Studies in synthetic carbonatite systems Solidus relationships for CaOMgOCO2H2O to 40 kbar and CaOMgOSiO2CO2H2O to 10 kbar

The system CaO-MgO-SiO2-CO2-H20 is an important model for many igneous and metamorphic processes, including the generation and differentiation of carbonatite and associated silicate magmas. We have experimentally established the vapor-saturated solidus for the system CaO-MgO-CO2-H20 from 595øC/1 kbar to <500øC/40 kbar, about 25øC lower than the corresponding temperatures for the CaOCO•-H•O system. Brucite replaces periclase on the vapor-saturated solidus at about 750 bars pressure, remaining as the stable phase to pressures of at least 40 kbar; no dolomite or magnesite was encountered. At a pressure between 35 and 40 kbar, the assemblage portlanditc + brucite + aragonite + vapor changes to one containing phase W, a previously unreported Ca-Mg carbonate. At 20 kbar, the vapor-saturated liquid contains at least 24 wt % H•O. The vapor-saturated solidus for the system CaO-MgO-SiO•-CO2H20 ranges from 613øC/1 kbar to 565øC/10 kbar, experimentally indistinguishable from that for CaOMgO-CO•-H•O, about 10øC lower than that for CaO-CO•-H•O, and about 25øC lower than that for CaO-SiO2-CO•-H•O. In the quinary system, monticellite is replaced by dellaitc and an unidentified silicate on the vapor-saturated solidus above 4.1 kbar. MgO-poor liquids are similar in composition to the magma from which the Magnet Cove calcite carbonatite crystallized. More magnesian magmas would produce dolomite at moderate depths, such as at Aln/3 Island.