Ultramafic xenoliths in basalt from the western United States are dominantly spinel Iherzolite in which at least four types of gabbroic and pyroxene-rich bands have formed. These bands closely resemble compositional bands in some alpine peridotite, and we infer a similar mode of origin: partial fusion over a range of T-P conditions during diapiric rise of a part of the upper mantle. The sequence of formation of bands, established by crosscutting relationships, is appropriate to crystallization of the products of melting in spinel Iherzolite that has risen from a level of spinel- and garnet-pyroxenite stability in the upper mantle to the lower crust where gabbroids are stable. Both the textures and compositions of the bands have been modified by repeated deformation and partial melting.

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