Origin of mantle peridotite and komatiite by partial melting

Theoretical calculations and some preliminary experimental observations show that the liquidus and solidus temperatures for each of a wide range of upper mantle peridotite and komatiite compositions become coherent at high pressures. This suggests that these materials may have been liquids in eutectic‐like equilibrium (rather than peritectic‐like) with mantle assemblages. It is shown that the major element geochemistry of 83 mantle peridotites and 61 komatiites define a trend which is not primarily due to an olivine control process; rather it is interpreted to represent the pressure‐induced compositional trace of eutectic liquids in equilibrium with mantle assemblages from 4 to 15 GPa. If the bulk Earth is chondritic in composition these phase equilibrium constraints would imply that the upper mantle had formed from the whole mantle as an ultrabasic partial melt, the transition zone and lower mantle being the complementary eclogite and pyroxenite residua. This is consistent with mineralogical interpretations of seismic data for the present‐day Earth which call for a peridotite upper mantle and pyroxene‐like transition zone and lower mantle.