Devolatilization and Trace Element Mobility During Subduction-Zone Metamorphism: Record in Alpine Blueschist-Eclogite Suites

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0079331BeboutThe effects of subduction-zone metamorphism on the chemical and isotopic compositions of oceanic crust and sediments subducted into the mantle have not been adequately considered in models of ocean-atmosphere-mantle geochemical cycling. In the research funded by this grant, the P.I. and his students will evaluate the nature of these effects through combined field and geochemical study of fluid loss and chemical alteration in metamorphic rocks in the Italian Alps thought to have been subducted to 50-120 km depths. The two specific field localities include a well-studied traverse of variably metamorphosed shales and carbonate rocks exposed in the Cottian Alps, and exposures of metamorphosed oceanic crustal rocks (and their overlying sediment cover) in the Valtournenche area. These localities are well-suited because of their particularly well-preserved metamorphic mineral assemblages and chemical compositions representing the effects of deep subduction. In addition, the lithologies at the two localities are representative of those thought to contribute geochemically to the production of arc lavas, and the carbonate-rich metasedimentary sections will afford insight into the decarbonation history of similar sections in modern subduction zones such as the Central America and Banda arc-trench systems. This planned study of devolatilization based on subduction-zone metamorphic suites will contribute to our knowledge of the cycling of volatiles such as carbon dioxide and nitrogen between Earth's oceans/atmosphere and mantle (information important in models of long- and short-term atmosphere evolution).