Abstract
The phase relations in the Na 2 CO 3 -(Fe 0.87 Mn 0.06 Mg 0.07 )CO 3 system have been studied in Kawai-type multi-anvil experiments using graphite capsules at 6.0 GPa and 900–1400 °C. Subsolidus assemblages comprise the stability fields of Na 2 CO 3 + Na 2 Fe(CO 3 ) 2 and Na 2 Fe(CO 3 ) 2 + siderite with the transition boundary at X (Na 2 CO 3 ) = 50 mol%. Intermediate Na 2 Fe(CO 3 ) 2 compound has rhombohedral R 3 eitelite structure with cell parameters a = 4.9712(16), c = 16.569(4) A, V = 354.61(22). The Na 2 CO 3 -Na 2 Fe(CO 3 ) 2 eutectic is established at 1000 °C and 66 mol% Na 2 CO 3 . Na 2 Fe(CO 3 ) 2 disappears between 1000 and 1100 °C via incongruent melting to siderite and a liquid containing about 55 mol% Na 2 CO 3 . Siderite remains a subliquidus phase at 1400 °C at X (Na 2 CO 3 ) ≤ 30 mol%. The ternary Na 2 CO 3 -FeCO 3 -MgCO 3 system can be built up from the corresponding binary systems: two systems with intermediate Na 2 (Mg,Fe)(CO 3 ) 2 phase, which melts congruently at the Mg-rich side and incongruently at the Fe-rich side, and the (Mg,Fe)CO 3 system with complete solid solution. The phase relations suggest that the maximum contribution of FeCO 3 component into the lowering solidus temperatures of Na-bearing carbonated mantle domains could not exceed several tens of degrees Celsius.
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