MARGINS: The Sr-Nd-Pb-Hf Isotope Evolution of the Mariana Island Arc

Columbia University, New York NY

Investigators

Abstract

The Sr-Nd-Pb-Hf isotope evolution of the Mariana Island Arc Intellectual Merit. Volcanic arcs represent a complex blend produced from pre-existing crustal basement, subducting slab and adjacent mantle reservoirs. Although temporal variations in arc magmatic outfluxes (compositions and volumes) must somehow be coupled to secular changes of the Earth's surface and mantle reservoirs, it remains unclear how they vary with subduction dynamics (crustal thickness, slab dip, convergence rate etc.). This project aims to investigate the causes of the variable magmatic outflux of the Mariana arc in the NW Pacific. The Mariana arc/backarc system is part of the intra-oceanic Izu Bonin-Mariana (IBM) arc that initiated ~49 million years ago. It is well known for its variable K2O-enrichment over time at rates that match to secular geological change. To understand what forces such Mariana 'fluctuations', it is proposed to determine chemical variations in Mariana volcanic rocks from the mid-Oligocene to Recent time. This rock record is preserved by tephra deposits in the marine sediments with high temporal precision and resolution (~1 Ma). While the lavas of the early (~32-48 Ma) and present-day arc (<1 Ma) provide an essential framework, only the tephras provide comprehensive information on the crucial intermediate period of arc evolution. Fresh volcanic particles (glasses, scoria, pumice, juvenile lithics) from the fraction >35 _m will be separated from minimally altered tephra layers, and these samples will be analyzed for Sr-Nd-Pb-Hf isotope ratios and major and trace elements of fresh volcanic glasses and particles. The project bears on one of the fundamental science themes of the Subduction Factory Initiative: 'How do forcing functions such as convergence rate and upper plate thickness regulate production of magma and fluid from the Subduction Factory?'. On a regional scale, the new data will fill an important gap in the overall evolution of the IBM system that will provide new perspectives on the evolution of the IBM convergent margin. On a global scale, the study of secular arc evolution will aid in the understanding how the Earth's reservoirs interact, and how this may be linked to arc magmatic fluxes and global environmental change. Broader Impacts. The project will promote scientific partnership between U.S. and Australian scientists. The support for women in the geosciences will be furthered. Straub is a female researcher with a disability who has already made substantial contributions to the IBM evolution as part of a long-held goal of her research. Straub will produce an online MARGINS mini-lesson to share her expertise on the use of tephras as petrogenetic tool. It is planned to involve undergraduate students in this project research.