Melting and subsolidus phase relations in the system Na2CO 3-MgCO3±H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle

Anton Shatskiy, Pavel N. Gavryushkin, Igor S. Sharygin, Konstantin D. Litasov, Igor N. Kupriyanov, Yuji Higo, Yuri M. Borzdov, Ken Ichi Funakoshi, Yuri N. Palyanov, Eiji Ohtani

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36 Citations (Scopus)


Phase relations in the Na2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1400 °C temperatures. Sub-solidus assemblages are represented by Na 2CO3+Na2Mg(CO3)2 and Na2CO3+Na2Mg(CO3) 2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The Na2CO3-Na 2CO3+Na2Mg(CO3)2 eutectic is established at 1200 °C and 29 mol% MgCO3. Melting of Na 2CO3 occurs between 1350 and 1400 °C. We propose that Na2CO3+Na2Mg(CO3)2 disappears between 1200 and 1250 °C via congruent melting. Magnesite remains as a liquidus phase above 1300 °C. Measurable amounts of Mg in Na 2CO3 suggest an existence of MgCO3 solid-solutions in Na2CO3 at given experimental conditions. The maximum MgCO3 solubility in Na-carbonate of about 9 mol% was established at 1100 and 1200 °C. The Na2CO3 and Na2CO3+Na2Mg(CO3)2 compounds have been studied using in situ X-ray coupled with a DIA-type multi-anvil apparatus. The studies showed that eitelite is a stable polymorph of Na2CO3+Na2Mg(CO3)2 at least up to 6.6 GPa and 1000 °C. In contrast, natrite, γ-Na 2CO3, is not stable at high pressure and is replaced by β-Na2CO3. The latter was found to be stable at pressures up to 11.7 GPa at 27 °C and up to 15.2 GPa at 1200 °C and temperatures at least up to 800 °C at 2.5 GPa and up to 1000 °C at 6.4 GPa. The X-ray and Raman study of recovered samples showed that, under ambient conditions, β-Na2CO3 transforms back to γ-Na2CO3. Eitelite [Na2CO 3+Na2Mg(CO3)2] would be an important mineral controlling insipient melting in subducting slab and upwelling mantle. At 6 GPa, melting of the Na2CO3+Na2Mg(CO 3)2+MgCO3 assemblage can be initiated, either by heating to 1300 °C under "dry" conditions or at 900-1100 °C under hydrous conditions. Thus, the Na2CO3+Na 2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under "dry" conditions and cause formation of the Na- and Mg-rich carbonatite melts similar to those found as inclusions in olivines from kimberlites and the deepest known mantle rock samples-sheared peridotite xenoliths (190-230 km depth).

Original languageEnglish
Pages (from-to)2172-2182
Number of pages11
JournalAmerican Mineralogist
Issue number11-12
Publication statusPublished - 2013
Externally publishedYes


  • Alkaline carbonates
  • Earth's mantle
  • Eitelite
  • High-pressure experiment
  • In situ X-ray diffraction
  • Natrite
  • Raman

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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