Experimental Investigations of the Role of H2O in Subduction Zone Processes

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Technical description:Three projects will be carried out. 1) Experimental studies mantle melting in the presence of H2O will determine the chemical composition of key minerals and the composition of the melt. These results will be used to understand the processes that transfer H2O from the subducted oceanic lithosphere into mantle wedge peridotite. 2) A systematic investigation of shallower mantle melting from 1 to 2 GPa (30 to km 60 depth in the Earth), will be carried out. These experiments will use mantle analog compositions and controlled variations in the amount of H2O. Their purpose is to provide constraints for the development of quantitative models of flux melting and reactive porous flow in the mantle wedge. 3) Experimental studies on crustal- level fractional crystallization of arc andesites to rhyolites. These experiments will be carried out at pressures of 100 to 300 MPa over a range of oxygen fugacities under H2O-saturated conditions and will calibrate an orthopyroxene-melt thermometer and hygrometer. This thermometer-hygrometer will be widely applicable to andesite-rhyolite magmas. 4) Parallel geochemical and petrologic investigations on andesite and dacite lavas from Mt. Shasta and Newberry volcano in the Cascades, USA to provide quantitative information on crustal differentiation processes. In all of the experimental studies we will develop quantitative, thermodynamically based analytical tools that allow estimation of melting/crystallization processes where H2O is involved. The effects of H2O flux through the subduction zone environment are of fundamental importance for understanding the chemical evolution of the Earth's crust-mantle system over the last 4.5 billion years. This information on the influence of H2O on melting and crystallization processes will allow us to develop better models that couple mantle flow with subduction. Ultimately, the research has relevance for the origin of the Earth?s oceans and the evolution of the atmosphere. Broader significance and importance:This work will advance our understanding of the processes that lead to the formation of magmas in subduction zone environments. H2O is a key ingredient in the formation of these magmas that erupt from volcanoes (like Mt. St. Helens) or cool and crystallize at depth to form the Earth's continental crust. By understanding water's influence on the temperature and chemical reactions of this magma generation process, we will gain a better understanding of where and how chemical elements are concentrated in these magma and where and how much H2O is recycled through the Earth's interior. This is a fundamentally important problem in Earth Science that has implications for a number of practical matters. Two of immediate public interest are: 1) the formation of ore deposits that concentrate elements of economic importance (e.g. Cu, Zn, Pb) where they can be extracted and 2) H2O is the active ingredient in producing the dangerous volcanic eruptions that are the signature of subduction volcanoes: knowing the amount of H2O that can be incorporated is fundamental fro understanding how these eruptions work.