Noble Gases in Archean Barites: Precise Determination of the Isotopic Fractionation of Atmospheric Xenon 3.48 Ga Ago.

Following the evolution of the composition of the atmosphere informs on the entire geological evolution of our planet. The discovery that Archean atmospheric xenon was isotopically fractionated compared to modern atmospheric xenon paved the way for using this noble gas as a tracer of hydrogen escape on the primitive Earth. The curve of the evolution of the isotopic composition of atmospheric xenon remains, however, poorly defined. Recent studies proposed that the evolution was discontinuous with brief episodes of escape and fractionation of xenon separated by up to several 100 Ma long pauses. Similarly, some major unknowns remain regarding the progressive depletion of xenon in the atmosphere due to the proposed escape mechanism. In this study, we report the noble gas elemental ratios and isotopic compositions of noble gases released from a ca. 3.48 Ga old barite sample from the Dresser Formation (North Pole, Australia) by stepwise crushing in high vacuum. All samples released xenon enriched in light isotopes relative to heavy isotopes compared to modern atmospheric xenon but with various degrees of isotopic fractionation. Krypton is enriched in heavy isotopes relative to light isotopes in some crushed samples. After correction for Kr and Xe loss, results show that 3.48 Ga ago atmospheric xenon was fractionated by -19.1 ± 1.8‰ per atomic mass unit (u). This value is more negative than that reported previously for 3.3 Ga old atmospheric xenon. A new curve for the evolution of the isotopic composition of atmospheric xenon is proposed. Xenon is also enriched relative to Kr in the gas released from the measured samples. While this is consistent with the scenario of a progressive selective escape of xenon from the Archean atmosphere, the exact abundance of Xe in the paleoatmosphere remains elusive.