Liquid immiscibility and the origin of alkali-poor carbonatites

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 isshown to occur on the j oins 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 CaCO 3 ( + MgCO3) progressively tonatrocarbonate. A carbonate melt cooling in isolation will suffer crystal fractionation, the residual liquid producing the rarer ferrocarbonatites, tc., whilst the crystal accumulate of calcite (dolomite) plus other phases uch as magnetite, apatite, baryte, pyrochlore, etc., are the raw material for the coarse-grained intrusive carbonatites commonly found in ring complexes