Partitioning of Noble Gases Between Crustal Minerals: Implications for Geochronology

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Noble gas based thermochronometers (notably 40Ar/39Ar and U-Th/He) represent an important method of constraining rate and time in geologic processes - particularly regarding the thermal and tectonic evolution of the crust and surface of the Earth. Quantitative models have recently been developed which can predict and quantify "excess Ar" - a common yet poorly understood problem in standard thermochronology. In order for the models to be fully implemented and tested in natural systems, Ar (and He) partitioning data among crustal phases is needed. While broad constraints exist for mineral/fluid partitioning, equally important mineral/mineral partitioning data is scant and/or controversial. The proposed research is designed to: 1) Measure the relative partitioning of Ar (and other noble gases, He and Ne) between common crustal minerals, and in so doing, 2) test the hypothesis that certain minerals, such as quartz, may act as important sinks for Ar within the crust, thus playing a key role in limiting excess Ar in other surrounding minerals. Piston-cylinder experiments will be conducted to measure the equilibrium partitioning of Ar between common crustal phases including quartz, micas, and feldspars. The experiments will employ reactor-produced 37Ar to monitor noble gas partitioning among minerals. Analysis of equilibrated mineral surfaces will be performed using UV-laser and/or Rutherford Backscattering. Future field-based applications of this research are anticipated. The broader impacts of the proposed activities include the education and training of a graduate student as well as the development of the PI's collaborative research program involving colleagues at Rensselaer Polytechnic Institute and at the Open University, England.