Shuster, David L. and Vasconcelos, Paulo M. and Heim, Jonathan A. and Farley, Kenneth A. (2005) Weathering geochronology by (U-Th)/He dating of goethite. Geochimica et Cosmochimica Acta, 69 (3). pp. 659-673. ISSN 0016-7037 http://resolver.caltech.edu/CaltechAUTHORS:20121120-133722013
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Nine samples of supergene goethite (FeOOH) from Brazil and Australia were selected to test the suitability of this mineral for (U-Th)/He dating. Measured He ages ranged from 61 to 8 Ma and were reproducible to better than a few percent despite very large variations in [U] and [Th]. In all samples with internal stratigraphy or independent age constraints, the He ages corroborated the expected relationships. These data demonstrate that internally consistent He ages can be obtained on goethite, but do not prove quantitative ^4He retention. To assess possible diffusive He loss, stepped-heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic ^3He. The ^3He release pattern indicates the presence of at least two diffusion domains, one with high helium retentivity and the other with very low retentivity at Earth surface conditions. The low retentivity domain, which accounts for ∼ 5% of ^3He, contains no natural ^4He and may represent poorly crystalline or intergranular material which has lost all radiogenic ^4He by diffusion in nature. Diffusive loss of ^3He from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate, so probably represents diffusion from individual micrometer-size goethite crystals. The ^4He/^3He evolution during the incremental heating experiments shows that the high retentivity domain has retained 90%–95% of its radiogenic helium. This degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to Earth surface temperature and held for the appropriate duration. Considering both the high and low retentivity domains, these data indicate that one of the samples retained 90% of its radiogenic ^4He over 47.5 Ma and the other retained 86% over 12.3 Ma. Thus while diffusive-loss corrections to supergene goethite He ages are required, these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton-irradiation ^4He/^3He method.
|Additional Information:||© 2005 Elsevier Ltd. Received March 19, 2004; accepted in revised form July 30, 2004. Associate editor: T. M. Harrison. We thank L. Hedges and C. Paine for sample preparation and J. Sisterson and E. Cascio for their expertise with the proton irradiation. We thank P. Reiners and an anonymous reviewer for their constructive comments on the manuscript. This work was supported by the National Science Foundation and UQ-AGES (University of Queensland Argon Geochronology in Earth Sciences) Laboratories. D.L.S. was supported by a N.S.F. Graduate Research Fellowship.|
|Official Citation:||David L. Shuster, Paulo M. Vasconcelos, Jonathan A. Heim, Kenneth A. Farley, Weathering geochronology by (U-Th)/He dating of goethite, Geochimica et Cosmochimica Acta, Volume 69, Issue 3, 1 February 2005, Pages 659-673, ISSN 0016-7037, 10.1016/j.gca.2004.07.028. (http://www.sciencedirect.com/science/article/pii/S0016703704006076)|
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|Deposited By:||Ruth Sustaita|
|Deposited On:||20 Nov 2012 21:54|
|Last Modified:||20 Nov 2012 21:54|
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