(U-Th)/Ne chronometry

Production of Ne-21 from the reaction O-18(c( 11)Ne-21 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 Ne-21 from this reaction using a thin target approach and compared the results against thick target situations. The (Ne-21/He-4) production ratio in these minerals is about 4 x 10(-8), but varies with oxygen content and Th/U ratio. Ne-21 has a stopping range of about I gm compared to about 20 mu m for alpha particles; thus the (Ne-21/He-4) production ratio also depends on crystal size when the crystals are small enough that a ejection is important. Using a Monte Carlo model we computed the effects of ejection on the (Ne-21/He-4) ratio for various geometries and grain sizes. We also present measurements of the (Ne-21/He-4) ratio on few mg aliquots of well-dated volcanic apatites and zircons for which the duration of retention of 2 'Ne and 4 He is the same. Values of (4.4 +/- 1.1) x 10(-8) and (3.5 +/- 1.2) x 10(-8) for apatite and zircon, respectively, are in agreement with the theoretical values of (4.16 +/- 0.14) x 10(-8); (3.54 +/- 0.13) x 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 Ne-21 production is well understood and can be used to measure the neon retention time for slowly cooled minerals. The He-4 and Ne-21 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 degrees C. (c) 2006 Elsevier B.V. All rights reserved.