The origin and degassing history of the Earth’s atmosphere revealed by Archean xenon
Authors: Guillaume Avice, Bernard Marty, Ray Burgess
Journal: Nature Communications
Publication Date: 18 May, 2017
Department of: Earth and Environmental Sciences
The contribution of comets explains the xenon mystery of the Earth’s atmosphere
Noble gases are chemically inert elements that act as powerful tracers for investigating the origin and evolution of the Earth’s atmosphere. Xenon (Xe) is the heaviest stable noble gases and has nine isotopes. The Xe isotope composition of the modern atmosphere is characterised by being elementally depleted relative to other noble gases and isotopically fractionated, features that are not represented in any other solar system body (meteorites or the Sun) and are difficult to explain. Researchers from the University of Manchester with collaborators in France, have obtained high precision Xe isotope analyses of ancient air trapped in fluid inclusions in rocks from the Barberton area, South Africa. The researchers showed that the fluid inclusions have been preserved intact for 3.3 billion years and have a Xe isotope “fingerprint” indicative of a hitherto theoretical component known as U-Xe. U-Xe is quite different from Xe found in the Earth’s mantle, meteorites or the solar wind, and must have been added to the Earth’s atmosphere during the final stages of terrestrial accretion. As primitive volatile-rich objects in the solar system, comets may potentially may carry such an exotic primordial component, possibly inherited from other planetary systems formed in the vicinity of our Sun. The work therefore suggests that some of the Earth’s atmosphere may have been brought to the planet by comets billions of years ago.
- Xenon (Xe) is a very minor constituent of the atmosphere and has nine stable isotopes. Some isotopes are “primordial” – present in amounts unchanged since the Earth formed, others are formed by radioactive decay of isotopes of iodine, uranium, thorium and plutonium, with amounts that have increased over geological time.
- U-Xe is an abbreviation of Ur-Xe meaning ‘primitive’ or ‘earliest’ to reflect its candidacy as primordial terrestrial xenon. Its existence was derived theoretically; unusually the work was only ever published in pre-print form but reference to U-Xe is widely found in the literature.
- The researchers also looked at the 129Xe anomaly in the ancient atmosphere. This isotope was formed in the mantle during the first 100 million years after Earth’s formation by radioactive decay of now extinct 129I (half-life 16Ma), and since outgassed to the atmosphere. By comparing the magnitude of the 129Xe anomaly in the ancient atmosphere, relative to modern air, a degassing rate 8-10 times higher than the present was estimated, consistent with more vigorous mantle convection in the past.