Isotopic Helium as an Indicator of the Terestrial Exposure Age of Minerals and Implications for Surface Processes

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3He accumulates in minerals exposed at the earth's surface because cosmic rays cause spallation of smaller nuclei from target nuclei. Because the cosmic ray flux and production rates are fairly well constrained and the cosmogonic component of 3He in minerals can be determined, measurement of 3He and 4He abundances in minerals at the earth's surface can be used to calculate the exposure ages of minerals. This knowledge can be used to study a variety of processes that take place at the earth's surface on a time scale of 100 years to several million years. Measurements involve the 3He and 4He abundances in rocks whose exposure ages are fairly well constrained by geologic evidence and by conventional K-Ar or 14C dating methods. Samples will be cored from glaciated surfaces (Sierras) and recent Lava flows (W. United States, Hawaii). In the Scripps Isotopic Laboratory, researchers will test radiogenic gas separation techniques and the possible efficacy of neon for radiogenic dating. Knowledge of the exposure age of surfaces will have important applications in Quaternary stratigraphy and geomorphology, in the study of chemical and mechanical erosion rates, and may have applications to understanding seismic recurrences and other mass movement phenomena.