SOME COMMENTS ON ISOTOPIC STRUCTURE OF TERRESTRIAL XENON

The problem of the origin of the upper-mantle Xe is discussed and a st raightforward model of lower- and upper-mantle mass exchange is presen ted. The upper-mantle Xe isotope composition is assumed to be that mea sured in continental CO2 gases. This assumption is supported by a regr ession analysis of isotope compositions of Xe in MORB glasses. The pos sibility is investigated that the lower mantle contains Xe isotope abu ndances that are similar to unfractionated solar Xe, as now appears to be the case for both He and Ne. Abundances of the heavy isotopes in u nfractionated Xe are relatively lower than in air Xe. A mixture of the unfractionated Xe with plutogenic Xe-131-136(Pu) may yield Xe with re lative abundances of heavy Xe isotopes similar to atmospheric Xe. This makes Xe(Pu) invisible in so far as air Xe is used as a reference com position. The abundance of Xe-131-136, solar-like Xe-130, and I-129-d erived Xe-129 in mantle Xe are used to derive concentrations of fissi on plutogenic these components in the lower mantle. A steady-state ass umption for highly incompatible elements in the upper mantle with resi dence times of 1 Ga is used to estimate the concentration of uranogeni c fission Xe(U) Xe-136(U)/Xe-129(I)=0.357, inferred from Xe-136/Xe-130 -Xe-129/Xe-130 correlation for mantle-derived samples, allow a lower m antle-upper mantle mass flux to be estimated. This flux is within the range from 0.05 . 10(13) to 1.2 . 10(13) kg/a, the upper limit being s imilar to independent estimates from U-He-Ne systematics. It is, howev er, by two orders of magnitude lower than the present-day mass flux of subducted slab.