Contemporary models of carbonatite formation

Kogarko L.N.

Vernadsky Institute of geochemistry and analytical chemistry, Moscow, Russia

 

At present the carbonatites origin is discussed in literature and on conferences. Three models were presented 1 - carbonatite melts were result of partially melting of carbonatised mantle source; 2 - carbonatites are result of liquid immiscibility between silicate and carbonate melts; 3 - carbonatitea are residual liqwuids and were formed during long crystalisation differentiation.

Geodinamical conditions of carbonatites were also discussed Bell (1997) using isotopic signatures of Canadian and African carbonatites proposed that carbonatites of these regions closely connected to plume activity. Our investigations of geochemistry and isotopic features of Maimecha-Kotui rocks showed that the age of carbonatites and other roicks were 250 Ma (lead isochrone). This mean that carbonatites and Siberian super plume manifestated simulteniously and genetically closely connected. Accocding to Gwalani (2003) postcretatious carbonatites of India cjnnected to Decan plum activity.

Plum-Litocphere  metasomatical interaction is very important in origin of carbonatites.

Mantle metasomatism is a very powerful process of global geochemical differentiation. The large-scale mass transportation in the upper mantle is closely connected with the origin of carbonatitic magmatism. We investigated metasomatised mantle nodules from many regions.

In metasomatised zones primary generation olivine, orthopyroxene, spinel and clinopyroxene are crossed by numerous veinlets of fine-grained assemblage which replaces first generation minerals especially orthopyroxene. This finegrained zones contain second generation clinopyroxene, olivine, carbonate, glass, sulfides sometimes apatite, armalcolite, krishtonite. The primary carbonate is Ca-rich with an atomic Ca/(Mg+Ca+Fe+Na) ratio in the range of 0.85-0.96.

The  carbonate metasomatism led to wehrlitization according to the reactions:

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

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

These results support a two stage model of Ca-rich carbonatite formation:

1st stage - metasomatic wehrlitization and carbonatization of mantle rocks;

2nd stage - partial melting of the carbonate-bearing wehrlitic substrate, resulting in generation of calciocarbonatites.