Melt Sources and Timescales in Western Pacific Island Arcs

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GillEAR-0106206Radioactive disequilibria in the 238U and 235U decay chains will be measured and interpreted for young volcanic rocks from several western Pacific island arcs (Marianas, Sangihe [Indonesia], and Japan). Specifically, disequilibria between 238U-230Th, 230Th-226Ra, and 235U-231Pa will be measured. Results will be integrated with other geochemical data to be obtained for the same rocks, especially trace element concentrations measured by ICPMS and Sr-Nd-Pb isotope ratios measured by TIMS. These supplemental can fingerprint components which may be in the source region of the magmas. Special attention will be placed on determining whether results constrain the physical processes of melting (such as the porosity, upwelling rate, and melt productivity of mantle in the wedge between the arc and underlying subducting lithosphere), as is the case in other major sites of planetary magmatism. Such processes are expected to differ in subduction zones where melting occurs by fluxing from subducted waters and may be much faster. The alternative is that the radioactive disequilibria are dominated by the source components too. By comparing results from three quite different arcs, we will explore the role of different tectonic environments and source components. Results for Japan will be integrated with seismic tomographic images of melting there, which are the best in the world, but it will also be necessary to work closely with Japanese colleagues familiar with the volcanoes being studied in order to take the local geological context fully into account. Results will be disseminated through a University of California K12 partnership program.