[1980] and Kyser and Rison [1982]. The present study also has shown that the nucleogenic 2•Ne production from the 24Mg(n,00 reaction is only about one-fifth the previous estimate. The contribution of nucleogenic 21Ne from the 24Mg(n,00 reaction is insignificant compared with nucleogenic 2•Ne production from 180(o•,n). Our estimate of the nucleogenic 2•Ne/radiogenic 4He production ratio both in the mantle and in the crust is 4.5 x 10 -8. This production ratio is quite consistent with nucleogenic 21Ne/radiogenic 4He 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%.

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We report a full suite of radiogenic, nucleogenic and fissiogenic noble gas compositions obtained by step-heating experiments from early Archaean Greenland zircons. We estimated activation energy (E a ) of diffusion and closure temperatures (T c ) for radiogenic 4 He* and fissiogenic 86 Kr* and 136 Xe* in zircons. These data demonstrate that the combined study of (U-Th)/He and (U-Th)/Kr-Xe can provide powerful geochronological information on cooling ages and crystallization ages from the same samples.

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