Mantle Composition as a Function of Crustal Tectonics, Southwestern U.S.

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This is a study of the geochemistryof mantle-derived, spinel peridotite xenoliths from localities in the Basin and Range Province and the adjacent Colorado Plateau, as well as the transition zone between the two provinces. The heart of the study is the anlysis and modelling of the REE and Sr and Nd isotopic compositions of clinopyroxene seperated from the bulk peridotites, but xenolith population systematics and mineral composition will be investigated and will be particularly important in choosing representative samples for study. In addition to standard analytical techniques, secondary ion mass spectrometry will be utilized to measure relative REE abundances in single cpx grains. Present-day mantle composition beneath the two tectonic provinces will be characterized and for each locality a model will be developed. This will include observed heterogeneity (or homogeneity) and where possible, the inferred age and nature of fractionation events. Ultimately models for all localities will be integrated to provide a regional model for the systematic variation of mantle composition as it relates to large-scale crustal tectonism.