Reconstructing Slab and Mantle Fluxes During th Eocene-Oligocene Evolution of the Izu-Bonin Volcanic Arc

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ABSTRACT - Goldstein (0453515)Intellectual Merit: This research investigates geologic factors, such as tectonics, subduction, and changes in the compositional evolution of the upper mantle, that affect volcanism through time, an objective being to determine how these factors relate to changes in volcanic chemical signals. To accomplish this task, samples of fragmental volcanic rocks (tephra) from Eocene to Oligocene deep sea sediment cores from the Izu Bonin-Mariana arc in the western Pacific were targeted for a host of geochemical and isotopic analyses. Justification of this work comes from the fact that volcanic arc lavas and tephra contain geochemical components that reflect melts, volatiles, and melt processes that occur as the ocean floor subducts and the subducting slab reassimilates into the mantle. Such processes are responsible for many of the chemical characteristics of present day seawater and the atmosphere and are related to the genesis of major earthquake events. One of the most completely characterized records of arc volcanism comes from the Izu Bonin-Mariana arc which has been active for ~ 50 million years. At that location, however, there is a gap in the oldest part of the magmatic record (Eocene-Oligocene) because lavas erupted in the early stages of arc construction are buried and generally inaccessible to study. Nevertheless, tephra from eruptions that took place during this time gap, are easily accessible from deep sea sediment cores because they are airfall deposits that are deposited over large geographical areas. A complete record of such tephra has been collected during coring expeditions of the NSF-funded Ocean Drilling Program (ODP). This research program uses tephra samples from ODP cores that occur in the Eocene-Oligocene time gap to identify secular changes in Izu Bonin-Mariana arc during this time interval using major element and trace element analyses. In addition, Sr-Nd-Pb-Hf isotopic ratios will be determined for the same samples to infer changing slab and mantle magma sources. The use of tephra to unravel volcanic arc evolution is a new approach in this area and geochemical analyses will focus on elements and ratios that are sensitive to both the composition of slab fluids and melt components. In addition to the tephra, similar geochemical analyses will be performed on select Oligocene lavas to tie the tephra results to the lavas erupted in the region. The tephra samples used in this study have been carefully selected for freshness to prevent analysis of altered materials for which the original chemical signature has changed. Broader Impacts: This research is an essential element of the NSF-funded MARGINS subduction factory initiative. It supports two female researchers, one of which is hearing disabled and has performed pioneering work in establishing the chemical evolution of the Izu-Bonin arc through tephra studies, and the other who is returning to the full-time workforce. The work also supports analytical infrastructure at Columbia University and engages an undergraduate student in research.