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Geochemical evidence for high volatile fluxes from the mantle at the end of the Archaean

Abstract : The exchange of volatile species-water, carbon dioxide, nitrogen and halogens-between the mantle and the surface of the Earth has been a key driver of environmental changes throughout Earth's history. Degassing of the mantle requires partial melting and is therefore linked to mantle convection, whose regime and vigour in the Earth's distant past remain poorly constrained(1,2). Here we present direct geochemical constraints on the flux of volatiles from the mantle. Atmospheric xenon has a monoisotopic excess of Xe-129, produced by the decay of extinct I-129. This excess was mainly acquired during Earth's formation and early evolution(3), but mantle degassing has also contributed Xe-129 to the atmosphere through geological time. Atmospheric xenon trapped in samples from the Archaean eon shows a slight depletion of Xe-129 relative to the modern composition(4,5), which tends to disappear in more recent samples(5,6). To reconcile this deficit in the Archaean atmosphere by mantle degassing would require the degassing rate of Earth at the end of the Archaean to be at least one order of magnitude higher than today. We demonstrate that such an intense activity could not have occurred within a plate tectonics regime. The most likely scenario is a relatively short (about 300 million years) burst of mantle activity at the end of the Archaean (around 2.5 billion years ago). This lends credence to models advocating a magmatic origin for drastic environmental changes during the Neoarchaean era, such as the Great Oxidation Event.
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Bernard Marty, David Bekaert, Michael Broadley, Claude Jaupart. Geochemical evidence for high volatile fluxes from the mantle at the end of the Archaean. Nature, Nature Publishing Group, 2019, 575 (7783), pp.485-488. ⟨10.1038/s41586-019-1745-7⟩. ⟨insu-02925137⟩



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