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

In situ -produced cosmogenic helium ( 3 He c ) provides a new tool for constraining histories of Quaternary geomorphic surfaces. Before general application of the technique, however, the systematics and production rates of 3 He c must be well understood. In a companion study, 3 He and 10 Be 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 3 He in quartz that has important implications for geochronology based on 3 Hec. In contrast to previous results from olivine and clinopyroxene, crushing quartz in vacuo releases helium with high 3 He/ 4 He ratios (up to 148 × R a , where R a is the atmospheric 3 He/ 4 He ratio), indicating that crushing cannot be used to determine the isotopic composition of trapped (i.e., noncosmogenic) helium in quartz. Analysis of 3 He in different size fractions of the same samples indicates significant 3 He loss not predicted by existing 3 He 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.