SESSION NO. 50 50-4 9:10 AM Farley, K.A. [53636] SESSION NO. 50, 8:00 AM APPLICATIONS AND LIMITATIONS OF (U-TH)/HE DATING FOR QUATERNARY GEOCHRONOMETRY Monday, July 28, 2003 FARLEY, K.A., Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, CA 91125,
[email protected] The recent resurgence of interest in dating by ingrowth of 4He from U and Th decay has been S30. New Developments in Quaternary Numeric driven by thermochronometry applications, but a few studies have demonstrated its strengths and Dating Methods weaknesses for “absolute” dating, especially of very young materials. The main attraction of 4He for this application is that ingrowth in many minerals is rapid relative to blanks and detection limits; e.g., in just 1 Myr the 4He produced in 1 mg of material with 1 ppm U at secular equilibrium is Reno Hilton Resort and Conference Center, Carson 3&4 >100x typical blank. Furthermore, it is commonly possible to trade-off lower U, Th contents against larger sample size, so even materials with ppb levels of parent nuclides may be datable. However the method suffers from two major limitations: 1) Some materials diffuse He at Earth 50-1 8:10 AM Steig, Eric J. [55603] surface temperatures, including volcanic glass and whole rock, quartz, feldspars, and Mn oxides, THE MYSTIQUE OF ICE CORE CHRONOLOGIES while apatite, zircon, titanite, garnet, fluorite, amphibole, pyroxene, and some Fe oxides are STEIG, Eric J., Quaternary Research Center, University of Washington, 19 Johnson Hall, thought to be sufficiently retentive to be potential dating targets (though “excess He” may be a Box 351360, Seattle, WA 98195,
[email protected] problem in some of these minerals).