One of the most important chemical species in magmas, H2O, affects a host of scientific and societal problems related to volcanism at subduction zones. Water is critical not only to the physics of subduction and eruption, but also to the processes that lead to melting and magma production in the mantle. This project seeks to develop the first comprehensive study of the volatile content (H2O, CO2, SO2, Cl, F) of Tonga arc magmas, or order to test the hypotheses regarding: (1) the composition of slab fluids, (2) their distribution in the mantle, and (3) their effect on melting. The Tonga arc provides several well-defined tests of hypotheses relating to the highest tectonic plate convergence rate in the world, anomalously high mantle temperatures, and cross-strike compositional variations along more than 200 km, which includes volcanism in the Lau Basin. This project addresses questions of melting and mantle composition by measuring volatile concentrations in melt inclusions using both ion and electron microprobes. These data will be combined with major and trace element compositions from electron microprobe and laser-ICPMS. Data will be used to develop quantitative relationships between slab fluid compositions and mantle melting progress. Samples are currently in-hand from recent research cruises supported by Australian organizations, with continuous coverage of the 1000 km long, largely submarine Tonga arc.
Broader Impacts This project complements the science plans of other major initiatives in NSF-GEO, including RIDGE 2000 in the Lau Basin, and the MARGINS Subduction Factory experiment. One important outcome will be a comprehensive phenocryst dataset from the Tonga arc area, which is poorly represented in current on-line databases. This project leverages infrastructure and ship-time costs contributed from Australian sources. Results from this project will be incorporated into the undergraduate and graduate classrooms at Boston University, and will support the Ph.D. thesis of a female graduate student and a new research direction for a senior female faculty members at Boston University in Massachusetts.
