Abstract
The evolution of Earth's atmosphere over billions of years provides important constraints on the geological history of our planet. However, major challenges in studying this have been finding samples in which ancient atmospheric signals are reliably preserved and precisely determining the age of the trapped atmosphere. Hydrothermal minerals can preserve fluid inclusions that formed from air-saturated waters. However, the use of hydrothermal minerals as paleo-atmospheric proxies is often limited by contamination by deeply-sourced crustal gases (e.g., radiogenic noble gases) and the difficulty of providing precise age constraints on the crystallization of the hydrothermal minerals.In this study, we tested the hypothesis that meteorite impact craters could provide an untapped archive of atmospheric evolution by investigating noble gases (Ne, Ar, Kr and Xe) contained in fluid inclusions within post-impact hydrothermal minerals from the Rochechouart impact structure (France). Impact craters can be dated using a variety of methods and because meteorite impacts cause localized fracturing of country rocks and the development of minor thermal anomalies, they are often associated with short-lived (up to a few Ma, depending on the size of the crater) and shallow hydrothermal systems dominated by surface-derived hydrothermal fluids.We found that the elemental ratios of noble gases in quartz from Rochechouart are close to the elemental ratios of modern Air. Furthermore, most noble gas isotope ratios are close to modern atmosphere, except for rare deviations caused by radiogenic ingrowth, nucleogenic effects and/or minor mass fractionation. After a minor correction for post-entrapment decay of 40K, an upper limit for the 40Ar/36Ar ratio of the ∼200 Ma atmospheric component trapped in Rochechouart quartz is determined as 292.7 ± 3.6 (2σ s.d.), which is significantly lower than the modern value of 298.6 ± 0.3. A Mesozoic 40Ar/36Ar ratio lower than 292.7 ± 3.6 is in line with the existing paleo-atmospheric dataset and existing models of the temporal evolution of the atmospheric 40Ar/36Ar ratio and conclusively proves the preservation of an ancient atmospheric component in rocks from Rochechouart. Given that meteorite impacts have occurred throughout the entire Earth's history they might collectively provide a unique, yet unexplored, repository of the atmospheric evolution. The same approach might also be applicable to investigating the atmospheric evolution of other terrestrial planets.
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