HR: 14:55h
AN: V13F-06    [Abstracts]
TI: Cosmogenic 3He in Fe Oxide Weathering Products
AU: * Shuster, D L
EM: dshuster@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: Caltech, Div. Geological and Planetary Sciences 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Vasconcelos, P M
EM: paulo@earth.uq.edu.au
AF: University of Queensland, Department of Earth Sciences, Brisbane, QLD 4072, Australia
AU: Stone, J O
EM: stone@geology.washington.edu
AF: University of Washington, Dept. Earth and Space Sciences 4000 15th Avenue NE, Seattle, WA 98195,
AB: We report ³He and ⁴He concentrations in samples of high-purity goethite and hematite from the uppermost 10 vertical meters of Fe-oxide cement (canga) atop a deep weathering profile at Cajaras, Brazil (6° S, 720m elev.) in the Amazon. ³He concentrations in goethite range from 2x10⁸ atoms/g near the surface to 2x10⁷ atoms/g at 10 meters depth, and in hematite range from 3x10⁸ to 5x10⁷ atoms/g, respectively. With a few exceptions, samples of both minerals show a clear exponential decrease in ³He concentration with increasing depth. However, the samples show no clear relationship between ⁴He concentration and depth, or between ³He and ⁴He. Although we consider other potential production mechanisms, we believe the ³He is dominantly cosmogenic in origin and was produced in situ. For instance, nucleogenic ³He from neutron capture by ⁶Li does not contribute more than ~10⁴ atoms/g of ³He. This low value arises from (i) the low Li content of the Fe oxides (~50 ppb), (ii) the extremely low productivity of neutrons in Fe oxides that are low in light elements and U+Th, and (iii) the comparative youth of the Fe oxides (e.g., goethite (U-Th)He ages < 50 Ma). Assuming a ³He production rate of 100 atoms/g/yr, the surface ³He concentrations require that the Fe oxides have resided near the surface for at least ~3 million years (if the erosion rate is zero), but an exposure age greater than ~10 Myr is implied at erosion rates less than 1 m/Myr. For a mean rock density of approximately 2.5 g/cm³, the apparent attenuation length-scale is much higher than expected for purely spallogenic ³He. Thus it is likely that muon-induced reactions significantly contribute to the measured ³He concentrations at depths greater than ~ 200 g/cm². Due to their ubiquity as chemical weathering products, cosmogenic nuclides in goethite and hematite offer great potential for the study of surface processes and landscape evolution in environments dominated by chemical weathering.
DE: 1039 Alteration and weathering processes (3617)
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting