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ORIGIN AND EVOLUTION OF ATMOSPHERIC XENON: ATMOSPHERIC AND GEODYNAMICAL IMPLICATIONS G. Avice1, B. Marty1, R. Burgess2, 1CRPG-CNRS, Université de Lorraine, Vandoeuvre-lèsNancy, France ([email protected], [email protected]). 2School of Earth, Atmospheric and Environmental Sciences, University of Manchester ([email protected]). Introduction: The origin of volatile elements on Earth remains poorly understood. Among other noble gases, atmospheric xenon presents striking features. Firstly, it is elementaly depleted relative to lighter noble gases (Ne, Ar, Kr) which follow a similar depletion pattern than chondrites relative to the solar composition. Secondly, atmospheric Xe is strongly isotopically fractionated (30-40 ‰.u-1) relative to cosmochemical components (SW-Xe, U-Xe, Q-Xe). These two features form the xenon paradox [1]. It has also been recognized for decades that neither Solar (SW-Xe) nor Chondritic (Q-Xe) cosmochemical components can be the ancestors of atmospheric xenon due to remaining 134 Xe & 136Xe excesses after correction for the massdependent fractionation described above. U-Xe, a theoretical component has thus been defined as the starting isotopic composition of the Earth's atmosphere [2,3]. Recent studies demonstrated that Archean atmospheric Xe had an isotopic composition intermediate between potential primordial components and the modern atmosphere [4,5]. These results suggest a longterm escape and isotopic fractionation of atmospheric Xe [6]. However, some key points remain to be clarified: i) What is the starting isotopic composition of the Earth's atmosphere? ii) What is exactly the isotopic evolution of atmospheric with time? iii) When did atmospheric xenon reach its modern isotopic fractionation? Samples & Methods: Samples from the Barberton greenstone belt (South Africa) are quartz crystals from hydrothermal veins intruding surrounding mafic lavas and cherts. A total of 27 analyses on 7 different samples by using a recent Helix MC Plus (Thermofisher) mass spectrometer permitted to determine the isotopic composition of xenon in these samples at the permil level. The isotopic compositions of other noble gases (Ne, Ar, Kr) and nitrogen were also determined in these samples. Samples duplicates from Barberton were neutron-irradiated and dated following the Ar-Ar method. We also analyzed quartz samples of various ages (3.2 Ga to 500 Ma) in order to follow the evolution of the isotopic composition of atmospheric Xe with time. Results: The 3.2 Ga-old atmosphere. Ar-Ar results demonstrate a time of fluid entrapment of 3.2 Ga for Barberton samples together with an atmospheric 40Ar/36Ar ratio of 210 ± 29 (1σ) at this time. N2-Ar correlations on the same samples indicate a partial pressure of atmopheric nitrogen in the Archean atmosphere lower or similar to the modern one. Neon isotopic ratios are atmospheric-like. Importantly, krypton is isotopically normal and show no deviation relative to the isotopic composition of the modern atmosphere. These two results confirm the absence of any mantle-derived component in these samples that thus recorded the isotopic composition of ancient atmospheres. Isotopic fractionation of Xe at the permil level. Barberton Xe presents an isotopic fractionation of 13.3 ± 1.8 ‰.u-1 (2σ) relative to the isotopic composition of the modern atmosphere (Fig. 1). This confirms that Archean Xe had an isotopic composition intermediate between cosmochemical ancestors and the modern atmosphere [4,5]. Furthermore, this result was obtained on samples originating from a different geological area compared to previous studies [4,5] and thus demonstrates that the isotopic fractionation of Archean Xe reflects a global atmospheric signal and not a local effect. Interestingly, after correction for this massdependent fractionation, the 129Xe* excess, due to the radioactive decay of extinct 129I (t1/2=15.7 Ma), is lower than in the modern atmosphere and allows us to compute a degassing rate of 9 ± 5 mol.a-1 for 129Xe* degassed from the silicate Earth during the last 3.2 Ga. This degassing rate is at least one order of magnitude higher than the modern one (≈ 0.45 mol.a-1 [7,8]). Such a discrepancy might be due to a higher convection regime in the past sustained by a higher heat flux in the early Earth's interior [9]. Fig. 1: Isotopic composition of Xe in Barberton quartz samples (3.2 Ga) in delta notation normalized to composition of the modern atmosphere. 130 Xe and to the isotopic U-Xe as the starting isotopic composition. The precise determination of the isotopic ratios of Xe in Barberton samples allows us to compute what was the starting isotopic composition of atmospheric Xe (Fig. 2). This starting isotopic composition was similar to the theoretical U-Xe [2,3]. This observation calls for a contribution to the Earth's atmosphere by some extraterrestrial material different from chondrites. Comets are promising candidates for carrying this unusual starting isotopic composition to the Earth since these objects present high noble gases to H2O ratios [10]. from the atmosphere to the outer space [12]. In these conditions, atmospheric xenon was probably also easily ionized. K. Zahnle proposed a model in which Xe ions escape ar lifted up by H ions and escape along open lines of the terrestrial magnetic field [13]. If this model is correct, the progressive isotopic fractionation of atmospheric xenon might reflect the escape of hydrogen from the early atmosphere that finally led to a global oxidation of the Earth's atmosphere [14]. Fig. 3: Evolution of the isotopic fractionation (in delta notation relative to the isotopic composition of the modern atmosphere) of atmospheric Xe with time. The starting isotopic composition is U-Xe (red star). The question mark represents the lack of knowledge on the mode of transition from U-Xe to the 3.5 Ga-old atmosphere. Fig. 2: Three-isotope plot of Xe demonstrating that only the massdependent isotopic fractionation of an isotopic composition similar to U-Xe (red range) (1), followed by the addition of products of the spontaneous fission of 238 U (2) are able to reproduce the isotopic composition of Xe measured in Barberton samples (black dot). Evolution of the isotopic composition of atmospheric Xe. Results obtained on quartz with ages bracketed between 2.7 Ga and 400 Ma allow us to build the curve of the evolution of the isotopic composition of atmospheric Xe with a much higher resolution than in previous studies (Fig. 3). Our results confirm that the isotopic fractionation of atmospheric xenon was established through long-term geological processes acting over more than 2.5 Ga. Interestingly, the isotopic fractionation marked a pause of 500 Ma, between 3.2 Ga and 2.7 Ga. The modern isotopic composition of atmospheric Xe was established around 2 Ga. Previous models calling for early episodes of hydrodynamic escape, preferential Xe retention in the mantle and/or cometary contribution must be revisited. How to selectively escape Xe during several Ga? The EUV flux from the Sun was several orders of magnitude higher in the Archean [11]. Such a high flux promoted the ionization and escape of hydrogen ions Conclusions: This study demonstrates the need for U-Xe as the starting isotopic composition of atmospheric xenon. This component may have been brought to the Earth by volatile-rich (cometary?) bodies. Furthermore, the evolution of the isotopic composition of atmospheric xenon with time is now documented with a good temporal and isotopic resolution. Even if the mechanism responsible for the long-term escape of Xe remains unknown, this evolution might be linked to hydrogen escape episodes and thus to major changes in the composition of the Earth's atmosphere. References: [1] Ozima M. and Podosek F. 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