Abstract
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 language | English |
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Pages (from-to) | 2172-2182 |
Number of pages | 11 |
Journal | American Mineralogist |
Volume | 98 |
Issue number | 11-12 |
DOIs | |
Publication status | Published - 2013 |
Externally published | Yes |
Keywords
- Alkaline carbonates
- Earth's mantle
- Eitelite
- High-pressure experiment
- In situ X-ray diffraction
- Natrite
- Raman
ASJC Scopus subject areas
- Geophysics
- Geochemistry and Petrology