Carbonate melts at the lower mantle conditions: to superdeep diamonds genesis

Spivak A.V.*, Solopova N.A. *^, **, Litvin Yu.A.*, Dubrovinsky L.S.**

* Institute of Experimental Mineralogy RAS, Chernogolovka, Russia, spivak@iem.ac.ru

** Bayerisches Geoinstitut, University Bayreuth, Bayreuth, Germany

 

PT conditions of СаСО₃, MgCO₃, and Na₂CO₃ melting, stability of the melts, and their decomposition were investigated at static pressures up to 46 GPa and temperatures up to 4000 K with the use of diamond anvil cell and laser heating. PT phase diagrams were plotted for СаСО₃, MgCO₃ and Na₂CO₃ system.  Evidences of congruent melting of Ca-, Mg-carbonates were obtained above 2000 K and at 11-43 GPa. Meanwhile, Na-carbonate melts congruently above 1200 K and up to 46 GPa. It was specified that the carbonate melts are stable within widely ranges of temperatures and pressures. High temperature boundaries of melting fields were determined with using of static high pressure and high temperatures experiments. It was shown, that at high temperature boundaries Ca-, Mg-, and Na-carbonates decompose with formation of corresponding oxides and CO₂ (probably fluid). CO₂ decomposes with formation of solid carbon – diamond or graphite, depending on the thermodynamic and kinetic conditions of nucleation. These processes may be explained by a two-stage decomposition reaction:

 

СаСО₃ = СаО + CO₂; CO₂ = C + O₂ (1),

MgСО₃ = MgО + CO₂; CO₂ = C+O₂ (2),

Na₂СО₃ = Na₂О + CO₂; CO₂ = C + O₂ (3).

 

It was determined that melts of СаСО₃ - С, MgCO₃ – С and Na₂CO₃ – С systems are effective diamond forming as melt-solution media at low temperature boundaries of melting fields at 18-25 GPa and 1700-2300 K. 

These results are promising for applications to model multicomponent compositions of carbonate-oxide-silicate parental media of superdeep diamonds and their primary mineral inclusions.

 

The study was supported by The Ministry of education and science of Russian Federation, projects 8317 and 16.740.11.0621and RFBR grants 13-05-00835, 12-05-33044 и 11-05-00401.