Could the oldest rocks under our feet be the remains of a planet that is now extinct? This is the kind of question that geochemists love. And it seems that the pieces may have fallen together for the explanation for Earth’s oldest rocks. Scientists now claim that parts of Earth’s “proto-Earth,” its precursor that is now extinct, have been preserved for the last 4.5 billion years.
Proto-Earth originated from a disk of gas and dust, forming roughly 4.56 billion years ago, with accretion similar to primitive meteorites. Only an estimated 80 to 100 million years later, with the protoplanet Theia approximately the size of Mars, there was the giant impact with the resulting formation of our Moon. This process resulted in the melting of much of Earth’s interior. Until this study, one might assume there were not chemical traces from proto-Earth.
However, a group of scientists, headed by Nicole Nie, a planetary scientist from MIT, have refuted that idea with a subtle but diagnostic chemical signature—a deficiency of the isotope potassium-40 from the earliest rocks that are found on our planet. Potassium is a naturally abundant element that occurs with three isotopes—potassium-39, potassium-40, and potassium-41. While the isotopes of potassium-39, potassium-41 are abundant in most rocks, potassium-40 is only traced in minute quantities. Nie and her team have analyzed these isotopes from powder samples of Greenland and Canadian ancient cratons and volcanic rocks from Hawaii, where a large amount of mantle material is extracted. Their findings were unexpected, with even lower amounts of potassium-40 compared to most surface rocks, where no known geological phenomenon could have caused such a discrepancy.
This result follows previous research that has made comparisons of meteorite compositions with that of Earth. The potassium isotope composition for meteorites formed at different stages and in different locations in the Solar System is unique. Anomalies exist that, in the context of Earth rocks, would indicate pre-impact composition. The signature of the potassium-40 deficiency of ancient Earth rocks is that of matter that survived the chemical resetting of the giant impact.
To pick up on such fine traces, it was necessary to dissolve rock powders in acids to separate potassium and then determine isotopic ratios with extreme sensitivity using multi-collector mass spectrometry. To measure this trace, it’s like trying to pick out a single dark speck in a big field of light grains of sand. Computer simulations of the impact that formed the Moon, subsequent bombardment by asteroid impacts, and convection currents in Earth’s interior demonstrated that if proto-Earth’s early potassium-40 content was low, this would increase it just to rock levels, as found in today’s earth materials.
To pick up on such fine traces, it’s necessary to dissolve rock powders in acids to separate samples of potassium and then determine isotopic ratios with extreme sensitivity using multi-collector mass spectrometry.
In the context of planetary formation, this finding is supported by high-resolution isotope geochemistry dating back to events in the early solar system. Potassium-40 has a relatively long half-life; therefore, its presence influenced the radiogenic warming process in early differentiation. In addition, its volatility level makes K-40 a sensitive indicator of mass loss or mass accumulation during accretion and impacts. The deficit level in the rocks indicated that accreted material in the early Earth contained a combination of material unlike those accreted later because of the lunar-forming collision.
To extract an understanding of those early chemical processes, specific geological settings are necessary. Volcanism due to mantle plumes, as occurred around Hawaii, may carry more unprocessed material near or onto the surface. Similarly, areas around cratons, as around Greenland in its Isua supracrustal belt, are home to some of Earth’s least-altered crust. By cross-comparing sediment cores from those areas with samples from meteorites, scientists have reassembled the elements of early Earth, although the parent meteorites themselves are yet to be found.
The study is also connected to advanced giant impact simulations. Standard simulations, where the material composing the Moon comes mostly from Theia, generate certain differences in isotopes between the Earth and the Moon. Though most elements have similar ratios, possibly as a consequence of post-impact mixing, the volatility characteristics of potassium isotopes imply its ratio preserved independently. This is consistent with regions of the proto-Earth’s mantle left isolated for over a billion years.
As for what None meant by the importance the data represented, he said, “Earth’s bulk composition has elemental and isotopic characteristics that cannot be fully reconciled with a mixture of known primitive meteorite compositions. One potential explanation for this is that the proto‑Earth accreted materials with isotopic signatures distinct from those accreted after the Moon‑forming giant impact.” The lack of potassium-40 isotopes provides the first ever direct indication that the materials mentioned exist to date.
