Windows into MORB Petrogenesis: Measuring U-Series Disequilibria in MORB from Transforms

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This proposal funds the analysis of uranium series (U-series) disequilibria in mid-ocean ridge basalt samples erupted within transform faults that offset mid-ocean ridges. Because mixing of mantle melts is less pronounced at this tectonic setting relative to the ridge axis, samples from transforms provide a window to observe the diversity of basalts produced by melting of the earth's mantle. U-series disequilibria data provide the ability to study the melting process by constraining the rate of formation and ascent of magmas through the mantle including quantification of the rate of solid upwelling. This grant will fund the determination of (230Th)/(238U), (226Ra)/(230Th) and (231Pa)/(235U) in 25-30 samples from the Cox, Blanco, Garrett and Siqueiros Transforms. These data will in turn be combined with complementary U-series data from adjacent portions of mid-ocean ridge and major and trace element data in order to evaluate the following questions. 1) What is the effect of mantle cooling due to transform offsets on how the mantle melts? 2) Is there an observable relationship between the amount of 230Th-238U disequilibria and the major element characteristics of primitive basalts? 3) Does the solid upwelling rate beneath the transform differ significantly from the adjacent ridge axis? Answers to these questions will lead toward the goal of providing an integrated view of the genesis of mid-ocean ridge basalt.