Chemical Dynamics of Mantle Lithosphere from Th Isotope Systematics

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9418323 Reid The origin of Th-Pb isotope variations in mantle lithosphere beneath the western USA will be investigated by examining basalts from three areas: the Western Great Basin (WGB), Snake River Plain (SRP), and western Colorado Plateau and its transition zone. These areas highlight complementary and contrasting processes that may have exerted significant control on the ultimate Th-Pb characteristics of the lavas including lithosphere formation and mineralogy (all areas), subduction-related metasomatism of mantle sources (WGB and SRP), and plume-related enrichment of mantle sources (SRP). Result obtained to date reflect a limited sampling of two of these areas but show that Th isotope disequilibria can be related to processes of partial melting and to characterizing mantle reservoirs and their origin. Th isotope data will be obtained for late Pleistocene to Holocene basalts from tectonomagmatic provinces in the western USA selected for their likelihood to yield important constraints on the nature and evolution of mantle lithosphere. In addition to Th, these basalts will be characterized for their Sr, Nd, and Pb isotope compositions. Within the framework of complete major and trace element and isotope analyses, disequilibrium in U-Th isotope systematics will be used to consider the conditions of partial melting and possible origins for lithospheric signatures in basaltic magmas.