Melting phase relations of simplified carbonated peridotite at 12-26 gpa in the systems CaO-MgO-SiO₂-CO₂ and CaO- Mgo-al₂O₃-SiO₂-CO₂: Highly calcic magmas in thetransition zone of the Earth

Phase equilibrium data pertaining to melting of simplified carbonated peridotite in the systems CaO-MgO-SiO₂-CO₂ and CaO- MgO-A_l2O₃-SiO₂-CO₂ at pressures of 10-26 GPa, corresponding to~300-750 km depths in the Earth, are presented. In both the studied systems, liquid compositions, with changing crystalline phase assemblage, are carbonatitic throughout the studied pressure range. In the system CMS-CO₂, melting phase relations are isobarically invariant; liquid is in equilibrium with forsterite +clinoenstatite+clinopyroxene+magnesite, forsterite+majorite +clinopyroxene+magnesite, wadsleyite+majorite+clinopyroxene+magnesite, ringwoodite+majorite+calcium-silicate perovskite+magnesite, magnesium-silicate perovskite+periclase +calcium-silicate perovskite+magnesite at 12, 14, 16, 20, and 26 GPa, respectively. In the system CMAS-CO₂, a phase assemblage consisting of forsterite+orthopyroxene+clinopyroxene+magnesite+garnet+melt from 10 to 14 GPa is isobarically invariant. However, owing to the disappearance of orthopyroxene at pressures greater than 14 GPa, from 16 and up to at least 26 GPa, the solidus of simplified carbonated peridotite spans a divariant surface in pressure-temperature space. The liquid coexists with wadsleyite+ clinopyroxene+garnet+magnesite, ringwoodite+calcium-silicate perovskite+garnet+magnesite, and magnesium-silicate perovskite+ periclase+calcium-silicate perovskite+magnesite at 16, 20, and 26 GPa, respectively. A curious, and as yet unexplained, feature of our study is an abrupt drop in the solidus temperature between 14 and 16 GPa that causes a small amount of melting of carbonated mantle in the Transition Zone of the Earth. In the systems CMS-CO₂ and CMAS-CO₂ liquid compositions at 16 and 20 GPa are highly calcic bona fide carbonatites; however, these liquids revert to beingmagnesiocarbonatites at 10-14 and 26 GPa. In the system CMS-CO₂, at 16 GPa we locate an isobaric invariant point consisting of wadsleyite+clinopyroxene+anhydrous B+magnesite+melt.The presence of anhydrous B at 16 GPa and 1475°C is interesting, as it lies outside the composition space of the mantle peridotite analog we have studied.However, despite the presence of twohighlymagnesian silicate crystalline phases, wadsleyite and anhydrous B, at 16 GPa and 1475°C, the liquid composition remains calcic with molar Ca-number [Ca/(Ca+Mg)~100] of about 63.The melting reactions at 16 and 20 GPa (with or without anhydrous B) show that lime-bearing crystalline silicatesplay afairly large part ingeneratingand controlling the composition of the liquids. At 16 GPa, in the system CMS-CO₂, we also report an experimental run at 1575°C, in which liquid coexistswith onlywadsleyite andmajorite.The liquid composition is less calcic (Ca-number 54) than that for other runs at lower temperatures, but is still more calcic than liquids at 10-14 and 26 GPa in both the studied systems. At present, the likely cause for these changes in the reported phase relations is not known. For normally assumed mantle temperatures, melting in theTransitionZone of the Earth, owing to the presence of carbonate, is probably unavoidable.The depth range of the drop in the carbonated peridotite solidus closely matches that of commonly observed lowseismic velocities at~400-600 km depth in the Earth.