The overall composition of Earth, both elementally and isotopically, cannot be completely explained by any combination of known primitive meteorite materials. One possible explanation is that the proto-Earth originally accreted materials with unique isotopic characteristics—distinct from those that were incorporated after the Moon-forming giant impact.
In this study, we present high-precision, mass-independent potassium isotopic analyses conducted using thermal ionization mass spectrometry on a range of terrestrial rocks representing both ancient and modern crust and mantle sources. The results support the hypothesis that portions of Earth’s mantle preserve a pre-giant-impact signature.
Our data show that certain mafic Archean rocks derived from the Hadean–Eoarchean mantle—including samples from Isua, Nuvvuagittuq, and the Kaapvaal Craton—along with some modern ocean island basalts from La Réunion Island and the Kama’ehuakanaloa volcano in Hawaii, display an average 40K (potassium-40) deficit of about 65 parts per million relative to other terrestrial samples.
This 40K deficit clearly separates these samples from the bulk silicate Earth and all known meteorite groups, and it cannot be explained by conventional magmatic or geochemical processes.
We therefore propose that this isotopic anomaly reflects the presence of primordial mantle reservoirs—remnants of the proto-Earth—that avoided complete mixing following the Moon-forming impact. These ancient mantle domains likely persist today in the deep mantle and may still influence modern hotspot volcanism.
