In the course of this exercise, you have learned about the spatial and isotopic variations within the most recent mafic volcanic activity to have affected the western United States. You have discovered that eruptions of mafic magma during the last 5 million years of geologic history were not randomly located, but have been concentrated along linear or curvilinear zones through the region, some of which correspond to tectonic features such as the Cascades volcanic chain, the Yellowstone hotspot track, and various extensional zones associated with tectonic activity in the Basin and Range.
The isotopic compositions of these mafic rocks are not random either, but appear to define two clusters. The low 87Sr/86Sr cluster is believed to originate from the asthenospheric mantle, a part of the mantle which is actively convecting and has partially melted many times in the past. In contrast, the high 87Sr/86Sr cluster represents a much more ancient, non-convecting (more rigid) mantle which is separate from the convecting mantle (probably residing as deep lithosphere underlying ancient continental crust). Compared to convecting asthenospheric mantle, lithospheric mantle is believed to be enriched in incompatible elements.