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Publication Detail
Zircon <sup>4</sup>He/<sup>3</sup>He thermochronometry
  • Publication Type:
    Journal article
  • Publication Sub Type:
    Journal Article
  • Authors:
    Tripathy-Lang A, Fox M, Shuster DL
  • Publication date:
    01/01/2015
  • Pagination:
    1, 14
  • Journal:
    Geochimica et Cosmochimica Acta
  • Volume:
    166
  • Status:
    Published
  • Print ISSN:
    0016-7037
Abstract
© 2015 Elsevier Ltd. Multiple thermochronometric methods are often required to constrain time-continuous rock exhumation for studying tectonic processes or development of km-scale topography at Earth's surface. Here, we explore 4He/3He thermochronometry of zircon as a method for constraining continuous time-temperature (t-T) paths of individual samples through a temperature range that is complementary to methods such as 40Ar/39Ar thermochronometry of K-feldspar and 4He/3He thermochronometry of apatite. For different cooling rates and diffusion domain size, the temperature sensitivity of zircon 4He/3He thermochronometry ranges from slightly less than 100°C to slightly greater than 250°C; a typical sample provides continuous thermal constraints over ~100°C within that range. Outside these temperatures, 4He in zircon will either be quantitatively retained or completely lost by volume diffusion. As proof-of-concept, we present stepwise release 4He/3He spectra and associated U and Th concentration maps measured by laser ablation ICP-MS analysis of individual crystal aliquots of Fish Canyon Tuff (FCT) zircon and of a more complex setting in the Sierra Nevada batholith that experienced reheating from a proximal basaltic intrusion, the Little Devil's Postpile (LDP). The FCT zircon 4He/3He release spectra are consistent with a 4He spatial distribution dominated by alpha-ejection from crystal surfaces. The spatial distributions of U and Th measured in the same crystals do not substantially influence 4He/3He release spectra that are predicted for the known thermal history, even when incorporating spatially variable diffusivity due to accumulation of radiation damage. Conversely, the LDP 4He/3He release spectra are strongly influenced by the observed parent nuclide zonation. A three-dimensional (3D) numerical model of 4He production and diffusion, which incorporates crystal geometry, U and Th zonation, and spatially variable He diffusion kinetics, substantially improves the fit between measured and modeled 4He/3He release spectra for the independently known thermal history of the sample. We conclude that zircon 4He/3He thermochronometry provides accurate thermal constraints through geologic time, but certain applications of this technique may require 3D numerical modeling approaches.
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