Melting phase relations of a fertile lherzolite KLB-1 have been studied in the pressure range from 1 atm to 14 GPa (140 kbar). Olivine is the liquidus phase at all pressures studied. The second mineral to crystallize changes with increasing pressure; chromian spinel (1 atm), Ca-poor orthopyroxene (up to 3 GPa), pigeonitic clinopyroxene (up to 7 GPa), pyrope-rich garnet (above 7 GPa). The melting temperature interval of the peridotite is more than 600¿ C wide at 1 atm but narrows to about 150¿ C at 14 GPa. The partial melts along the peridotite solidus become increasingly more MgO-rich as pressure increases throughout the pressure range studied. At 5--7 GPa, the partial melts formed within 50¿ C of the solidus contain more than 30 wt % MgO and are very similar to Al-undepleted-type peridotitic komatiite which is common in Archean volcanic terrains. Due to the increase of enstatite component in clinopyroxene solid solution at high pressure and temperature, the orthopyroxene liquidus field narrows as pressure increases and disappears at 3.5 GPa. Harzburgites which are common in the basal peridotite in ophiolite suites may have been produced as residues by partial melting at relatively shallower depths (100 km). A diapiric model is consistent with the genesis of komatiite magma by partial melting of mantle peridotite at 150--200 km depth. Based on the following observations, (1) convergence of the liquidus and solidus of the peridotite at pressures >14 GPa, (2) the near solidus partial melt composition very close to the bulk rock at 14 GPa, and (3) change in liquidus mineral from olivine to majorite garnet at pressures between 16 and 20 GPa in preliminary experiments, it is proposed that the upper mantle peridotite was generated originally as a magma (or magmas) by partial melting of the primitive earth at 400--500 km depth.