Nucleogenic production of Ne isotopes in Earth's crust and upper mantle induced by alpha particles from the decay of U and Th

The production of nucleogenic Ne in terrestrial crust and upper mantle by a lpha particles from the decay of U and Th was calculated. The calculations are based on stopping powers for the chemical compounds and thin-target cro ss sections. This approach is more rigorous than earlier studies using thic k-target yields for pure elements, since our results are independent of lim iting assumptions about stopping-power ratios. Alpha induced reactions acco unt for >99% of the Ne production in the crust and for most of the Ne-20,Ne -21 in the upper mantle. On the other hand, our Ne-22 value for the upper m antle is a lower limit because the reaction Mg-25(n,alpha)Ne-22 is signific ant in mantle material. Production rates calculated here for hypothetical c rustal and upper mantle material with average major element composition and homogeneously distributed F, U, and Th are up to 100 times higher than dat a presented by Kyser and Risen [1982] but agree within error limits with th e results by Yatsevich and Honda [1997]. Production of nucleogenic Ne in "m ean" crust and mantle is also given as a function of the weight fractions o f O and F. The alpha dose is calculated by radiogenic He-4 as well as by th e more retentive fissiogenic Xe-136. U and Th is concentrated in certain ac cessory minerals. Since the ranges of alpha particles from the three decay chains are comparable to mineral dimensions, most nucleogenic Ne is produce d in U- and Th-rich minerals. Therefore nucleogenic Ne production in such a ccessories was also calculated. The calculated correlation between nucleoge nic Ne-21 and radiogenic He-4 agrees well with experimental data for Earth' s crust and accessories. Also, the calculated Ne-22/He-4 ratios as function of the F concentration and the dependence of Ne-21/Ne-22 from O/F for zirc on and apatite agree with measurements.