Melting of a model chondritic mantle to 20 GPa

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 more^1,2. 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.