Radiogenic and fissiogenic Xe

The Xe budget provides strong evidence for early losses of noble gases from the atmosphere (see below), and isolation of atmospheric Xe from mantle Xe (see Xe isotopes and a nonresidual upper mantle section). 

Plutonium. Pu is a highly refractory element represented by a single isotope, 

However, the contribution of fissiogenic heavy isotopes are more difficult to calculate fractionated chondritic and solar Xe have greater proportions of Xe and Xe than is actually seen in the atmosphere and so cannot serve as the primordial terrestrial composition (see Pepin 2000). While no other suitable common solar system compositions have been found that provide the nonradiogenic heavy isotope composition, multi-dimensional isotopic correlations of chondrite data have been used to define a composition, U-Xe, that when mass-fractionated yields the light-isotope ratios of terrestrial Xe and differs from atmospheric Xe by a heavy isotope component that has the composition of Pu-derived fission Xe (Pepin 2000). The fractionated U-Xe ratios of Xe/ °Xe = 6.053 and Xe/ °Xe = 2.075 are the present best estimates of the isotopic composition of nonradiogenic terrestrial Xe (see further discussion in Pepin and Porcelli 

Therefore, 6.8 0.30% of atmospheric Xe ( Xeatm = 1.7x10 atoms) and 4.65 0.5% of atmospheric Xe ( Xeatm = 3.81 x 10 atoms) are radiogenic. The Xeatm is 20% of the total Xe produced by Pu in the BSE. However, the Xeatm is only 0.8% of the total Xe produced since 4.57 Ga such a low value cannot be accounted for by incomplete degassing of the mantle nor from any uncertainties in the estimated amount of Xe, and requires losses to space. Note that an alternative composition for atmospheric nonradiogenic Xe, obtained by Igarashi (1995), requires that 2.8 1.3% of the atmospheric Xe is added later. However, the composition of this component has relative proportions of the other heavy Xe isotopes that do not match the spectrum of either Pu or and so cannot be used to determine the fissiogenic Xe abundance of the atmosphere. 

Xe closure ages. The depletion of radiogenic Xe in the atmosphere due to losses from the Earth to space must have occurred during early Earth history, when such heavy species could have been lost either from protoplanetary materials or from the growing Earth. Full accretion of the Earth is believed to have occurred over -100 Ma (Wetherill 1975). Over this time, almost all of the and a considerable fraction of the Pu, had decayed to daughter Xe isotopes that could have been lost to space (Fig. 2a,b). Wetherill 

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Figure 11. Evolution of terrestrial Xe, Kr, Ar and Ne from primordial atmospheric distributions to present-day compositions by Giant Impact (Gl)-driven fractionation, addition of outgassed (OG) solar Kr, Ar and Ne, a later stage of solar EUV-powered Ne escape fractionation, and subsequent outgassing of radiogenic and fissiogenic Xe and nucleogenic Ne (after Pepin 1997). The 6 Xe and 5 Kr representations are defined by 5 = 1000 [(R/Rref) - 1] %o here R = Xe/ °Xe and Kr/ Kr respectively, and the references Rj-ef are the corresponding isotope ratios in the nonradiogenic Earth atmosphere for Xe (Pepin 1991) and the present Earth atmosphere (Basford et al. 1973) for Kr. Xe data from Table 1 solar and atmospheric Kr from Wider (2002, Table 5, refs. 2 and 4) solar Ar/ Ar = 5.80 0.06, °Ne/ Ne = 13.84 0.04, and Ne/ Ne = 0.0334 0.0003 from Palma et al. (2002).

Ocean island basalts and xenoliths show much smaller anomalies in Xe/ °Xe and Xe/ °Xe ratios than MORBs (Fig. 15). The small excesses in radiogenic and fissiogenic Xe appear to be correlated, but it is impossible to establish this firmly given... 

Figure 9. Xe isotope compositions of terrestrial precursors and present reservoirs. U-Xe, fractionated to match the light Xe isotopes (Fig. 7), provides an initial composition of the atmosphere, to which radiogenic Xe and fissiogenic e have been added. Similar fractionation of solar wind (SW) Xe produces Xe that is too heavy to supply the atmosphere. MORE typically have Xe/ °Xe and e/ °Xe ratios that are greater than the atmosphere due to radiogenic and fissiogenic additions and are correlated, with the range likely due to variable atmospheric contamination of samples. The most precise measurement of mantle Xe is for CO2 well gas. When uranogenic Xe is subtracted, the...

Figure 13. Post-escape evolution of Xe in the Earth s atmosphere (Pepin 2000). Escape-fractionated U-Xe defines an initially nonradiogenic terrestrial Xe composition (NEA-Xe, Table 1) to which radiogenic and fissiogenic components generated by decay of and in the crust and mantle were subsequently added by planetary outgassing.

A suitable initial composition for the atmosphere has been deduced from meteorites. Multi-dimensional isotopic correlations of chondrite data have been used to constrain a range of compositions that, when mass-fractionated, yields the light-isotope ratios of terrestrial Xe. In order to match the terrestrial heavy Xe isotope ratios, addition of radiogenic I and a heavy Xe isotope component is required. Constraining the composition of the heavy isotope component to known fission spectra then defines the U-Xe composition and identifies " Tu-derived fissiogenic Xe as the heavy isotope component (see Primordial Xe section). This is compatible with meteorite data that... 

The model is consistent with the isotopic evidence that upper mantle xenon does not have a simple direct relationship to atmospheric xenon. The radiogenic xenon presently seen in the atmosphere was degassed from the upper portion of the solid Earth prior to the establishment of the present upper mantle steady state xenon isotope compositions and concentrations. The lower mantle ratios are established early in Earth history by decay of I and Pu decay produces a relatively small fraction of fissiogenic nuclides (Porcelli and Wasserburg, 1995b). The xenon daughters (now in the upper mantle) of the shortlived parents are supplied from the lower mantle. The MORE Xe/ Xe ratio (when corrected for air contamination) has no radiogenic contributions... 

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