PRODUCTION OF NUCLEOGENIC NEON IN THE EARTH FROM NATURAL RADIOACTIVE DECAY

Nucleogenic neon isotope production resulting from a decay of naturall y occurring U and Th was calculated for typical compositions of the ma ntle and crust using a simple non-Monte Carlo code for neutron transpo rt. The (alpha,n), (alpha,p), and (n,alpha) reactions, leading to the production of neon isotopes in the outgoing reaction channel, are take n into account in the calculations. In the first stage of calculation, neon production from (alpha,n) and (alpha,p) reactions on O-17, O-18, and F-19 is considered, and the neutron energy spectrum from (alpha,n ) reactions on the major elements is derived. This spectrum is further used for calculation of the neon isotope yield from the reactions Mg- 24(n,alpha)Ne-21, Mg-25(n,alpha)Ne-22, and Na-23(n,alpha)Ne-20. The ca lculated production of nucleogenic neon is dominated by (alpha,n) reac tions with oxygen in the Earth's mantle and crust. The present calcula tions give Ne-21/He-4 production ratios that are approximately half th ose of the previous estimates by Risen [1980] and Kyser and Risen [198 2]. The present study also has shown that the nucleogenic Ne-21 produc tion from the Mg-24(n,alpha) reaction is only about one-fifth the prev ious estimate. The contribution of nucleogenic Ne-21 from the Mg-24(n, alpha) reaction is insignificant compared with nucleogenic Ne-21 produ ction from O-18(alpha,n). Our estimate of the nucleogenic Ne-21/radiog enic He-4 production ratio both in the mantle and in the crust is 4.5 x 10(-8). This production ratio is quite consistent with nucleogenic N e-21/radiogenic He-4 ratios actually observed in crustal fluid samples ((4.6 +/- 0.8) x 10(-8)). The error in calculation of the nucleogenic neon production, resulting from a random statistical variation of the entry neutron yields and cross sections, is estimated to be less than 5%.