MANTLE XENOLITHS OF THE NORTH AMERICAN SUBCONTINENTAL MANTLE: MAGNETIC PROPERTIES AND PETROLOGIC AND TECTONIC IMPLICATIONS

The subcontinental mantle of North America is heterogeneous. Thus the mantle xenoliths from this source are also heterogeneous. Their magnetic properties reflect variations in nature and abundance of ferromagnetic phases that reflect different fO₂ and distinct temperatures and depths of equilibration. Here we present part of a new database that encompasses xenoliths from the Cascadia arc, San Andreas Fault area (Coyote Lake), from margins of the Rio Grande Rift (San Carlos and Kilbourne Hole) and from stable platforms (Knippa and Bearpaw Mountains). We show that these xenoliths exhibit distinct magnetic properties that reflect different magnetic assemblages and cooling histories. We further demonstrate that the stepwise demagnetization of their Natural Remanent Magnetization (NRM) provides invaluable assistance in identifying primary vs. secondary-alteration of magnetic minerals in mantle xenoliths. The implications of this research are that the subcontinental mantle may constitute a significant source for long-wavelength magnetic anomalies that had, to this point, been entirely neglected. This should be of a particular importance in cratonic domains.