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

摘要

During the investigation of a harzburgite nodule from the Montana Clara Volcano (Canaiy 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: 2MgSiO_3 + CaMg(CO_3)_2 = 2Mg_2SiO_4 + CaMgSiO_4 + 2CO_2 3CaMg(CO_3)_2 + CaMgSi_2O_6 = 4CaCO, +2Mg_2SiO_4 + CO_2 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 tile metasoinatised 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 earbonatitic magmas during the processes of the partial melting of carbonatized merasomatized oceanic mantle. The development of carbonatito magnmtism 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 carhonate-silicate immiscibility under cnistal conditions is represented by phonoliles (Polar Siberia, Maimecha-Kotui) containing carbonate globules.Wo investigated some dykes of uarbonntitic massif Dolbykha which comprise olivine and melilite nephelinites, nosean. calcite and cancrinite phonoliles. calcitc trachytes and calcite carbonatites. Some ultra alkaline phonolitic dykes contain carbonate-bearing globules.Olobules consist of polycrystalline calcite aggregate and contain albitc, mica, apatite. Sr-lueshite. zircon, ancylite, ilmenite and strontianiteT. There are phenocrysts of albite, mica and ihnenite in photiolites. There are also albitc, mica, calcite and nepheline present in the groundnmss. The analysis of these materials in the light of experimental data on the liquid immiscibility in carhonate-silicate systems suggests that the separation of carbonatite melts from phonolitic ones took place due to the immiscibility in liquid state. We propose thai originally carbonate melts contains significantly higher alkali concentrations which were subsequently lost in fluid phase due to incongruenl 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 carbonatito melts was the formation of liquid immiscibility which may take place in mantle or cnistal conditions.