| Subducted materials suffer one of two fates, either to return to the surface as a melt or fluid driving arc volcanism, or to remain in the residual slab that is transported into the earth's deep interior. This work addresses these consequences of subduction by evaluating the initial composition of subducting altered oceanic crust (AOC) and the composition of erupted arc lavas in the Mariana-Izu-Bonin subduction zone.; Uranium (U), thorium (Th), and lead (Pb) make useful natural tracers of subduction zone processes. Based partly on Pb isotopes, many studies suggest that subducted AOC ultimately resides in the deep mantle source of hotspot volcanoes. This study reports comprehensive measurements of U, Th, and Pb in AOC prior to subduction and considers its effect on long-term Pb isotope evolution in the mantle. Oceanic crust is geochemically altered by seawater, causing five-fold enrichment in U. A comparison with Mariana arc lavas indicates subducted AOC loses 45% of its Pb and <10% of its U to the volcanic arc, whereas greater U loss occurs to the back-arc. Together, these processes raise AOC parent/daughter ratios, U/Pb and Th/Pb, by a factor of two, which are precisely the shifts required to explain the Pb isotopic variations in hotspots.; Water is also critical to study in an arc setting, as both a vehicle of mass transfer from the dehydrating slab to the overlying mantle and as the driver of mantle melting. This study uses melt inclusions to determine the pre-eruptive volatile concentrations of Mariana arc lavas. Elements sensitive to mantle melting (e.g., titanium) correlate with the mantle water content both in the Marianas and in global arcs and back-arcs, showing that water indeed fuels mantle melting. Regional variations in this correlation also illuminate local controls, such as mantle temperature or arc crustal thickness, on the melting process. Melt inclusions also preserve geochemically heterogeneous melts. Mariana melt inclusions define clear mixing arrays that extend between a sediment-like end-member and one enriched in fluid-mobile elements. Along with water concentrations, these inclusions provide a means of calculating the compositions of the different slab-derived components contributing to Mariana arc volcanism. |
