Melts in the mantle modeled in the system CaO-MgO-SiO2-CO2 at 2.7 GPa

The effect of CO2 on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO2-CO2 at 2.7 GPa. The experiments provide isotherms for the vapor-saturated liquidus surface, bracket piercing points for field boundaries on the surface, and define the positions and compositions of isobaric invariant liquids on the boundaries (eutectics and peritectics). CO2-saturated carbonatitic liquids (>80% carbonate) exist through approximately 200 degrees C above the solidus, with a transition to silicate liquids (>80% silicate) within similar to 75 degrees C across a plateau on the liquidus. Carbonate-rich magmas cannot cross the silicate-carbonate liquidus field boundary, so the carbonate liquidus field is therefore a forbidden volume for liquid magmas. This confirms the fact that rounded, pure carbonates in mantle xenoliths cannot represent original liquids. A P-T diagram is constructed for the carbonation and melting reactions for mineral assemblages corresponding to Iherzolite, harzburgite, websterite and wehrlite, with carbonate, CO2 vapor (V), or both. The changing compositions of liquids in solidus reactions on the P-T diagram are illustrated by the changing com-positions of eutectic and peritectic liquids on the liquidus surface. At an invariant point Q (similar to 2.8 GPa/ 1230 degrees C), all peridotite assemblages coexist with a calcite-dolomite solid solution (75 +/- 5% CaCO3) and a dolomitic carbonatite melt [57% CaCO3 (CC), 33% MgCO3 (MC), 10% CaMgSi2O6 (Di)], with 63% CC in the carbonate component. At higher pressures, dolomite-lherzolite, dolomite-harzburgite-V, and dolomite-websterite-V melt to yield similar liquids. Magnesian calcite-wehrlite is the only peridotite melting to carbonatitic liquids (more calcic) at pressures below Q (similar to 70 km). Dolomitic carbonatite magma rising through mantle to the near-isobaric solidus ledge near Q will begin to crystallize, releasing CO2 (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite.