Phase relations in the system Na 2 CO 3 –CaCO 3 –MgCO 3 at 3 GPa with implications for carbonatite genesis and evolution

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Abstract

The phase relations in the system Na 2 CO 3 −CaCO 3 −MgCO 3 have been studied at 3 GPa and 700–1285 °C using a Kawai-type multianvil press. At 700 °C, the system has five intermediate compounds: dolomite, Mg-bearing Na 2 Ca 4 (CO 3 ) 5 burbankite, Na 2 Ca 3 (CO 3 ) 4 , Na 4 Ca(CO 3 ) 3 , and eitelite. As temperature increases to 800 °C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na 2 Ca 4 (CO 3 ) 5 disappears. At 850 °C, Na 4 Ca(CO 3 ) 3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875 ± 25 °C to form Na 2 Ca 3 (CO 3 ) 4 . Incongruent melting of eitelite to magnesite and liquid, occurs at 925 ± 25 °C. Mg-bearing shortite melts incongruently at 950 ± 50 °C, producing Na 2 Ca 3 (CO 3 ) 4 and liquid. Na 2 Ca 3 (CO 3 ) 4 disappears at 1000 °C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na 2 Ca 3 (CO 3 ) 4 , Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850 °C and 52Na 2 CO 3 ∙48(Ca 0.62 Mg 0.38 )CO 3 . The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3 GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na 2 Ca 2 (CO 3 ) 3 (Amm2) shortite, Na 2 Ca(CO 3 ) 2 (P2 1 ca) nyerereite, Na 2 Mg(CO 3 ) 2 (R3¯) eitelite (0.1 GPa) → Na 2 (Ca 0.97–0.98 Mg 0.02–0.03 ) 4 (CO 3 ) 5 (P6 3 mc), Na 2 (Ca ≥0.91 Mg ≤0.09 ) 3 (CO 3 ) 4 (P1n1), Na 2 (Ca ≥ 0.81 Mg 0≤0.19 )(CO 3 ) 2 (R3¯) nyerereite, Na 2 (Ca 0.77–0.93 Mg 0.07–0.23 ) 2 (CO 3 ) 3 (Amm2) shortite, Na 4 (Ca 0.90–0.98 Mg 0.02–0.10 )(CO 3 ) 3 (Ia3d), Na 2 (Mg ≥0.9 Ca 0≤0.1 )(CO 3 ) 2 (P2 1 ca) eitelite (3 GPa) → Na 2 (Ca ≥0.87 Mg 0≤0.13 ) 4 (CO 3 ) 5 (P6 3 mc), Na 2 (Ca ≥0.89 Mg ≤0.11 ) 3 (CO 3 ) 4 (P1n1), Na 4 (Ca ≥ 0.7 Mg 0≤0.3 )(CO 3 ) 3 (Ia3d), Na 2 (Mg ≥0.92 Ca 0≤0.08 )(CO 3 ) 2 (P2 1 ca) eitelite (6 GPa). Using the present results at 3 GPa and previous data at 6 GPa in the Na 2 CO 3 −CaCO 3 −MgCO 3 system, we constrained isopleths of the Na 2 CO 3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200 km toward sodic dolomitic with the ~52 mol% Na 2 CO 3 at 80–120 km depths.

Original languageEnglish
Pages (from-to)74-89
Number of pages16
JournalLithos
Volume330-331
DOIs
Publication statusPublished - 1 Apr 2019

Keywords

  • Carbonatite
  • Continental lithosphere
  • High-pressure experiments
  • Mantle metasomatism
  • Na CO -CaCO -MgCO , Na-Ca carbonates
  • Na2CO3-CaCO3-MgCO3, Na-Ca carbonates
  • RAMAN-SPECTRA
  • BEARING ECLOGITE
  • HIGH-PRESSURE
  • UDACHNAYA-EAST KIMBERLITE
  • LITHOSPHERIC MANTLE
  • CRYSTAL-STRUCTURE
  • GARNET LHERZOLITE
  • CALCITE CARBONATITE
  • MELTING RELATIONS
  • RICH MELTS

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