Indexed on: 01 Jun '93Published on: 01 Jun '93Published in: Mineralogy and Petrology
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.