Surface-Exposure Chronology Using in Situ Cosmogenic 3He in Antarctic Quartz Sandstone Boulders

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Abstract
In situ-produced cosmogenic helium (3Hec) provides a new tool for constraining histories of Quaternary geomorphic surfaces. Before general application of the technique, however, the systematics and production rates of 3Hec must be well understood. In a companion study, 3He and 10Be data from sandstone and granite boulders in the Dry Valleys region of Antarctica have been used to constrain the ages of an important moraine sequence formed by the Taylor Glacier. Data from these deposits also provide information about the systematics of 3He in quartz that has important implications for geochronology based on 3Hec. In contrast to previous results from olivine and clinopyroxene, crushing quartz in vacuo releases helium with high 3He/4He ratios (up to 148 × Ra, where Ra is the atmospheric 3He/4He ratio), indicating that crushing cannot be used to determine the isotopic composition of trapped (i.e., noncosmogenic) helium in quartz. Analysis of 3He in different size fractions of the same samples indicates significant 3 He loss not predicted by existing 3He diffusion data for quartz. The origin of the discrepancy is not clear, but loss from these samples is not as significant as suggested by the limited data of previous studies.