PRIMARY CA-RICH CARBONATITE MAGMA AND CARBONATE-SILICATE-SULFIDE 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 thi s 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 commo n carbonatite type occurring in crustal complexes. The carbonate in th e Montana Clara harzburgite host is restricted to wehrlitic alteration zones and is intimately associated with a second generation of minera ls, mainly olivine, clinopyroxene and spinel, with glass of syenitic c omposition, and with Fe-Cu-rich sulphides. The metasomatic assemblage was formed by reaction of a sodium-bearing dolomitic melt, derived fro m a somewhat deeper level in the upper mantle, with the harzburgite mi neral assemblage at a pressure of 15 kbars, or lower. As a result of t he reaction the residual carbonatite melt became more enriched in calc ium. The calciocarbonatite and sulphide phases almost invariably form globules in the silicate glass, indicating the existence of three immi scible liquids under upper mantle conditions. Several alkaline complex es contain carbonatites occurring with syenitic rock types and its see ms feasible that the formation of such close associations might have b een influenced by processes of liquid immiscibility which took place u nder upper mantle conditions.