Distant Sources: Tiny inclusions of krypton gas in magma have provided surprising insights into the early days of our planet. Because they reveal that the young earth also received volatile material from the outer solar system in its earliest growth period. This contradicts the popular theory that most of the volatile elements of our planet were only entered after the moon-forming collision 4.5 billion years ago.
Our planet grew like its neighbors through a gradual agglomeration of dust and rocks in the primeval cloud. But around 4.5 billion years ago there was a catastrophic collision with a Mars-sized protoplanet that created the moon and in which a large part of the earth’s material could have evaporated. Because the outer layers of the earth were significantly changed as a result, it is debatable which volatile elements of our planet originate from the time before the catastrophe and where they once came from.
Time capsule from the beginning of the earth
Lava samples from Iceland and the Galapagos Islands, which Sandrine Péron from the University of California at Davis and her colleagues have examined, are now providing new insights into the early history of the earth. The volcanoes on these islands are fed by mantle plumes, which carry magma from the deepest areas of the lower mantle to the surface. The rocks at this depth may have remained largely unchanged since before the lunar collision.
Péron and her team have therefore looked for inclusions in the volcanic, vitrified basalt rock of ordinary islands, in which elements and minerals from this deep, primeval layer of the earth are preserved. In particular, they were interested in isotopes of the element krypton. Because this noble gas has six stable isotopes, the proportions of which give clues to their origin, as the team explains: Krypton from the dust of the solar primeval cloud has a higher proportion of lighter isotopes, while krypton from carbon-rich, chondritic asteroids contains more heavy isotopes.
On the trail of the krypton isotopes
For their study, Péron and her colleagues developed a method with which they could isolate the rare krypton gas from the inclusions, separate it from the other noble gases argon and xenon and analyze it for its isotope ratio. “This is the first study that can precisely measure all krypton isotopes from the Earth’s mantle, including the extremely rare isotopes Kr-78 and Kr-80,” says Péron.
The result: The krypton from the lower mantle showed a different isotope ratio than the noble gas contained in the earth’s atmosphere. This confirms that the krypton circulating in the upper layers of the earth and the atmosphere today does not go back to the early days of the planet. “Instead, a large part of the atmospheric noble gases was only entered after the last major exchange of gases between the earth’s mantle and the atmosphere – after the moon-forming collision,” the researchers write.
Something else was surprising, however: “Unexpectedly, the samples from the deep earth mantle have a deficit of the heavy isotope Krypton-89,” report Péron and her team. That sheds new light on the origin of this oldest, most pristine rock material on our planet. The lack of krypton-89 suggests that the dust from the primeval cloud and lumps from the inner solar system were not the only sources of material on the young earth.
“Our results require that volatile elements were supplied from other sources very early on in planet formation,” explains Péron. The comparison of the isotope proportions showed that the primordial earth must have already absorbed material from the outer solar system as a protoplanet. Even while the primordial cloud existed and the planets were growing, there was a transport of material from the outer edge of the cloud to the inside.
Dust and planetesimals from far outside
Specifically, before the formation of Jupiter and the other outer planets, ice-rich dust grains could have drifted inward from the outer solar system. “In addition, during the formation of the gas giants and their migration, planetesimals enriched with volatile elements were thrown onto orbits that cross the earth’s orbit,” explains the team.
In any case, the deepest, most primeval reservoir of krypton and other volatile elements comes, at least in part, from regions far beyond the Earth’s orbit. “Our study thus provides important information on the sources and timing of the accretion of volatile elements in the young earth,” says Péron. “That helps us to better understand how our earth came about, but also other planets in the solar system and around other stars,” says Péron. (Nature, 2021; doi: 10.1038 / s41586-021-04092-z)
Those: University of California – Davis