Phase relations in the system FeCO3-CaCO3 at 6 GPa and 900-1700 °C and its relation to the system CaCo3-FeCO 3-MgCO3

Anton Shatskiy, Yuri M. Borzdov, Konstantin D. Litasov, Igor N. Kupriyanov, Eiji Ohtani, Yuri N. Palyanov

Research output: Contribution to journalArticlepeer-review

25 Citations (Scopus)

Abstract

The subsolidus and melting phase relations in the CaCO3-siderite system have been studied in multianvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900-1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R3̄c, while CaCO3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO3 from aragonite, Pmcn, to calcite, R3̄c, structure, i.e., the presence of FeCO3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO 3-CaCO3 join extends from 1580 °C (FeCO3) to 1670 °C (CaCO3) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO3. At X(Ca) = 0-30 mol%, 6 GPa and 1500-1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X(Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO2 in (Fe,Ca)CO3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO3-MgCO3-FeCO 3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO > 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO < 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO3 phase relations.

Original languageEnglish
Pages (from-to)773-785
Number of pages13
JournalAmerican Mineralogist
Volume99
Issue number4
DOIs
Publication statusPublished - 2014 Apr
Externally publishedYes

Keywords

  • Ankerite
  • Aragonite
  • Earth's mantle
  • High pressure
  • Phase relations
  • Raman
  • Siderite

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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