Laser-heated diamond-cell experiments on Allende meteorite, a natural analog for undifferentiated terrestrial planetary mantles, yield a melting curve increasing from ~2200 to 3200 K over the pressure range of 15--70 GPa, significantly below that of peridotite that is representative of the Earth's current upper mantle. Samples quenched from high temperatures and pressures reveal structures that may mimic the initial stages of differentiation between core and mantle materials. Micron-scale chemical analyses support the idea that sulfur is enriched, and silicon, oxygen and aluminum are depleted, in the (potentially core-forming) metal relative to the silicate phases. ¿ 1998 American Geophysical Union