Collaborative Research: The Role of Mafic Crust in Orogenic Magmatism - New Perspectives from Re-Os Isotopes

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9980525ShireySeventy-five to eighty percent of the new crust that is generated at the world's orogenic arcs in the Cenozoic can be considered to belong to "juvenile" island or continental arcs, and such settings likely represent the major environments where new material is added to the crustal inventory of the Earth. However, intracrustal recycling through re-melting of juvenile arc basement is virtually undetectable in such settings because there is virtually no isotopic contrast using traditional isotopic tracers such as O, Sr, Nd, and Pb isotopes. Assessment of the role of intracrustal recycling is critical for calculation of net crustal growth rates. This work will focus on the Re-Os isotope system, which offers an exceptional means to evaluate intracrustal processing in juvenile arcs. The motivation for the proposed research lies in the recognition that basaltic magmas develop extreme fractionations in Re/Os ratios, which will produce radiogenic 187Os/188Os ratios that far exceed those of the mantle, over periods of several to 10's of m.y. The initial target of the proposed research is the Cascade Arc, NW USA, which is an ideal place to test our this hypothesis because the Quaternary centers are constructed on voluminous Miocene to Pliocene mafic sequences, and interpretation of the Re-Os isotope data will not be confused by a) the presence of old (pre-Cenozoic) sources of Os, b) significant subducted sediment input,and c) subduction of old oceanic crust.