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

Abstract.

Evidence is presented for a carbonate-immiscibility phenomena in the upper mantle based on data from a set of strongly metasomatized xenoliths in a basanitic lava flow from Fernando de Noronha Island (southwest Atlantic). A petrological and geochemical study of lherzolitic and harzburgitic xenoliths reveals that the oceanic mantle of this region has been affected by very strong carbonate metasomatism. The metasomatism led to wehrlitization of the primary mantle mineral assemblage (ol, opx, sp). The wehrlitization was the result of interaction between a possibly ephemeral sodic dolomitic melt or fluid with the mantle peridotite according to the following reactions, which include sodic components:

4MgSiO₃+CaMg(CO₃)₂=2Mg₂SiO₄+CaMgSi₂O₆+2CO2

3CaMg(CO₃)₂+CaMgSi₂O₆=4CaCO₃+2Mg₂SiO₄+2CO₂

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 interrelationships between the glass, sulfide and carbonate inclusions permit speculation that silicate, sulfide, and Ca-rich carbonatite melts were in equilibrium with each other and originated from partial melting of metasomatized and wehrlitized peridotite underneath Fernando de Noronha Island. These results support a two-stage model of Ca-rich carbonatite formation: first stage – metasomatic wehrlitization and carbonatization of mantle rocks; second stage – 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.