The Role of Sulphide-Carbonate-Silicate and Carbonate-Silicate Liquid Immiscibility in the Genesis of Ca-Carbonatites

Abstract

During the investigation of a harzburgite nodule from the Montana Clara Volcano (Canary island archipelago) the evidence of the primary carbonate melt was discovered. This carbonate is enriched in calcium and it occurs together with glass containing sulphide globules. In addition to primary olivine and orthopyroxene there are pockets of fine-grained minerals belonging to the metasomatic second generation (more magnesium olivine, sodium-bearing clinopyroxene. less aluminous spinels). The metasomatic assemblage was formed by reaction of sodium-bearing dolomitic melt with the harzburgite according to the reactions: 2 MgSiO 3 + CaMg ( CO 3 ) 2 2 Mg 2 SiO 4 + CaMgSi 2 O h 2 CO 2 3 CaMg ( CO 3 ) 2 + CaMgSi 2 O 6 = 4 CaCO 3 + 2 Mg 2 SiO 4 + CO 2 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429070877/6179bb61-7023-400d-b003-6c487c25041b/content/eq23.tif"/> The calciocarbonatite and sulphide phase almost invariably form globules in the silicate glass indicating the existence of three immiscible liquids under upper mantle conditions resulting from melting of the metasomatised mantle material during the uprising and adiabatic decompression. Our experiments reveal carbonate-silicate-sulphide immiscibility. Therefore the investigated mineral assemblage including carbonate and glass can be considered as a micro model of the generation of the Ca-rich carbonatitic magmas during the processes of the partial melting of carbonatized merasomatized oceanic mantle. The development of carbonalite magmatism on Canary Islands (Lanzarote). Cape Verde Islands is likely to be related to the partial melting of carbonatized mantle in the South Atlantic, which took place over a vast territory. Another example of carbonate-silicate immiscibility under crustal conditions is represented by phonolites (Polar Siberia. Maimecha-Kotui) containing carbonate globules. We investigated some dykes of carbonatitic massit Dolbykha which comprise olivine and melilite nephelinites. nosean. calcite and cancrinite phonolites. calcite trachytes and calcite carbonatites. Some ultra alkaline phonolitic dykes contain carbonate-bearing globules. Globules consist of polycrystalline calcite aggregate and contain albite. mica, apatite. Sr-lueshite. zircon, ancylite. ilmenite and strontianiteT. There are phenocrysts of albite. mica and ilmenite in phonolites. There are also albite. mica, calcite and nepheline present in the groundmass. The analysis of these materials in the light of experimental data on the liquid immiscibility in carbonate-silicate systems suggests that the separation of carbonatite melts from phonolitic ones took place due to the immiscibility in liquid state. We propose that originally carbonate melts containe significantly higher alkali concentrations which were subsequently lost in fluid phase due to incongruent dissolution of calcium-sodium carbonates in aqueous fluid at low temperatures. Our discovery of nyerereite in carbonatite of Polar Siberia confirms this assumption. Thus one of the very important mechanisms of the genesis of Ca-rich carbonatite melts was the formation of liquid immiscibility which may take place in mantle or crustal conditions.