Liquid immiscibility and the origin of alkali-poor carbonatites

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 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 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.

Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

“…The liquid miscibility relations in the carbonatitesilicate system have been studied intensively in the past decades (Koster VanGroos andWyllie 1963, 1966;Hamilton and Freestone 1979;Freestone and Hamilton 1980;Kjarsgaard and Hamilton 1988;Brooker and Hamilton wt. % Crn in c r e a s in g A l-c o n t e n t…”

The Nature and Origin of the REE Mineralization in the Wicheeda Carbonatite, British Columbia, Canada

“…2) liquid immiscibility (exsolution of the carbonatite melt from the silicate melt) (Freestone and Hamilton, 1980;Kjarsgaard and Hamilton, 1988;, or 3) directly from partial melting of carbonated mantle peridotite Eggler, 1974;Eggler, 1978;Eggler, 1989;Sweeney, 1994;Gittins and Harmer, 1997).…”

Silicate-carbonate-salt liquid immiscibility and origin of the sodalite-haüyne rocks: study of melt inclusions in olivine foidite from Vulture volcano, S. Italy

“…an.5) has Mg#=0.70 -0.65, is rich in Ti and Al, and is similar in composition to that of the transition zone of Cpx II-III. K-feldspar from melt inclusions in Cpx I, is orthoclase contains a minor anorthite component, and up to 0.5 wt.% FeO, 0.12 -0.13 wt.% BaO, and SrO ( 3 and is similar to that of alkaline trachyte. On heating, inclusions with or without salt aggregates behave differently.…”

“…Experimental studies of synthetic and natural alkaline systems [3][4][5][6][7] evidenced that liquid silicate-carbonate immiscibility occurs in a wide range of temperatures and pressures, and depends on the composition of parental silicate melt, degree of fluid saturation, and oxygen fugacity. An increase in the amount of alkalies and CO 2 pressure expands the immiscibility field [8][9][10][11].…”