Solidus and phase relations of carbonated peridotite in the system CaO-Al2O3-MgO-SiO2-Na2O-CO2 to the lower mantle depths

Melting phase relations have been determined in a model carbonated peridotite (5wt.% CO2) at 10.5-32.0GPa and 1300-1850 degrees C. The assemblage of silicate minerals coexisting with partial melts changes with pressures from forsterite/wadsleyite-clinoenstatite/akimotoite-garnet-clinopyroxene/Caperovskite at 10-20 GPa to Mg-perovskite-periclase-Ca-perovskite at 27-32 GPa. Magnesite is the only carbonate stable in peridotite through the studied pressure range. The solidus temperature was defined by the appearance of quenched carbonatite melt, which occurs at slightly lower temperature than that of disappearance of magnesite. Accordingly, solidus of carbonated peridotite in the present study is bracketed at 1380-1460 degrees C at 10.5 GPa, 1550-1650 degrees C at 16.5 GPa, 1620-1720 degrees C at 20 GPa, 1710-1850 degrees C at 27 GPa, and 1750-1890 degrees C at 32 GPa. The slope of solidus curve is more gradual at 10-32 GPa than at lower pressures. The solidus temperature was found to be in agreement with previous works on carbonated peridotite at pressures below 10GPa with comparable alkali and CO2 contents in the starting material. Partial melts formed by melting of carbonated peridotite at 10.5-32.0GPa have magnesiocarbonatitic compositions with moderate variations in Ca/Mg ratio and have high Na2O-contents. It has been demonstrated that alkali-rich magnesiocarbonatite melt can be generated by partial melting of carbonated peridotite at pressure up to at least 32 GPa, i.e. to the lower mantle depths. The generation of calciocarbonatite by melting of carbonated peridotite is unlikely in the deep mantle. Determined solidus temperatures allow stability of magnesite along the normal mantle geotherm, however minor heating or addition of potassium to the system can cause melting of carbonates. (C) 2009 Elsevier B.V. All rights reserved.