Zircon 4He/3He thermochronometry. (1st October 2015)

• Record Type: Journal Article
• Authors: Tripathy-Lang, Alka; Fox, Matthew; Shuster, David L.
• Is Part Of: Geochimica et cosmochimica acta. Volume 166(2015:Oct. 01)
• Page Start: 1
• Page End: 14
• Publication Date: 2015-10-01
• Subjects: CL Cathodoluminescence -- FCT Fish Canyon Tuff -- Dt/a2 diffusion kinetics -- eU effective uranium, eU = [U] + 0.235[Th] -- FT fission track -- FT fraction of alpha particles that come to rest within a crystal -- LAICPMS laser ablation inductively coupled plasma mass spectrometry -- LDP Little Devil's Postpile -- r radial coordinate -- Rstep 4He/3He ratio of each step -- Rbulk the 4He/3He ratio of the bulk sample -- t–T time–temperature -- Tbc bulk closure temperature -- To opening temperature -- Tr resetting temperature.; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.05.027
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: 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 4 He/ 3 He 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 40 Ar/ 39 Ar thermochronometry of K-feldspar and 4 He/ 3 He thermochronometry of apatite. For different cooling rates and diffusion domain size, the temperature sensitivity of zircon 4 He/ 3 He 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, 4 He in zircon will either be quantitatively retained or completely lost by volume diffusion. As proof-of-concept, we present stepwise release 4 He/ 3 He 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 4 He/ 3 He release spectra are consistent with a 4 He 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 4 He/ 3 He 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 4 He/ 3 He release spectra are strongly influenced by the observed parent nuclide zonation. A three-dimensional (3D) numerical model of 4 He 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 4 He/ 3 He release spectra for the independently known thermal history of the sample. We conclude that zircon 4 He/ 3 He thermochronometry provides accurate thermal constraints through geologic time, but certain applications of this technique may require 3D numerical modeling approaches.