9706526 Hirschmann Summary: The chemical differences in mid-ocean ridge basalts at one location on a ridge and along a ridge have been noted for some time. Various models of melting, mantle heterogeneity, melt migration through the mantle have been proposed to explain these differences. The PI hypothesizes that the differences are due to the presence pyroxenite veins in the mantle that melt preferentially and at greater depth than the bulk of the mantle. Thus this small amount of material (about 5% of the mantle) could contribute more than 20% of the melt that reaches the surface in some places, thus causing the chemical differences seen in the basalts. The PI plans to melt pyroxenites under mantle conditions to determine the compositions of melts that might contribute to the ocean ridge basalts.