Calculations of the sthermal evolution of the Earth during accretion suggest that the outer layer may once have been molten, perhaps to a depth of 1,000 km or more1,2. If such global melting did occur, fractionation processes could have produced a chemical stratification in the mantle. Recent technical developments in experimental petrology have made it possible to achieve temperatures in excess of 2,000°C at a pressure of 20 GPa 3,4, thereby permitting studies of multi-component systems under conditions that prevail in the Earth's mantle. We have conducted melting experiments using a composition in the five-component system CaO-FeO-MgO-Al 2O3-SiO2 (CFMAS) corresponding to a model chondritic mantle, and find that the liquidus phase changes from olivine to majorite at a pressure between 12 and 15 GPa. The liquid coexisting with majorite at 20 GPa has a peridotitic composition. In addition, because the CaO/A12O3 ratio of the liquidus majorite at 20 GPa is lower than that of the sub-solidus majorite, partial melting and majorite fractionation at the base of the upper mantle could produce a peridotitic liquid with a CaO/A12O3 ratio greater than that of the chondritic starting material, consistent with current views on mantle geochemistry.
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