ABSTRACT

The global process of subduction, which is the sinking of Earth's tectonic plates beneath other plates, is accompanied by physical and chemical processes that cause natural hazards such as earthquakes, magma generation, and volcanic eruptions. It is also the basis of the formation of many economically valuable minerals. Understanding subduction processes in detail is of critical importance for protecting human interests, locating resources, and building a basic knowledge of the Earth. This research focuses on understanding the initial stages of subduction, specifically the geochemical processes, that lead from incipient subduction to a self-sustaining subduction zone and volcanic arc. Samples come from a core drilled in the Izu-Bonin-Mariana area in the western Pacific Ocean as part of a coordinated drilling expedition of the International Ocean Discovery Program (IODP) in 2014. The expedition recovered over 1600 meters of core, preserving seafloor stratigraphy extending from the basaltic foundation of the subducting plate through overlying volcanic arc sediments. Goals of the research are to use the geochemical composition of the basement rocks and volcanic sedimentary processes to understand subduction initiation, the causes of magma formation, and the timing and extent of volcanic arc eruptions. Broader impacts of the work include collaboration with researchers from Japan, Switzerland and the UK. In addition the lead institution is a large, public, Hispanic- and minority-serving institution, which provides the potential for engaging under-represented minority students in the sciences in the research project. Additional impacts include support of an institution in an EPSCoR state (South Carolina) and outreach to Miami, Florida K-12 public schools.

One outcome of this project will be comprehensive geochemical analyses of volcanic rocks recovered on IODP Expedition 351, at Site 1438, which is located just west of the Kyushu-Palau Ridge, the site of Izu-Bonin Mariana (IBM) arc inception. Preliminary findings indicate that oceanic basement at ODP Site 1438 is close to the age of forearc basalts which form the base of the IBM forearc sequences. Prior to the expedition, it was thought that magmas formed at subducting margins were "forearc basalts" of tholeiitic character, similar but not identical, to mid-ocean ridge basalts, followed by boninites. Preliminary findings demonstrate that this initial magmatism may not have been limited to the forearc, but may instead have occurred over a broad area of the plate margin during subduction and arc initiation. This research produces whole-rock major element, trace element, and Hf-Nd isotope geochemical data for basement basalts at Site 1438, at ODP Site 1201, and at DSDP Site 447. Similar studies of clasts separated from volcaniclastic sedimentary rocks that overlie basement at Site 1438 will characterize the earliest products of arc stratovolcanoes and their compositional evolution through time. The proposed studies will establish the full extent of geochemical similarity between forearc and basement basalts. It will also document compositional changes with time that may be recorded at Site 1438. Results of this work will likely lead to a revision of current hypotheses about magma generation during subduction initiation. The overlying volcaniclastic section of Site 1438 provides an unprecedented opportunity to study the chemical evolution of an island arc, from subduction initiation through arc growth and maturity, through the waning/cessation of arc volcanism as the result of rifting and back-arc basin formation.

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