Carbonate Metasomatism of Oceanic Lithosphere and Carbonate-Silicate-Sulphide Liquid Immiscibility

Abstract

Mantle metasomatism is a very powerful process of global differentiation. The large-scale transportation of material in the upper mantle is closely connected with the origin of alkaline magmatism. For instance we investigated the isotopic signature of Kola Peninsula alkaline rocks (Kogarko et al 1986) and related gigantic rare metal deposits. These data demonstrated that they originated from a quite depleted mantle source. So there is a paradox: on the one hand these rocks are extremely enriched in rare elements, on the other, they have a depleted mantle signature. The enormous concentration of rare elements in alkaline rocks of this region is mostly related to large-scale processes of mantle metasomatism and is confined to a very narrow time span (371-361 Ma) so the primary signature stayed unchanged. There are many chemical types of mantle metasomatism. During many years we carried out the investigations of metasomatised mantle material from different regions of the globe. The petrologic and geochemical study of lherzolitic and harzburgitic xenoliths from melilitites of Montana Clara island (Canary archipelago ) and basanites of Fernando de Noronha island revealed that oceanic mantle of these regions has been affected by very strong carbonate metasomatism. Partial melting of carbonated metasomatized mantle results in carbonate-silicate-sulphide liquid immiscibility, which has been confirmed by our experiments.