Collaborative Research: Pa-231/U-235 Fractionation During Intra-Plate Mantle Melting and Magma Transport

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Asmerom EdwardsEAR-9980545 EAR-9980605Basalts are our physical link to the mantle, which is the largest silicate domain in the earth. Variations in the chemical and isotopic composition of basalts may result from differences in their mantle sources or processes related to melting and melt transport. Continental basalts, although volumetrically not significant, may provide unique insights into mantle melting processes. Particularly interesting are the processes related to the enigmatic lithospheric mantle melting and the role of the lithosphere during the dynamic melting of the underlying asthenosphere. Unlike long-lived isotopic systems, which provide time-integrated data, the short-lived uranium series isotopes may provide unique constraints on extent and timing of melting and melt transport. These constraints are possible because the source nuclide ratios are known (equal to the ratio of the half-lives of the nuclides in any chain) and the half-lives are of time-scales appropriate for mantle melting and transport. The two important uranium-series isotopic systems for the study of mantle melting are 230Th-238U and 231Pa-235U. The 231Pa-235U system can be a powerful complement to the more-developed 230Th-238U method once we gain understanding of the ways in which 231Pa and 235U behave during melting. At the present there are very few 231Pa-235U data on basalts. We expect results from this project will enlarge our understanding of 231Pa-235U fractionation during melting and melt transport. In addition to whole-rock data, we will obtain mineral/matrix Pa data. At the present there are no mineral/melt partitioning data for Pa. Uranium-series isotopic data from this project will provide important constraints for melting and magma transport models.