Abstract
Xenon (Xe) is an exceptional tracer for investigating the origin and fate of volatile elements on Earth. The initial isotopic composition of atmospheric Xe remains unknown, as do the mechanisms involved in its depletion and isotopic fractionation compared with other reservoirs in the solar system. Here we present high precision analyses of noble gases trapped in fluid inclusions of Archean quartz (Barberton, South Africa) that reveal the isotopic composition of the paleo-atmosphere at ≈3.3 Ga. The Archean atmospheric Xe is mass-dependently fractionated by 12.9±2.4 ‰ u−1 (± 2σ, s.d.) relative to the modern atmosphere. The lower than today 129Xe excess requires a degassing rate of radiogenic Xe from the mantle higher than at present. The primordial Xe component delivered to the Earth's atmosphere is distinct from Solar or Chondritic Xe but similar to a theoretical component called U-Xe. Comets may have brought this component to the Earth's atmosphere during the last stages of terrestrial accretion.
Highlights
Xenon (Xe) is an exceptional tracer for investigating the origin and fate of volatile elements on Earth
The results provide confirmation for the specific and long-term evolution of the isotopic composition of atmospheric Xe, since the fractionation determined here is significantly different from 21±6 % u À 1 (±2s, s.d.) Xe in North Pole, Pilbara barite with an age of 3.48±0.18 Ga (±2s, s.d.)[18] (Fig. 6)
A mass-dependent isotope fractionation process acted on atmospheric Xe isotopes from at least 3.3 Ga until the present day
Summary
Xenon (Xe) is an exceptional tracer for investigating the origin and fate of volatile elements on Earth. When corrected for mass-dependent isotope fractionation, atmospheric Xe is depleted in its heavy isotopes (134Xe and 136Xe) relative to Solar or Chondritic Xe, and cannot be related to any known cosmochemical component[13,14] These observations led to the definition of a theoretical primordial component labelled ‘U-Xe’ Recent studies of Archean barite and quartz samples from North Pole, Pilbara (NW Australia) demonstrated that, 3.5 to 3.0 Ga ago, atmospheric Xe had an isotopic composition less isotopically fractionated than the modern atmospheric Xe relative to any of the potential primordial components[17,18,19,20] These data suggest a progressive long-term evolution of the isotopic composition of atmospheric Xe by mass-dependent isotope fractionation, that may be due to ionization of atmospheric xenon[21] by ancient, ultraviolet-rich, solar radiation and progressive escape of Xe ions to space[22,23]. To investigate the ultimate origin of atmospheric xenon, we selected and analysed with unprecedented high precision Archean quartz samples from the Barberton Greenstone Belt (BGB), South Africa
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