HR: 1340h
AN: U33A-0003    [Abstracts]
TI: Experimental Determination of the Cosmogenic Ar Production Rate From Ca
AU: * Niedermann, S
EM: nied@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473, Potsdam Germany
AU: Schäfer, J M
EM: schaefer@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964 United States
AU: Wieler, R
EM: wieler@erdw.ethz.ch
AF: ETH Zurich, Isotope Geology and Mineral Resources, NO C61, CH-8092, Zurich Switzerland
AU: Naumann, R
EM: rudolf@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473, Potsdam Germany
AB: Cosmogenic 38Ar is produced in terrestrial surface rocks by spallation of target nuclides, in particular K and Ca. Though the presence of cosmogenic Ar in Ca-rich minerals has been demonstrated earlier [1], is has proven difficult to establish its production rate. To circumvent problems connected to 36Ar production by 35Cl neutron capture and different production rates from K and Ca, we have analyzed the noble gases in seven pyroxene separates (px) from the Antarctic Dry Valleys which are essentially free of Cl and K. The px were obtained from dolerite rocks, for which 3He and 21Ne exposure ages from 1.5 to 6.5 Ma have been reported [2]. The noble gases were extracted in two or three heating steps at GFZ Potsdam, yielding 38Ar/36Ar ratios up to 0.2283 ± 0.0008 (air: 0.1880). Ca (3.7-11.2 wt. %) is expected to be the only relevant target element for Ar production in the five pure px (<0.05% K); the production rate from Fe is at least two orders of magnitude lower than that from Ca [e.g. 3]. Assuming an 38Ar/36Ar production ratio of 1.5 ± 0.2, we obtain cosmogenic 38Ar concentrations between 130 and 530x10⁶ atoms/g. The 38Ar production rate was calculated based on 21Ne exposure ages [2], corrected for elevated nuclide production in Antarctica due to prevailing low air pressure and for the revised 21Ne production rate from Si. We obtain values between 188 ± 17 and 243 +110/-24 atoms (g Ca)^-1 a^-1 at sea level and high (northern) latitudes for four out of the five pure px, while one yields a very high value of 348 ± 70 atoms (g Ca)^-1 a^-1. Values above 250 atoms (g Ca)^-1 a^-1 are also obtained from two less pure px containing 0.3 and 0.9% K and from one feldspar/quartz accumulate, indicating that the production rate from K may be higher than that from Ca. The weighted mean (excluding the outlier) of ~200 atoms (g Ca)^-1 a^-1 is in excellent agreement with Lal's [3] theoretical estimate. [1] Renne et al., EPSL 188 (2001) 435. [2] Schäfer et al., EPSL 167 (1999) 215. [3] Lal, EPSL 104 (1991) 424.
DE: 1130 Geomorphological geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 4918 Cosmogenic isotopes (1150)
SC: Union [U]
MN: Fall Meeting 2005