A new view of Xe and Ne in the Paleoarchean atmosphere from paired noble gas isotopic and trace element analyses of Barberton cherts

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Here, we include the main text and supplementary data tables associated with the manuscript "A new view of Xe and Ne in the Paleoarchean atmosphere from paired noble gas isotopic and trace element analyses of Barberton cherts," submitted to GCA. In this paper, we present high precision noble gas abundances and isotopic ratios paired with ion images for select trace elements from four cherts of the 3.26-3.55 Ga Onverwacht Group in Barberton, South Africa. These data demonstrate that cherts are capable of trapping ancient gases with compositions comparable to previous studies of hydrothermal quartz or barites (e.g., Avice et al., 2018, GCA), and provides critical new insight: the trapped ancient compositions of all samples have been variably overprinted by modern atmospheric gas, and estimates of the extent of past mass-fractionation are truly lower limits. Using a multi-component mixing model, we show that the ~3.3 Ga atmosphere was characterized by a Xe composition fractionated by a minimum of 19 ‰/amu relative to the modern atmospheric composition, nearly twice as strongly fractionated as the most recent estimate (Broadley et al., 2022; GCA). Our data indicate very limited contributions of Xe produced after emplacement by Ba, Te, and U. Our study cautions that both hydrothermal quartz and chert are susceptible to post-depositional alteration that must be understood in order to accurately characterize how the terrestrial atmospheric Xe isotope composition changed over Earth history, mostly likely due to atmospheric loss to space. In addition, Onverwacht chert Ne isotopic data suggest a sub-atmospheric Paleoarchean 20Ne/22Ne composition (< 9.7). SIMS data were gathered using the Cameca 7f-geo at Washington University in St. Louis. Noble gas data were collected using the Nu Noblesse HR 5F5M at Washington University in St. Louis. Below, find an excel file containing data tables associated with this manuscript.

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• Washington University in St. Louis

  MO, Saint Louis

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