Deep-mantle krypton reveals Earth’s early accretion of carbonaceous matter

• Published in: Nature (London) 2021-12, Vol.600 (7889), p.462-467
• Main Authors: Péron, Sandrine, Mukhopadhyay, Sujoy, Kurz, Mark D., Graham, David W.
• Format: Article
• Language: English
• Subjects: 101/58; 704/2151/209; 704/445/209; 704/445/845; 704/445/849; Accretion; Air pollution; Atmosphere - chemistry; Carbon - analysis; Carbon dioxide; Carbonaceous materials; Carbonaceous meteorites; Carbonaceous rocks; Contamination; Depletion; Deposition; Earth; Earth mantle; Earth, Planet; Ecuador; Environmental aspects; Evolution, Chemical; Evolution, Planetary; Gases; Geologic Sediments - chemistry; Helium; Helium - analysis; Humanities and Social Sciences; Iceland; Isotope composition; Isotopes; Isotopes - analysis; Krypton; Krypton - analysis; Krypton isotopes; Mantle; Meteorites; Meteoritic composition; Meteoroids; Meteors & meteorites; multidisciplinary; Neon; Neon - analysis; Neutrons; Nitrogen - analysis; Outer solar system; Outgassing; Planet formation; Plumes; Rare gases; Science; Science (multidisciplinary); Solar system; Tungsten; Tungsten - analysis; Volatile compounds; Volatiles; Xenon - analysis
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-04092-z

Establishing when, and from where, carbon, nitrogen and water were delivered to Earth is a fundamental objective in understanding the origin of habitable planets such as Earth. Yet, volatile delivery to Earth remains controversial 1 – 5 . Krypton isotopes provide insights on volatile delivery owing to their substantial isotopic variations among sources 6 – 10 , although pervasive atmospheric contamination has hampered analytical efforts. Here we present the full suite of krypton isotopes from the deep mantle of the Galápagos and Iceland plumes, which have the most primitive helium, neon and tungsten isotopic compositions 11 – 16 . Except for 86 Kr, the krypton isotopic compositions are similar to a mixture of chondritic and atmospheric krypton. These results suggest early accretion of carbonaceous material by proto-Earth and rule out any combination of hydrodynamic loss with outgassing of the deep or shallow mantle to explain atmospheric noble gases. Unexpectedly, the deep-mantle sources have a deficit in the neutron-rich 86 Kr relative to the average composition of carbonaceous meteorites, which suggests a nucleosynthetic anomaly. Although the relative depletion of neutron-rich isotopes on Earth compared with carbonaceous meteorites has been documented for a range of refractory elements 1 , 17 , 18 , our observations suggest such a depletion for a volatile element. This finding indicates that accretion of volatile and refractory elements occurred simultaneously, with krypton recording concomitant accretion of non-solar volatiles from more than one type of material, possibly including outer Solar System planetesimals. The krypton isotopic pattern of Earth’s deep mantle indicates that volatile-rich material from the outer Solar System was delivered early in Earth’s accretion history.