Compositional Domains in Altered Oceanic Crust

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The generation, alteration, subduction and recycling of mid-ocean ridge basalts results in first order chemical transport between seawater, the mantle, volcanic arcs and continental crust. The study of chemical processes in each phase of this oceanc rustal life cycle requires knowledge of the previous one. A most significant missing link is the poor understanding of the bulk chemical changes during alteration of the oceanic crust prior to subduction. This study will involve careful documentation of the primary and secondary mineralogy, modal abundance estimates and an integrated chemcial and isotopic analysis of samples from DSDP Sites 417A, 417D, and 418A. The PIs will determine a number of alteration sensitive parameters, including the concentrations of K2O, H2O, CO2, B, and the isotopic ratios of delta O18, Sr87/86, and deltaB11. A profile of these parameters for similary lithologies will provde insights into the decay of the effects of seafloor alteration with depth. By monitoring these parameters in the upper portions of the crust, quantitative estimates of the amount of seawater entering the oceanic crust will be possible. Sites 417A, 417D, and 418A have been studied extensively, however, there is no data on stable or radiogenic isotopes and only a few trace elements, and there are too few analyses of extremely altered pillow margins. The PIs will provide these analyses.