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The role of liquid immiscibility in the genesis of carbonatites — An experimental study

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Abstract

The two-liquid field between alkali-carbonate liquids and phonolite or nephelinite magmas from the Oldoinyo Lengai volcano has been determined between 0.7 and 7.6 kb and 900°–1,250° C. The miscibility gap expands with increase in \(P_{CO_2 }\) and decrease in temperature. Concomitantly there is a rotation of tie-lines so that the carbonate liquids become richer in CaO. The element distribution between the melts indicates that a carbonate liquid equivalent in composition to Oldoinyo Lengai natrocarbonatite lava would have separated from a phonolitic rather than a nephelinitic magma. CO2-saturated nephelinites coexist with carbonate liquids much richer in CaO than the Lengai carbonatites, but even so these liquids have high alkali concentrations. If the sövites of hypabyssal and plutonic ijolite-carbonatite complexes originated by liquid immiscibility, then large quantities of alkalis have been lost, as is suggested by fenitization and related phenomena. The miscibility gap closes away from Na2O-rich compositions, so that the tendency to exsolve a carbonatite melt is greater in salic than in mafic silicate magmas. The two-liquid field does not approach kimberlitic compositions over the range of pressures studied, suggesting that the globular textures observed in many kimberlite sills and dykes may be the result of processes other than liquid immiscibility at crustal pressures.

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Author notes

  1. I. C. Freestone

    Present address: British Museum Research Laboratory, Great Russell Street, WC1B 3DG, London, England

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  1. Department of Geology, University of Manchester, M 13 9 PL, Manchester, England

    I. C. Freestone & D. L. Hamilton

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  1. I. C. Freestone
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  2. D. L. Hamilton
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Freestone, I.C., Hamilton, D.L. The role of liquid immiscibility in the genesis of carbonatites — An experimental study. Contr. Mineral. and Petrol. 73, 105–117 (1980). https://doi.org/10.1007/BF00371385

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