Carbonate metasomatism of the oceanic mantle beneath Fernando de Noronha Island, Brazil

Evidence is presented for a carbonate-immiscibility phenomena in the upper mantle based on data from a set of strongly metasomatized xenoliths in a ba sanitic lava flow from Fernando de Noronha Island (southwest Atlantic). A p etrological and geochemical study of lherzolitic and harzburgitic xenoliths reveals that the oceanic mantle of this region has been affected by very s trong carbonate metasomatism. The metasomatism led to wehrlitization of the primary mantle mineral assemblage (ol, opx, sp). The wehrlitization was th e result of interaction between a possibly ephemeral sodic dolomitic melt o r fluid with the mantle peridotite according to the following reactions, wh ich include sodic components: 4MgSiO(3) + CaMg(CO3)(2) = 2Mg(2)SiO(4) + CaMgSi2O6 + 2CO(2) 3CaMg(CO3)(2) + CaMgSi2O6 = 4CaCO(3) + 2Mg(2)SiO(4) + 2CO(2) The olivine has abundant micro-inclusions consisting of Na-Al-Si-rich glass , Fe, Ni and Cu-monosulfide, Ca-rich carbonate and dense CO2. The interrela tionships between the glass, sulfide and carbonate inclusions permit specul ation that silicate, sulfide, and Ca-rich carbonatite melts were in equilib rium with each other and originated from partial melting of metasomatized a nd wehrlitized peridotite underneath Fernando de Noronha Island. These resu lts support a two-stage model of Ca-rich carbonatite formation: first stage - metasomatic wehrlitization and carbonatization of mantle rocks; second s tage - partial melting of the carbonate-bearing wehrlitic rock resulting in the formation of immiscible silicate, sodic carbonate and sulfide liquids and the ultimately generation of calciocarbonatites.