Relationships between silicate melts and carbonate-precipitating melts in CaO-MgO-SiO 2 -CO 2 -H 2 O at 2 kbar

TLDR: In this article, phase fields intersected by three joins in the System CaO-MgO-SiO-CO_2-H_2O at 2 kbar were investigated experimentally to determine the melting relationships and the sequences of crystallization of liquids co-precipitating silicate minerals and carbonates.

Abstract: Phase fields intersected by three joins in the System CaO-MgO-SiO_2-CO_2-H_2O 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 SiO_2 to three mixtures of CaCO_3-MgCO_3-Mg(OH)_2 with compositions on the primary field for calcite, between the composition CaCO_3 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°C which is either calcite + akermanite + forsterite + vapor = monticellite + liquid calcite + monticellite + forsterite + vapor = akermanite + liquid. The eutectic liquid has high MgO/CaO and CO_2/H_2O and only 2–3% SiO_2 (estimated 15–20% MgCO_3, 35–40% CaCO_3, 40–45% Mg(OH)_2, and 5–6% Mg_2SiO_4). The composition joins intersect a thermal maximum for akermanite + 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 precipitation of monticellite and akermanite 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.