HR: 1340h
AN: V13A-0512    [Abstracts]
TI: (U-Th)/Ne Chronometry
AU: * Gautheron, C E
EM: gautheron@geol.u-psud.fr
AF: Division of Geological and Planetary Sciences, MS100-23, California Institute of Technology, Pasadena, CA CA91125 United States
AU: Tassan-Got, L
EM: tassango@ipno.in2p3.fr
AF: Institut Physique Nucléaire, Bat 102, Université Paris XI, Orsay, 91405 France
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, MS100-23, California Institute of Technology, Pasadena, CA CA91125 United States
AB: Nuclear production of ²¹Ne, like ⁴He, in U and Th rich minerals such as apatite, zircon, monazite and titanite can potentially be used for chronometry. Due to the absence of ²⁴Mg from the accessory minerals of interest, the production of ²¹Ne through neutron-induced reactions can be neglected and this production only occurs through the reaction ¹⁸O(α,n)²¹Ne. As the closure temperature is higher for neon than for helium, this gives a possible insight to the thermal history of minerals. To test the possibility of this new chronometer, a review of the available cross section data has been done, permitting a reevaluation of the ²¹Ne production from this reaction, using a thin target approach and testing the validity of the results against thick target situations. The important factors of the simulation are the cross section and the stopping distance values for a mineral characterized by its chemical composition and density. The (²¹Ne/⁴He) production ratio in these minerals is about 4× 10^-8, but varies with oxygen content and Th/U ratio. Although the ²¹Ne has a stopping range of about 1 μm compared to about 20 μm for α particles, preferential α ejection compared to ²¹Ne lowers (²¹Ne/⁴He) at grain edges. For a bulk crystal the (²¹Ne/⁴He) production ratio depends on crystal size when the crystals are small. Using a Monte Carlo model we computed the effects of ejection on the (²¹Ne/⁴He) ratio for various geometries and grain sizes. The simulation can be run by any user through a graphical interface accessible from the Web at the address: http://h0.web.u-psud.fr/UThHeNe_MonteCarloSimulation/. We also present measurements of the (²¹Ne/⁴He) ratio on few mg aliquots of well-dated volcanic apatites and zircons. Values of (4.43± 1.1)× 10^-8 and (3.46± 1.2)× 10^-8 for apatite and zircon respectively are in agreement with the theoretical values of (4.16± 0.14)× 10^-8; (3.54± 0.13)× 10^-8. Based on our production rate estimates the Durango apatite and Fish Canyon Tuff zircon give Ne ages of 34.2± 8.6 Ma and 28.0± 12.2 Ma respectively, which are in agreement with independently known ages. Our results demonstrate that (U-Th)/Ne can be used as a chronometer in volcanic samples. Additionally, the ⁴He and ²¹Ne content of zircons from the deeply exhumed crustal section in Gold Butte, Nevada (crystallization age of 1.4 Ga) imply (U-Th)/Ne ages of 963± 164 and 777± 122 Ma, far older than their He ages of 16.7± 1.3 and 19.1± 1.5 Ma respectively. To explain the age difference, a neon closure temperature for zircon around 400°C is derived.
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
DE: 1744 Tectonophysics
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005