Volatile Recycling in Subduction Zones and Formation of Primitive Arc Magmas: A Case Study of the Trans-Mexican Volcanic Belt

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EAR-0309559WallaceSubduction zones play an important role in global geochemical cycles because they provide a pathway for crustal materials to be recycled into the Earth's mantle. The materials include many volatile compounds such as water, carbon, sulfur, and chlorine. Subducted crustal materials become heated by the overlying mantle as they descend, and the volatiles are at least partially released by metamorphic devolatilization reactions, after which they migrate into the hot mantle above the subducted slab. This process causes the overlying mantle to melt because water lowers the melting temperature of mantle rocks. Thus recycling of volatiles in subduction zones is responsible for widespread formation of magma in Earth's mantle, resulting in chains of volcanoes such as those of the circum-Pacific "Ring of Fire". The objectives of this study are to investigate how volatile elements (H, C, S, Cl) are recycled in subduction zones and the role of volatiles in magma generation in the mantle wedge above subducting slabs. Because volatiles are almost completely degassed from magma during eruption, it is difficult to measure original concentrations directly. To circumvent this problem, the PI will analyze tiny inclusions of melt, quenched to glass upon eruption, that are trapped in olivine crystals in volcanic rocks. Such inclusions commonly preserve their pre-eruption volatile contents if they remain sealed in the crystals during eruption. The PI will collect samples from a number of subduction-related volcanoes in Mexico and analyze trapped inclusions in olivine using several techniques to measure the concentrations and isotopic compositions of volatiles, as well as other major and trace elements. The PI will use the results to test some fundamental ideas about volatile recycling and magma generation in subbduction zones and to construct quantitative models of volatile fluxes. The resulting data set will be the most extensive in terms of spatial coverage and most complete in terms of volatile element concentrations and isotope ratios for any subduction zone region in the world. In this project, the PI will integrate research and education by the involvement of a graduate and undergraduate student in fieldwork, sample analysis, geochemical modeling, hypothesis testing, and presentation of results at scientific meetings and through publications. These students will learn to use a number of sophisticated analytical techniques. The project, particularly the fieldwork, will involve collaboration with scientists and students in Mexico, thus providing a mutually beneficial scientific and cultural experience for them and their U.S. counterparts. Because of this collaboration, the project is partially supported by NSF's Americas Program of the Office of International Science and Engineering.