New data on the system Na2CO3–CaCO3–MgCO3 at 6 GPa with implications to the composition and stability of carbonatite melts at the base of continental lithosphere

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Abstract

Subsolidus and melting phase relationships in the system Na 2 CO 3 –CaCO 3 –MgCO 3 have been studied at 6 GPa and 900–1250 °C using a Kawai-type multianvil press. At 900 and 1000 °C, the system has four intermediate compounds: Na 2 Ca 4 (CO 3 ) 5 burbankite, Na 2 Ca 3 (CO 3 ) 4 , Na 4 Ca(CO 3 ) 3 , and Na 2 Mg(CO 3 ) 2 eitelite. The Na-Ca compounds dissolve noticeable amounts of Mg component, whereas eitelite dissolves a few percents of Ca component: Na 2 (Ca ≥0.91 Mg ≤0.09 ) 4 (CO 3 ) 5 , Na 2 (Ca ≥0.94 Mg ≤0.06 ) 3 (CO 3 ) 4 , Na 4 (Ca ≥0.67 Mg ≤0.33 )(CO 3 ) 3 , and Na 2 (Mg ≥.93 Ca ≤0.07 )(CO 3 ) 2 . At 1050 °C, the system is complicated by an appearance of dolomite. Na-Ca burbankite decomposes at 1075 ± 25 °C to aragonite plus Na 2 Ca 3 (CO 3 ) 4 . Na 4 Ca(CO 3 ) 3 and eitelite disappear via congruent melting between 1200 and 1250 °C. Na 2 Ca 3 (CO 3 ) 4 remains stable through the whole studied temperature range. The liquidus projection of the studied ternary system has eight primary solidification phase regions for magnesite, dolomite, calcite-dolomite solid solutions, aragonite, Na 2 Ca 3 (CO 3 ) 4 , Na 4 Ca(CO 3 ) 3 , and Na 2 CO 3 solid solutions. The system has five ternary peritectic reaction points and one minimum on the liquidus at 1050 °C and 48Na 2 CO 3 ·52(Ca 0.75 Mg 0.25 )CO 3 . The minimum point resembles a eutectic controlled by a four-phase reaction, by which a liquid transforms into three solid phases upon cooling: Na 2 (Ca 0.94 Mg 0.06 ) 3 (CO 3 ) 4 , Na 4 (Ca 0.67 Mg 0.33 )(CO 3 ) 3 , and Na 2 (Mg 0.93 Ca 0.07 )(CO 3 ) 2 eitelite. Since at 6 GPa, the system has a single eutectic, there is no thermal barrier preventing continuous liquid fractionation from alkali-poor toward Na-rich dolomitic compositions. Cooling of the Na-Ca-Mg carbonatite melt from 1400 to 1100 °C within the lherzolite substrate will be accompanied by magnesite crystallization and wehrlitization keeping calcium number of the melt at 40 and shifting the Na 2 CO 3 content to ≥40 mol%. In the case of the eclogitic wall rock, the cooling will be accompanied by dolomite crystallization keeping calcium number of the melt at 60–65 and shifting the Na 2 CO 3 content to ≥30 mol%.

Original languageEnglish
Pages (from-to)50-60
Number of pages11
JournalChemical Geology
Volume515
DOIs
Publication statusPublished - 20 Jun 2019

Keywords

  • Continental lithosphere
  • Eitelite, burbankite, carbonatite
  • High-pressure experiments
  • Mantle metasomatism
  • Na CO -CaCO -MgCO , Na-Ca carbonates
  • Na2CO3-CaCO3-MgCO3, Na-Ca carbonates
  • BEARING ECLOGITE
  • UDACHNAYA-EAST KIMBERLITE
  • INCLUSIONS
  • GARNET LHERZOLITE
  • PERIDOTITE XENOLITHS
  • PIPE
  • MANTLE
  • DIAMOND FORMATION
  • PHASE-RELATIONS
  • CANADA-GOLD-MINE

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