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–1). 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.