The history of surface volatile recycling into the mantle is poorly known. Rita Parai and Sujoy Mukhopadhyay now use mantle xenon isotope systematics to constrain the age of initiation of volatile regassing into the deep Earth. They conclude that significant recycling of atmospheric xenon into the deep Earth could not have occurred before about 2.5 billion years ago and that the mantle shifted from a net degassing to a net regassing regime after this time. Because xenon is largely carried into the Earth’s interior in hydrous mineral phases, the authors also infer that downwellings in the mantle were drier in the Archaean era relative to the modern day.
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