[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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In order to increase the sensitivity of static-mode noble gas mass spectroscopic analysis, we have developed and tested a gas re-circulation system, which uses a commercially available turbo molecular pump (TMP). During the analysis, the sample gases are directed from the collector end towards the ionization source part of the flight tube. Our preliminary results indicate that the apparent sensitivity of krypton and xenon can be increased by a factor of about 20 and 50, respectively. We also found that the gain factor of helium was somewhat smaller and was approximately 2. Such mass-dependence is quite consistent with the compressibility of TMP which is larger for the heavier molecules.

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