New evidence for chemical fractionation of radioactive xenon precursors in fission chains

Mass-spectrometric analyses of Xe released from acid-treated U ore reveal that apparent Xe fission yields significantly deviate from the normal values. The anomalous Xe structure is attributed to hemically ractionated ission (CFF), previously observed only in materials experienced neutron bursts. The least retentive CFF-Xe isotopes, ${}^{136}\mathrm{Xe}$ and ${}^{134}\mathrm{Xe}$, typically escape in 2:1 proportion. Xe retained in the sample is complimentarily depleted in these isotopes. This nucleochemical process allows understanding of unexplained Xe isotopic structures in several geophysical environments, which include well gasses, ancient anorthosite, some mantle rocks, as well as terrestrial atmosphere. CFF is likely responsible for the isotopic difference in Xe in the Earth's and Martian atmospheres and it is capable of explaining the relationship between two major solar system Xe carriers: the Sun and phase-Q, found in meteorites.

Figure 1

Release of radiogenic ${}^{136}\mathrm{Xe}$ from the original STR-Nk1 sample (top) and from the sample STR-Nk2 treated with $\mathrm{HN}{\mathrm{O}}_3$ for 3 h (bottom). About 2/3 of fission ${}^{136}\mathrm{Xe}$ has been lost as a result of the treatment and the low-temperature Xe release peak became apparent in the leached STR-Nk2.

Figure 2

Isotopic composition of Xe released from acid treated sample STR-Nk2 during stepwise pyrolysis. All temperature fractions tend to form linear arrays (solid lines) which differ from mass-fractionation lines (dashed). Stars show the compositions of Xe from spontaneous fission of ${}^{238}\mathrm{U}$ and neutron-induced fission of ${}^{235}\mathrm{U}$. Composition of Xe lost due to the acid treatment is calculated from the difference between the original and treated samples.

Figure 3

Composition of fission Xe released from the acid-treated sample STR-Nk2 at different extraction temperatures. These compositions are normalized to ${}^{136}\mathrm{Xe}$. Errors are 1σ.

Figure 4

Weighted average of Xe relative (to ${}^{136}\mathrm{Xe}$) isotopic excesses ${\delta }_M=100\%\times [{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{low}\mathrm{T}}/{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{total}}-1)]$ at low $\left(<800{}^{\circ }\mathrm{C}\right)$ extraction temperatures vs. half-lives of corresponding iodine isotopes, Xe precursors in fission chains. The correlation spans over 13 orders of magnitude in half-lives. Errors are 1σ.

Figure 5

Composition of fission Xe in $\mathrm{C}{\mathrm{O}}_2$ well gases from C-16, New Mexico [27], Motwan, Gujarat/India [31], and the lowest temperature extraction from the acid-treated sample STR-Nk2 (this work). Bars show fission yields from spontaneous fission of ${}^{238}\mathrm{U}$. All compositions are normalized to ${}^{136}\mathrm{Xe}$. Errors are 1σ.

Figure 6

Composition of fission Xe in Archean anorthosite from Fiskenaesset Complex in West Greenland. Xe in the coarse-grained fraction [32] is indistinguishable from of fission Xe from ${}^{238}\mathrm{U}$ (green bars). Xe from the fine-grained fraction of the same anorthosite block [7] has an anomalous pattern similar to the low-temperature Xe extraction of the acid-treated sample STR-Nk2 (this work). Errors are 1σ.

Figure 7

Isotope excess ${\delta }_M=100\%[{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{sample}}/{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{atm}.}-1)]$ in Xe released from boreholes in Rosvumchorr tunnel, Kola Peninsula [34], and in gas bubbles in groundwater from soil containing highly radioactive transuranic waste at DOE's Hanford site [35] (solid lines). Fission yields of Xe isotopes from ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$ (normalized to ${}^{136}\mathrm{Xe}$) are shown for comparison (dashed lines). In both cases the ${}^{136}\mathrm{Xe}$ excess is twice as large as the ${}^{134}\mathrm{Xe}$ excess.

Figure 8

Radiogenic Xe in precisely measured midocean ridge basalt glasses 2ΠD47 and 2ΠD26 [36] are not consistent with fission spectrum of ${}^{244}\mathrm{Pu}$ (blue bars). This inconsistency is likely due to a CFF process which modified a ${}^{244}\mathrm{Pu}$ fission yield. Similar modification is observed in low-temperature Xe extraction from acid-treated sample STR-Nk2 (this work) which is due to CFF modified yields of ${}^{238}\mathrm{U}$ (red bars). Errors are 1σ.

Figure 9

Xe isotopic excesses in terrestrial atmosphere derived by Takaoka [38], Igarashi [43] ${\delta }_M=100\%[({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})/{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{ref}.}-1)]$ (top) and CFF-Xe released at low temperature from acid-treated sample STR-Nk2 (bottom, this work). Terrestrial fission Xe is plotted relative to fission Xe from ${}^{244}\mathrm{Pu}$, and CFF-Xe from STR-Nk2 is shown relative to the total Xe composition in this sample. The similarity of the isotopic patterns suggests that terrestrial fission Xe is also formed by the CFF process.

Figure 10

Fractionated Martian atmosphere [44] relative to the Earth's atmosphere. ${\delta }_M=100\%[{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{Mars}}/{({}^M\mathrm{Xe}/{}^{136}\mathrm{Xe})}_{\mathrm{Earth}.}-1)]$.

Figure 11

Xe-Q can be modeled as fractionated by 6.5‰ /u solar Xe [48] with $\sim 3\%$ addition of ${}^{136}\mathrm{Xe},\sim 1.5\%$ of ${}^{134}\mathrm{Xe}$, and $<0.5\%$ of ${}^{132}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe}$ (top). The isotopic structure of the required for this model addition matches the pattern of Xe found in the gas bubbles released from the Hanford site [35] (bottom). This model does not require any presolar Xe components.