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Relationships between silicate melts and carbonate-precipitating melts in CaO-MgO-SiO2-CO2-H2O at 2 kbar

Die Beziehungen zwischen silikarischen Schmelzen und karbonatbildenden Schmelzen im System CaO-MgO-SiO2-CO2-H2O bei 2 kbar

Summary

Phase fields intersected by three joins in the System CaO-MgO-SiO2-CO2-H2O at 2 kbar were investigated experimentally to determine the melting relationships and the sequences of crystallization of liquids co-precipitating silicate minerals and carbonates. These joins connect SiO2 to three mixtures of CaCO3-MgCO3-Mg(OH)2 with compositions on the primary îield for calcite, between the composition CaCO3 and the low-temperature (650°C eutectic liquid co-precipitating calcite, dolomite and periclase. In the pseudo-quaternary tetrahedron calcite-magnesite-brucite-diopside, two of the significant reactions found are: (1) a eutectic at 650°C, calcite + dolomite + periclase + forsterite + vapor = liquid, and (2) a peritectic at 1038°Cwhich is either calcite + åkermanite + forsterite + vapor = monticellite + liquid calcite + monticellite + forsterite + vapor = åkermanite + liquid. The eutectic liquid has high MgO/CaO and CO2/H2O and only 2–3% SiO2 (estimated 15–20% MgCO3, 35–40% CaCO3, 40–45% Mg(OH)2, and 5–6% Mg2SiO4). The composition joins intersect a thermal maximum for åkermanite + forsterite + vapor = liquid, which separates high-temperature liquids precipitating silicates together with a little calcite, from low-temperature liquids precipitating carbonates with a few percent of forsterite; there is no direct path between the silicate and synthetic carbonatite liquids on these joins, but it is possible that fractionating liquid paths diverging from the joins may connect them. More complex relationships involving the pprecipitatioon off monticellite and åkermanite are also outlined. Magnetite-magnesioferrite may replace periclase in natural magmatic systems. The results indicate that the assemblage calcite-dolomite-magnetite-forsterite represents the closing stages of crystallization of carbonatites, whereas assemblages such as calcite-magnetite-forsterite and dolomite-magnetite-forsterite span the whole range of carbonatite evolution in terms of temperature and composition, and provide the link between liquids precipitating silicates and those precipitating carbonates.

Zusammenfassung

Phasenfelder, die durch den Schnitt von drei Verbindungslinien im System CaO-MgO-SiO2-CO2-H2Odefiniert werden, wurden experimentell bei 2 kbar untersucht, um die Schmelzbeziehungen und die Kristallisationsfolge von Schmelzen, die gleichzeitig silikatische und karbonatische Minerale ausscheiden, zu bestimmen. Diese Linien verbinden SiO2 mit drei Mischungen von CaCO3-M9CO3-Mg(OH)2 mit Zusammensetzungen im primären Calcitfeld, zwischen der Zusammensetzung CaCO3 und der tieftemperierten (650°C Calcit-, Dolomit- und Periklasbildenden eutektischen Schmelze. Zwei wichtige im ppseudo-quaternären Tetraeder Calcit-Magnetit-Brucit-Diopsid gefundene Reaktionen sind: (1) Ein Eutektikum bei 650°C Calcit + Dolomit + Periklas + Forsterit + Vapor = Liquid und (2) ein Peritektikum bei 1038°C mit entweder Calcit + Åkermanit + Forsterit + Vapor = Monticellit + Liquid oder Calcit + Monticellit + Forsterit + Vapo = Åkermanit + Liquid Die eutektische Schmelze zeigt hohe MgO/CaO und CCO2H2O Verhältnisse und nur 2–3% SiO2(geschätzter Anteil an MgCO315–20%, CaCO3 35–40%, Mg(OH)2 40–50% und Mg2SiO4 5–6%). Die Verbindungslinie schneidet ein thermisches Maximum von Åkermanit + Forsterit + Vapor = Liquid, das höher temperierte Schmelzen, die Silikate gemeinsam mit etwas Clacit ausscheiden, von tiefer temperierten Schmelzen trennt, aus denen sich Karbonate gemeinsam mit wenigen Prozenten Forsterit abscheiden. Es existiert keine direkte Verbindung zwischen silikatischen und synthetischen karbonatitischen Schmelzen entlang dieser Verbindungslinien, es wäre aber möglich, daß Fraktionierungspfade, die von diesen Verbindungslinien ausgehen, sie verbinden. Komplexere Beziehungen, die die Kristallisation von Monticellit und Åkermanit beinhalten, werden ebenfalls aufgezeigt. Magnetit-Magnesioferrit könntean die Stelle von Periklas in nnatürlichenmagmatischen Systemen treten. Die Ergebnisse weisen darauf bin, daß die Vergesellschaftung Calcit-Dolomit-Magnetit-Forsterit das Endstadium der Karbonatitkristallisation repräsentiert, während die Vergesellsschaftungen von Calcit-Magnetit-Forsterit bzw. Dolomit-Magnetit-Forsterit die gesamte Spannweite der Karbonatitevolution hinsichtlich Temperatur und Zusammensetzung umfassen und demnach ein Verbindungsglied zwischen silikat- und karbonatausscheidenden Schmelzen darstellen.

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OOtto, J.W., Wyllie, P.J. Relationships between silicate melts and carbonate-precipitating melts in CaO-MgO-SiO2-CO2-H2O at 2 kbar. Mineralogy and Petrology 48, 343–365 (1993). https://doi.org/10.1007/BF01163107

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Keywords

  • Calcite
  • CaCO3
  • Forsterit
  • Mg2SiO4
  • MgCO3