Since 1963 when 129Xe excess were first observed in CO2 samples (Butler et al., 1963) similar excesses had been found in other well gases and in MORB. The 129Xe anomaly has been attributed to the decayed primordial 129I that was incorporated into the early Earth's interior. Excesses of 131-136Xe were also observed and attributed mainly to spontaneous fission of 238U and (or) probably 244Pu, although the fission spectra of these heavy nuclei did not match precisely the natural Xe spectra. On this basis a number of mantle degassing models have been suggested with 129Xe/136Xe being an important modelling parameter. Nevertheless, up to now there are serious problems in terrestrial xenology. It is not clear (a) why terrestrial Xe is isotopically unique among the major solar system reservoirs, (b) why Xe and Kr are mass-fractionated relative to AVCC (or solar) in different directions, (c) why there is a lack of 244Pu derived Xe in samples that carry clear excess-129Xe although one would expect that Pu fission Xe is more abundant than Xe from primordial 129I because of the longer 244Pu half-life (compared to that of 129Xe) and the comparable initial
abundances of 244Pu and 129I. In general, it is unclear why in Xe-rich sediments that were involved in subduction and subsequent mixing within the mantle Xe anomalies are not completely erased.
These problems stimulated to search for a non-primordial origin of the 129Xe anomaly. It was suggested that the observed 129Xe excess is due to the decay of 129I that in turn is a fragment of either 238U spontaneous fission (Caffee and Hudson, 1987) or n-induced 235U fission (Meshik and Shukolyukov, 1986; Pravdivtseva et al., 1986). If fission of heavy nuclei occurs simultaneously with diffusion of the radioactive fission products I, Te, Sb, and Sn, and if certain portions of these products could migrate out of parent minerals into the atmosphere and only then decay into stable Xe isotopes, it finally will result in the observed isotopic shifts in atmospheric Xe with the shifts correlated to the half-lifes of precursor isotopes. Indeed, we have shown that such a correlation exists.
To test our approach we assembled all available published Xe data with noticeable 129Xe excess in three isotope plots where we regard mantle Xe as well as atmospheric Xe to be a mixture of AVCC-Xe, fission Xe and CFF-Xe. First, we calculated the isotopic composition of CFF-Xe from the slope of mixing lines between AVCC-Xe and CFF-Xe. Second, we subtracted AVCC-Xe from atmospheric
Xe assuming that all 130Xe is due to AVCC-Xe. Third, we graphically decomposed the reminder using various three-isotope plots. Finally, we calculated the components of atmospheric Xe to be simply 91% AVCC-Xe + 5.3% 238U fission Xe + 2.5% 235U n-induced Xe + 1.1% CFF-Xe (numbers are given for 136Xe).
Thus, neither isotopic mass-fractionation nor mysterious U-Xe are anymore needed to explain the Xe composition of the terrestrial atmosphere. Our air Xe is not unique indeed: It consists of real and experimentally identified components. The only problem with our new concept is the abundance of light Xe isotopes. Unfortunately, the data on light Xe isotopes in mantle samples and even in CO2 gases are rather uncertain. Although there were indications that mantle Xe has a higher 128Xe/130Xe than the atmosphere (Harper et al., 1995), most published light Xe isotope ratios are close to air. However, if we suppose that the main portion of L-Xe (or HL-Xe) is still in the solid Earth because the carrier of this component is not yet degassed (in contrast to the carrier of AVCC) this difficulty is removed as well. The proposed approach to terrestrial xenology has the attractive advantage not to require complex and sometimes very artificial scenarios for the early history of the Earth.
This work is supported by the INTAS foundation grant # 94-2397.
Butler et al., J. Geophys. Res. 68, 3283-3291 (1963).
Caffee, M.W. & Hudson, G.B., LPSC XVIII, 145 (1987).
Harper, C.L. Jr. et al., Meteoritics 30, 517 (1995).
Meshik, A.P. & Shukolyukov, Y.A., In Proceedings of XI Symposium on Isotope Geochemistry, 237-238 (Moscow, 1986), in Russian.
Pravdivtseva, O.V. et al., In Proceedings of XI Symposium on Isotope Geochemistry, 289-238 (Moscow, 1986), in Russian.