Mantle derived rocks-peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow in Hawaii, as well as peridotite, dunite, and eclogite xenoliths from Roberts Victor, Dutoitspan, Kilbourne Hole, and San Carlos diatremes-are weakly magnetic at room temperature. Saturation magnetization values range from 0.0l3 emu/gm to <0.001 emu/gm equivalent to ~0.0l to 0.00l wt % Fe3O4. A review of pertinent literature dealing with analysis of the minerals in mantle xenoliths provides evidence that metals and primary Fe3O4 are absent, and that complex Cr, Mg, Al, Fe spinels dominate the oxide mineralogy. These spinels would be non-magnetic at mantle temperatures. The crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. Curie depth estimates from aeromagnetic anomalies do not require a source of magnetization in the mantle. All of the available evidence supports the new magnetic results, indicating that the seismic Moho is a magnetic boundary. The source of magnetization is in the crust and the maximum Curie isotherm depends on the type of magnetic mineralogy and is located at depths which will vary with the regional geothermal gradient.