Crustal Imaging and Characterization Team
Noble gas isotope chemistry of fluid inclusions is a relatively new field that when combined with stable isotope, active gas and solute chemistry data has the potential to result in a quantum leap in the understanding of ore forming processes. As inert and conservative gas species, noble gases and their isotopes provide independent insights into volatile evolution in magma generation through magmatic and hydrothermal processes, independent of specific rock or gas buffered constraints. Melt-crystal-fluid solubility, partitioning, radiogenic, and nucleogenic production, and isotopically definable 'reservoirs' enable clear concentration and isotopic signatures of important ore-forming processes.
The task will provide high precision fluid inclusion gas analyses of the active gases (N2, CO2, CH4, H2, H2S, SO2, HCl, HF, H2O and light hydrocarbons) and noble gases and their isootpes (He, Ne, Ar, Kr, Xe). Composition and isotopes will be used to identify volatile evolution, magma degassing, and gas-rock buffering processes leading to mineralization in several metal deposit environments. The thermodynamics versus kinetics of key reactions will be examined. Geochemical and model driven paradigms of ore-forming processes will be advanced based upon new insights constrained by active and noble gas chemistry.
The Mineral Resources Program (MRP) and the National Cooperative Geologic Mapping Program benefit from the proportioned investment in this noble gas task which concentrates both on specific applied geochemical research and on instrumental and methods development focused to contribute noble gas and active gas geochemistry to several Minerals and Mapping projects.
In continuing research on magmatic volatile compositions, their coupling to magmatic processes, and roles played in ore forming processes, this fiscal year's efforts will provide composition data on very detailed evolution of magmatic volatiles in growth bands of alunite from Marysvale alunite ridge deposits. Special instrumental methodology enables both noble gas analysis and detailed redox thermochemical modeling of H2-H2O, H2S-SO2, CH4-CO2, rock and gas buffering. These efforts are keyed to related MRP projects in identifying 'key' indicators potentially useful for mineral resource assessment (e.g. Mt. Lassen, Florida Canyon, and others). This research includes efforts with colleagues to develop new technology for detailed fluid inclusion volatile gas chemistry analyses using new ion trap - time of flight (IT-TOF) hardware.
Additional efforts focus on noble gas nuclides 3He and 21Ne produced by cosmic neutronmuon interactions at surface exposures thus enabling cosmogenic surface exposure age geochronology. This is critical both for geochronology study of young age surficial processes, and for distinguishing between exposure nuclide components and deep mantle 3He and 21Ne in mineralizing processes. Both research objectives require the same underlying very low level analytical methods and detection limits.
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U.S. Geological Survey
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