Primary Ca-rich carbonatite magma and carbonate-silicate-sulphide liquid immiscibility in the upper mantle

A primary carbonate phase with Ca/(Ca+Mg) in the range 0.85–0.95 has been identified in a metasomatized, depleted harzburgite nodule from Montana Clara Island, Canary Islands; textural relations show that this carbonate represents a quenched liquid. Although magnesian carbonate melts have been described from upper mantle peridotites, this is the first reported occurrence of a primary magma within peridotite nodules which has the composition of calciocarbonatite, by far the most common carbonatite type occurring in crustal complexes. The carbonate in the Montana Clara harzburgite host is restricted to wehrlitic alteration zones and is intimately associated with a second generation of minerals, mainly olivine, clinopyroxene and spinel, with glass of syenitic composition, and with Fe−Cu-rich sulphides. The metasomatic assemblage was formed by reaction of a sodiumbearing dolomitic melt, derived from a somewhat deeper level in the upper mantle, with the harzburgite mineral assemblage at a pressure of 15 kbars, or lower. As a result of the reaction the residual carbonatite melt became more enriched in calcium. The calciocarbonatite and sulphide phases almost invariably form globules in the silicate glass, indicating the existence of three immiscible liquids under upper mantle conditions. Several alkaline complexes contain carbonatites occurring with syenitic rock types and its seems feasible that the formation of such close associations might have been influenced by processes of liquid immiscibility which took place under upper mantle conditions. Editorial responsibility: I. Parsons