UC Davis

Precise and accurate knowledge of the neon isotopic composition in Earth’s atmosphere and the Solar System abundance and fissiogenic mass spectrum of Xe from plutonium-244 are essential for a range of applications in planetary science. These values were initially determined decades ago and suffer from considerable uncertainties. Advances in noble gas mass spectrometry over recent years have improved the repeatability in Ne and Xe isotopic measurements and allow these values to be redetermined with higher precision.

In chapter 1, we redetermine the value of the atmospheric 22Ne/20Ne isotopic ratio using a gravimetrically-prepared artificial high-purity mixture of pure 20Ne and 22Ne as 0.10196 ± 0.00002 (1σ). The uncertainty of this ratio was reduced by a factor of 40 from the previous consensus value from 0.78% to 0.02% which increases the precision and accuracy of geochemical applications of Ne isotopes including cosmogenic exposure age dating.

In chapter 2, we present the results of an experiment to determine more precisely the mass spectrum of xenon isotopes 131, 132, 134, 136Xe produced by the spontaneous fission of 244Pu by repeatedly extracting fissiogenic Xe from the headspace of a flask containing 244Pu. We observed an unexpected isotopic fractionation effect, but statistical analysis of the collected data produces a mass spectrum with improved precision for the fissiogenic 131Xe/136Xe, 132Xe/136Xe, and 134Xe/136Xe ratios compared to literature estimates. The uncertainties have been reduced by factors of 6.5, 4, and 6.8 respectively for the three ratios compared to the sole previous experimental determination.The redetermined values are 131Xe/136Xe = 0.2555 ± 0.0034, 132Xe/136Xe = 0.8826 ± 0.0080, and 134Xe/136Xe = 0.9318 ± 0.0041 (1σ). Based on measurements of 133Xe, we calculate a 136Xe yield from 244Pu spontaneous fission of 6.8 ± 0.5% which is 21% higher than the literature value and necessitates a downward revision of literature 244Pu abundance calculations from 136Xe244. The revised spectrum reduces the uncertainties by up to a factor of 2 for deconvolutions of Xe components in meteorites and Earth’s mantle that contain information about the formation and early history of the Solar System. We calculate a I-Pu-Xe closure age of the depleted mantle source of 76.2 ± 9 Ma, which is 6% younger and 24% more precise than the value calculated with Alexander et al. (1971)’s fission Xe mass spectrum.

In chapter 3, we present measurements of the abundance and isotopic composition of Xe and the abundance of rare earth elements (REEs) in multiple aliquots of four eucrites. We determine the original abundance of 244Pu from the abundance of fissiogenic 136Xe and use a linear regression technique to determine the 244Pu/REE ratio in the eucrite parent body. Using the REE/238U ratios of CI chondrites we determine a Solar System 244Pu/238U ratio of 0.0051 ± 0.0005 (1σ) at the time the first Solar System solids condensed. This value agrees within uncertainty with the most cited literature value but disagrees with other literature determinations. The newly determined 244Pu/238U ratio has implications for I-Pu-Xe dating of Solar System reservoirs and investigations of galactic nucleosynthesis.