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Cosmogenic ^3He production rates in apatite, zircon and pyroxene inferred from Bonneville flood erosional surfaces

Amidon, William H. and Farley, Kenneth A. (2011) Cosmogenic ^3He production rates in apatite, zircon and pyroxene inferred from Bonneville flood erosional surfaces. Quaternary Geochronology, 6 (1). pp. 10-21. ISSN 1871-1014.

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^3He concentrations were measured in zircon, apatite and pyroxene separates from rhyolite surfaces along the Snake River near Twin Falls, Idaho. These fluted and potholed surfaces were presumably scoured by the Bonneville outburst flood ~17.5 ka. Several of the samples contain inherited ^3He from prior exposure, reflecting the complex incision history of the Snake River canyon and suggesting earlier flood events. Each individual mineral from the remaining surfaces yielded ^3He concentrations that are within error of each other. ^3He produced by neutron capture on ^6Li was established from shielded samples and a simple neutron production model. The remaining ^3He is spallogenic and was used to calibrate the production rate of cosmogenic ^3He in zircon, apatite and pyroxene by assuming minimal erosion since the Bonneville flood. In all three phases the resulting production rates are ~7–12% lower than previous ^3He production rate estimates obtained by reference to various production rates for ^(10)Be in quartz. This disagreement is partially reconciled here by recalculating previous results with a ^(10)Be production rate of 4.51 at g^(−1) a^(−1) in quartz. Adopting this revised rate brings three independent ^3He production rate estimates for zircon and apatite into agreement within 5%, with grand means of 103 ± 3 at g^(−1) a^(−1) in zircon and 133 ± 6 at g^(−1) a^(−1) in apatite. The major source of uncertainty in cosmogenic ^3He dating of these phases is now correction for ^3He produced by neutron capture on ^6Li. Calculations are presented for assessing the amount of uncertainty introduced by this correction as a function of Li concentration, cosmic-ray exposure age, and He closure age.

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Farley, Kenneth A.0000-0002-7846-7546
Additional Information:© 2010 Elsevier B.V. Received 4 January 2010; revised 24 March 2010; accepted 30 March 2010. Available online 7 April 2010. We acknowledge funding from NSF grant EAR-0921295 as well as field assistance from Kevin Uno and insightful discussions with Michael Lamb. Careful reviews by Pete Burnard and Samuel Niedermann greatly improved the manuscript. Editorial handling by: D. Bourlès.
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Subject Keywords:Helium; 3H; Tritium; Shoshone Falls; Twin Falls; Li; Thermal neutron
Issue or Number:1
Record Number:CaltechAUTHORS:20110202-120311957
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Official Citation:William H. Amidon, Kenneth A. Farley, Cosmogenic 3He production rates in apatite, zircon and pyroxene inferred from Bonneville flood erosional surfaces, Quaternary Geochronology, Volume 6, Issue 1, February 2011, Pages 10-21, ISSN 1871-1014, DOI: 10.1016/j.quageo.2010.03.005. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:21970
Deposited By: Tony Diaz
Deposited On:08 Feb 2011 03:58
Last Modified:03 Mar 2020 13:01

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