U-Th-Pb Systematics of the Lower Crust

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The 206Pb/204Pb isotopic compositions of oceanic basalts seem to be higher than the bulk Earth values since they plot to the right of the geochron. This is important because it shows that the U-Pb systems has behaved differently than the Sm-Nd and Rb-Sr systems during he evolution of the mantle. There have been numerous models proposed that could explain the anomalous Pb values of ocean basalts. One of these models proposes that U preferentially concentrates in the upper crust during intracrustal differentiation. Consequently any preferential recycling of the upper crust into the mantle would result in a mantle more radiogenic than the bulk Earth. This model requires U depleted lower continental crust with low U/Pb ratios. There are few data available that can constrain the U/Pb ratio of the lower crust and estimates of this value vary widely. This study will obtain U-Pb-Th elemental data for exposed granulites from the Grenville Province in Ontario, and for lower crustal xenoliths from New Mexico and Arizona in order to constrain the U/Pb and Th/Pb ratios of the lower crust. Lead isotopic data will also be obtained in order to constrain the timing of U depletion. It is important to analyze exposed granulites and xenoliths because they may represent different levels of the crust. These localities were chosen because of extensive previous work, which among other things constrain the lithologic characteristics and P, T of last equilibration of the samples. In the Grenville Province, samples are available that equilibrated over a ca. 5Kb range. These data will allow evaluation of the behavior of U-Th and Pb with pressure. The New Mexico and Arizona localities will allow estimation of U/Pb of the present day lower crust in those areas. The data obtained in this study will at least double the available U-Th-Pb data available for granulites worldwide and will help constrain the U/Pb ratio of the lower crust and Pb-isotope models of the evolution of the mantle.