Deep mantle krypton reveals Earth’s outer solar system ancestry

Krypton from the Earth’s mantle, collected from geological hot spots in Iceland and the Galapagos Islands, reveals a clearer picture of how our planet was formed, according to new research.

Scientists intend to use different isotopes of krypton to investigate and uncover the ingredients that made the Earth, which includes solar wind particles and meteorites from the inner and outer solar system.

According to new research conducted by the University of California, it indicates that the Earth’s volatile elements, which are essentials such as carbon, water and nitrogen, arrived as Earth was growing and developing into a planet. This contradicts the popular theory that most of Earth’s volatile elements were delivered near the end of Earth’s formation, which is marked by the giant impact of the moon-forming. Instead, the krypton isotopes indicate that planetesimals from the cold outer solar system bombarded the Earth early on, millions of years before the big crunch. The young Earth also adopted dust and gas from the solar nebula, the cloud surrounding the sun, and was attacked by meteorites.

“Our results require concurrent delivery of volatiles from multiple sources very early in Earth’s formation,” said Sandrine Péron, lead author of the study. Péron, currently a Marie Skłodowska-Curie Actions Fellow at ETH Zürich in Switzerland, conducted the research at UC Davis as a postdoctoral fellow working with Professor Sujoy Mukhopadhyay in the Department of Earth and Planetary Sciences.

“This study provides clues for the sources and timing of volatile accretion on Earth and will help researchers better understand how not only Earth formed, but also other planets in the solar system and around other stars,” Péron explained. This study was published December 15 in the journal Nature.

Primordial geochemistry

The volcanic hot spots in Iceland and the Galapagos are fed by magma plumes, rising from the deepest layer of the mantle, near its boundary with the Earth’s iron core. The elements and minerals in this deep layer are relatively unchanged since before the Moon-forming impact, like a time capsule of the early Earth’s chemistry which is more than 4.4 billion years old.

Mukhopadhyay’s lab specialises in producing precise measurements of noble gasses in rocks from the Earth and elsewhere. To sample deep mantle krypton, the researchers collected lava at hot spot plumes. The ancient gases rise to the surface in the erupting lava, getting trapped and entombed as bubbles in a glassy matrix when the lava sets to a solid, providing protection from outside contamination. However, even the most abundant krypton isotopes in these bubbles amount to only a few hundred million atoms, making their detection challenging, Mukhopadhyay stated.

Measuring mantle krypton

Péron designed a new technique for measuring mantle krypton with mass spectrometry, concentrating krypton from rock samples in an environment virtually free of air contamination and neatly separating it from argon and xenon. “Ours is the first study to precisely measure all krypton isotopes for the mantle, including the rarest krypton isotopes, Kr-78 and Kr-80,” she said.

The researchers discovered that the chemical fingerprint of deep mantle krypton closely resembled primitive, carbon-rich meteorites, which may have been delivered from the cold outer reaches of the solar system.

However, previous work by Mukhopadhyay, among others found that neon, another noble gas in the deep mantle, was derived from the sun. The two differing results suggest that at least two distinct volatile sources for the Earth’s mantle were delivered very early in its history. The researchers also noted less of the rare isotope Kr-86 in the deep mantle compared to known meteorites. The deficit in Kr-86 suggests that known meteorites alone may not account for all the mantle’s krypton.

To conclude, the new results also have implications for how the Earth’s atmosphere occurred. The ratio of different krypton isotopes in the deep mantle does not match the isotope ratio in Earth’s atmosphere, the researchers discovered. This means that some gases in the atmosphere, including noble gases like krypton, were delivered to Earth after the Moon-forming impact. Péron added that  otherwise, the Earth’s mantle and atmosphere would have the same isotopic composition due to isotopic equilibration following the impact.

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