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Liquid immiscibility and the origin of alkali-poor carbonatites

Published online by Cambridge University Press:  05 July 2018

B. A. Kjarsgaard  and
D. L. Hamilton
B. A. Kjarsgaard
Affiliation:
Department of Geology, The University, Manchester M13 9PL
D. L. Hamilton
Affiliation:
Department of Geology, The University, Manchester M13 9PL
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Abstract

The work on liquid immiscibility in carbonate-silicate systems of Freestone and Hamilton (1980) has been extended to include alkali-poor and alkali-free compositions. Immiscibility is shown to occur on the joins albite-calcite and anorthite-calcite at 5 kbar. These results make it possible to interpret ocellar structure between calcite-rich spheroids in lamproite or kimberlite host rock as products of liquid immiscibility. The common sequence of rock types found in carbonatite complexes of melilitite-ijolite-urtite-phonolite is interpreted as being the result of both fractional crystallization and liquid fractionation, the corresponding carbonatite composition changing from nearly pure CaCO3 (±MgCO3) progressively to natrocarbonate. A carbonate melt cooling in isolation will suffer crystal fractionation, the residual liquid producing the rarer ferrocarbonatites, etc., whilst the crystal accumulate of calcite (dolomite) plus other phases such as magnetite, apatite, baryte, pyrochlore, etc., are the raw material for the coarse-grained intrusive carbonatites commonly found in ring complexes.

Keywords

carbonatitesliquid immiscibilitycarbonate-silicate systems
Type
Recent Developments in Experimental Petrology and Mineralogy
Information
Mineralogical Magazine , Volume 52 , Issue 364 , March 1988 , pp. 43 - 55
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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