In: Bell K (ed) Carbonatites — genesis and evolution. Unwin Hyman, London, pp 70–88.
Keller J, Krafft M (1990) Effusive natrocarbonatite activity of Oldoinyo Lengai, June 1988. Bull Volcanol 52:629–645.
Kjarsgaard BA, Hamilton DL (1989) The genesis of carbonatites by immiscibility. In: Bell K (ed) Carbonatites — genesis and evolution. Unwin Hyman, London, pp 388–404.
Reid DL, Cooper AF (1992) Oxygen and carbon isotope patterns in the Dicker Willem carbonatite complex, southern Namibia. Chem Geol 94:293–305.
Taylor WR, Green DH (1987) The petrogenetic role of methane: effects on liquidus phase relations and the solubility of reduced C-H-O volatiles. In: Mysen BO (ed) Magmatic processes and physiochemical principles. Geochem Soc Spec Publ 1:121-138.
Wallace ME, Green DH (1988) An experimental determination of primary carbonatite magma composition. Nature 335:343–346.
Waters FG, Erlank AJ (1988) Assessment of the vertical extent and the distribution of mantle metasomatism below Kimberley, South Africa. J Petrol Spec Lithos Issue: 185-204.
Wyllie PJ (1989) Origin of carbonatites: evidence from phase equilibrium studies. In: Bell K (ed) Carbonatites — genesis and evolution. Unwin Hyman, London, pp 500–545.
Wyllie PJ, Huang W-L (1976) Carbonation and melting reactions in the system CaO-MgO-SiO2-CO2. Geology 3:621–624.
Wyllie PJ, Huang W-L, Otto J, Byrnes AP (1983) Carbonation of peridotites and decarbona-tion of siliceous dolomites represented in the system CaO-MaO-SiO2-CO2 to 30 kbar. Tectonophysics 100:359–388.
Author information
Authors and Affiliations
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 1995 Springer-Verlag Berlin Heidelberg
About this chapter
Cite this chapter
Sweeney, R.J., Falloon, T.J., Green, D.H. (1995). Experimental Constraints on the Possible Mantle Origin of Natrocarbonatite. In: Bell, K., Keller, J. (eds) Carbonatite Volcanism. IAVCEI Proceedings in Volcanology, vol 4. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-79182-6_14
Download citation
DOI: https://doi.org/10.1007/978-3-642-79182-6_14
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-642-79184-0
Online ISBN: 978-3-642-79182-6
eBook Packages: Springer Book Archive