Abstract
The problem of the origin of the upper-mantle Xe is discussed and a straightforward model of lower- and upper-mantle mass exchange is presented. The upper-mantle Xe isotope composition is assumed to be that measured in continental CO 2 gases. This assumption is supported by a regression analysis of isotope compositions of Xe in MORB glasses. The possibility is investigated that the lower mantle contains Xe isotope abundances 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 unfractionated Xe are relatively lower than in air Xe. A mixture of the unfractionated Xe with plutogenic 131–136Xe(Pu) may yield Xe with relative 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 composition. The abundance of fission plutogenic 131–136Xe ★, solar-like 130Xe, and 129I-derived 129Xe ★ in mantle Xe are used to derive concentrations of these components in the lower mantle. A steady-state assumption for highly incompatible elements in the upper mantle with residence times of 1 Ga is used to estimate the concentration of uranogenic fission Xe(U) generated in this reservoir. These concentrations and the ratio of 136 Xe(U) 129 Xe(I) = 0.357 , inferred from 136 Xe 130 Xe− 129 Xe/ 130 Xe correlation for mantle-derived samples, allow a lower mantle-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 similar to independent estimates from UHeNe systematics. It is, however, by two orders of magnitude lower than the present-day mass flux of subducted slab.
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