Collaborative Research: U-series constraints on the ages and petrogenesis of Lau Basin Lavas

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AbstractLangmuir/ Sims, 0732448; 0732480The investigators will measure U, Th and Ra isotopes on samples from throughout the length of the East Lau Spreading Center, a Ridge 2000 Integrated Study Site, with an emphasis on the ABE vent field, which is the designated "bulls-eye" for this region. These data will be obtained on samples with existing Pb, Sr, Nd and He isotopes, major elements, trace elements and water contents. The resulting data set will be a benchmark for backarc basin studies, will address major problems of ocean ridge and convergent margin science, and provide essential age constraints for the Lau ISS. U-series measurements have the potential to address three important scientific problems in this region:(1) Subduction fluids have high U contents, causing 238U excesses in convergent margin lavas. These excesses decay with time, and reflect the time scale of fluid transport through the mantle wedge at varying distances from the volcanic front. We will test the hypothesis ofdecreasing 238U excess (i.e. greater transport time) with increasing distance from the trench.(2) Back-arc basin and open ocean spreading centers have very different responses to water in the source, with back-arc spreading centers showing no apparent garnet influence, or evidence of effects of deep low degree melts. Langmuir et al. (2006) attribute these differences to the effects of active corner flow and a deep cold slab in the back-arc. Since garnet controls U-Th disequilibria at open ocean ridges, we predict smaller 230Th excesses at back-arcs compared to open ocean ridges. The northern ELSC samples have little or no subduction signature. These samples in particular permit a test of this hypothesis.(3) A host of problems related to ocean ridge formation require knowledge of time scales. Age constraints are essential to relate volcanic activity to tectonics, variations in the underlying magma chamber reflector, and temporal changes in lava compositions. They are an essential link between the volcanic system and the hydrothermal and biological activities that are supported and impacted by temporal and spatial fluctuations in volcanism. U-series are the sole means to quantify time constraints for young ridge volcanism. Our proposed work will add the dimension of time to the study of ridge processes in this region.Broader Impacts: The proposed work is part of a multi-disciplinary effort to understand the ocean ridge system from melt generation in the mantle to volcanism and life on the seafloor. The constraints on fluid fluxes at convergent margins will be pertinent for the large community interested in subduction processes, and to the NSF MARGINS program. Harvard undergraduate students work routinely in Langmuir's lab, and will be exposed to U-series analysis through this project. Langmuir and Sims will also prepare a set of U-series teaching modules. These modules will be part of Langmuir's 'Introduction to Petrology' and Sims' 'Introduction to Isotope Chemistry'. The web-based module will be hosted on their personal web sites, and integrated into the MIT OpenCourseWare as part of Sims' Isotope Chemistry course.