RELATIONSHIPS BETWEEN SILICATE MELTS AND CARBONATE-PRECIPITATING MELTS IN CAO-MGO-SIO2-CO2-H2O AT 2 KBAR

Phase fields intersected by three joins in the system CaO-MgO-SiO2-CO2 -H2O at 2 kbar were investigated experimentally to determine the melti ng relationships and the sequences of crystallization of liquids co-pr ecipitating silicate minerals and carbonates. These joins connect SiO2 to three mixtures of CaCO3-MgCO3-Mg(OH)2 with compositions on the pri mary field for calcite, between the composition CaCO3 and the low-temp erature (650-degrees-C) eutectic liquid co-precipitating calcite, dolo mite and periclase. In the pseudo-quaternary tetrahedron calcite-magne site-brucite-diopside, two of the significant reactions found are: (1) a eutectic at 650-degrees-C, calcite + dolomite + periclase + forster ite + vapor = liquid, and (2) a peritectic at 1038-degrees-C which is either calcite + akermanite + forsterite + vapor = monticellite + liqu id or calcite + monticellite + forsterite + vapor = akermanite + liqui d. 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% Mg2S iO4). The composition joins intersect a thermal maximum for akermanite + forsterite + vapor = liquid, which separates high-temperature liqui ds precipitating silicates together with a little calcite, from low-te mperature liquids precipitating carbonates with a few percent of forst erite; there is no direct path between the silicate and synthetic carb onatite liquids on these joins, but it is possible that fractionating liquid paths diverging from the joins may connect them. More complex r elationships involving the precipitation of monticellite and akermanit e are also outlined. Magnetite-magnesioferrite may replace periclase i n natural magmatic systems. The results indicate that the assemblage c alcite-dolomite-magnetite-forsterite represents the closing stages of crystallization of carbonatites, whereas assemblages such as calcite-m agnetite-forsterite and dolomite-magnetite-forsterite span the whole r ange of carbonatite evolution in terms of temperature and composition, and provide the link between liquids precipitating silicates and thos e precipitating carbonates.