The goals of this study are to further understanding of the large-scale chemical dynamics of the
mantle and the geochemical evolution of upper mantle in the SW USA. Preliminary measurements indicate significant ⁷Li/⁶Li variations on a variety of scales in peridotite xenoliths. These potentially contain information about metasomatic reactions, and recycling of surface materials within the deep Earth, but could also be subject to disturbance during transport of xenoliths to the surface.

⁷Li/⁶Li, and Li and other trace element contents will be determined at a fine spatial scale by
SIMS in olivine and pyroxene from the extensively studied suites of peridotite xenoliths at San
Carlos, Arizona, and Colorado Plateau diatremes. To evaluate transport-related disturbance,
⁷Li/⁶Li heterogeneity and zoning patterns will be related to proximity to grain boundaries, cracks, and xenolith margins, and to three contrasting volcanic environments hosting xenoliths at the Peridot Mesa volcano. To evaluate the effects of metasomatism in the San Carlos suite, ⁷Li/⁶Li variations will be determined in relation to modal composition, veins, major- and trace element composition of minerals, and degree of deformation and recrystallization. Xenoliths representing contrasting styles of metasomatism (Type-I, Type-II) at San Carlos will be analyzed.

⁷Li/⁶Li will be measured in previously studied xenoliths from Colorado Plateau diatremes
with mineralogical evidence of hydration and compared with data from Basin-and-Range
localities. The comparison will evaluate Li isotope variation introduced during subduction-related hydration of the upper mantle beneath the Colorado Plateau, and test the model for dehydration during low-angle subduction.

The results of this work will (1) establish the extent of Li isotopic disturbance during
transport and the degree to which bulk methods of analysis represent mantle values of δ⁷Li, (2)
test for recycled surface materials in the source regions of Type I and Type II metasomatic
melts/fluids with broad implications for mantle geochemistry and chemical dynamics, and (3)
contribute to understanding the relationship of subduction to tectonic and geochemical evolution
of the western US, in particular the role of water-rich fluids.