Noble Gas Partitioning Between Mineral Interiors And Grain Boundaries

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Farley and AsimowEAR-0125784Despite the ubiquitous use of noble gases as tracers of Earth processes and as chronometers, many fundamental aspects of their geochemical behavior are, surprisingly, poorly understood. Among the uncertain characteristics are the siting of noble gases in rocks and minerals, mineral/melt partitioning behavior, solubility and diffusivity in important and commonly analyzed minerals, and transport mechanism and rate through wet and dry rocks. This proposal seeks to address a particularly critical characteristic of noble gas geochemistry, about which neither data nor theoretical understanding exist: at equilibrium, how do noble gases partition between grain boundaries and crystals? Unlike all other elements, the noble gases are uncharged and rather large atoms that do not fit in any obvious way within most crystal lattices. Thus it is not unreasonable to propose that noble gases will partition into the defect-rich region between grains. This possibility is not commonly considered, yet if true, would have profound implications for several important fields of geochemistry, most notably including studies of mantle evolution and noble-gas-based geochronology. The proposed research project seeks to investigate in a quantitative and thermodynamically rigorous way the partitioning behavior of noble gases between mineral interiors and grain boundaries in rocks. The conceptually simple experimental technique relies on reactor-produced isotopes to eliminate the concerns regarding atmospheric adsorption and contamination that plague studies based on natural isotopes. In addition to producing otherwise rare isotopes, neutron irradiation provides an initial condition closely analogous to in-situ radiogenic noble gas production relevant in nature. Although isotopes of all noble gases can be produced in many different substrates in this way, this pilot study will focus on just one model system: the partitioning of 4He and 37Ar in polycrystalline aggregates of diopside, equilibrated after irradiation at elevated temperature and pressure in the piston-cylinder device. Along with partitioning measurements, samples will be characterized texturally and chemically with the electron microprobe, SEM, and transmission electron microscope to directly assess the physical nature, width, and chemistry of the grain boundary region.The proposed work will develop and explore the techniques necessary for establishing the role of grain boundaries in noble gas behavior through study of this one simple system. If the irradiation technique works for assessing He and Ar partitioning in diopside, it should be generally applicable for other noble gases and other materials, and as such has potential well beyond the initial pilot study.