Tagami, Takahiro and Farley, Kenneth A. and Stockli, Daniel F. (2003) (U-Th)/He geochronology of single zircon grains of known Tertiary eruption age. Earth and Planetary Science Letters, 207 (1-4). pp. 57-67. ISSN 0012-821X http://resolver.caltech.edu/CaltechAUTHORS:20121115-092559670
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We performed a systematic study of (U–Th)/He ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon He dating and to validate the resulting ages. Using laser He extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks: Fish Canyon Tuff (accepted age 27.8±0.7 Ma), Buluk Tuff (16.3±0.2 Ma), Tardree Rhyolite (58.4±0.7 Ma) and Utaosa Rhyolite (2.52±0.02 Ma). He ages of Fish Canyon Tuff, Buluk Tuff and Utaosa Rhyolite zircons are in good agreement with their reference ages: mean α emission-corrected ages were 28.6±1.4 (1σ), 16.1±0.8 and 2.61±0.18 Ma, respectively. However, the zircon He ages of Tardree Rhyolite are irreproducible and consistently too old, with a mean He age of 78.8±7.0 Ma. Spontaneous fission track densities of Tardree Rhyolite zircons indicate that about 80% of these zircons are strongly zoned in U, with high track density (i.e., high U content) in the core and low density on the rim. This causes an overestimation of the α ejection correction that likely explains the erratic and excessively high He ages from these zircons. Assessment of the degree and style of U zonation within a zircon population is thus prudent before application of the (U–Th)/He method. Step-heating experiments indicate that for most zircons He extraction is >99% complete after ∼1 h at 1300°C. However, for as yet unknown reasons occasional grains seem to retain substantial amounts of He under these conditions.
|Additional Information:||© 2003 Elsevier Science B.V. Received 10 July 2002; received in revised form 15 October 2002; accepted 11 December 2002. We thank Martha House and Lindsey Hedges for their help and suggestions. We also thank Chuck Naeser and Tony Hurford for supplying the standard zircons. T. T. thanks Kyoto University Foundation for leave to Caltech. This work was supported in part by the National Science Foundation.[BW]|
|Subject Keywords:||zircon; helium; geochronology; standard; laser heating|
|Official Citation:||Takahiro Tagami, Kenneth A. Farley, Daniel F. Stockli, (U–Th)/He geochronology of single zircon grains of known Tertiary eruption age, Earth and Planetary Science Letters, Volume 207, Issues 1–4, 28 February 2003, Pages 57-67, ISSN 0012-821X, 10.1016/S0012-821X(02)01144-5. (http://www.sciencedirect.com/science/article/pii/S0012821X02011445)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||15 Nov 2012 17:37|
|Last Modified:||15 Nov 2012 17:37|
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