Geochemical and geophysical investigations over the past decade suggest a laterally as well as radially heterogeneous upper mantle. The sources of this variability are mantle dynamics and interactions with the crust. The opportunities to sample these variations directly are limited within continental regions. However, the basalts of the Midcontinent Rift (MCR) System of North America are particularly attractive for studying subcontinental mantle.
The MCR is an 1100-Ma paleorift [Halls, 1978; Green, 1983; Van Schmus and Hinze, 1985] that extends for more than 2000 km across the North American midcontinent (Cover). Drill holes into the MCR to obtain samples of the basalt can be located to answer critical questions regarding the origin and evolution of this aborted Precambrian rift.