(U Th) / Ne chronometry

Details (U Th) / Ne chronometry (U Th) / Ne chronometry

Mar 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006e%26psl.243..520g&link_type=abstract

Earth and Planetary Science Letters, Volume 243, Issue 3-4, p. 520-535.

Computer Science

8

Scientific paper

Production of 21Ne from the reaction 18O(α, n)21Ne in U and Th rich minerals such as apatite, zircon, monazite and titanite can potentially be used for chronometry. Based on a review of the available cross section data we reevaluated the production of 21Ne from this reaction using a thin target approach and compared the results against thick target situations. The (21Ne / 4He) production ratio in these minerals is about 4 × 10- 8, but varies with oxygen content and Th / U ratio. 21Ne has a stopping range of about 1 μm compared to about 20 μm for α particles; thus the (21Ne / 4He) production ratio also depends on crystal size when the crystals are small enough that α ejection is important. Using a Monte Carlo model we computed the effects of ejection on the (21Ne / 4He) ratio for various geometries and grain sizes. We also present measurements of the (21Ne / 4He) ratio on few mg aliquots of well-dated volcanic apatites and zircons for which the duration of retention of 21Ne and 4He is the same. Values of (4.4 ± 1.1) × 10- 8 and (3.5 ± 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 and 28 ± 12 Ma, respectively, which are in agreement with independently known ages. Our results demonstrate that the 21Ne production is well understood and can be used to measure the neon retention time for slowly cooled minerals. 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 between the values given by the thermochronometers U Pb on apatite and Ar Ar on muscovite is derived, probably 400 ± 50 °C.

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