Hf-Nd isotope evidence for a transient dynamic regime in the early terrestrial mantle

Modern basalts have seemingly lost all 'memory' of the primitive Earth's ma ntle except for an ambiguous isotopic signal observed in some rare gases(1, 2). Although the Earth is expected to have reached a thermal steady state w ithin several hundred million years (refs 3, 4) of accretion, it is not kno wn how and when the initial chemical fractionations left over from planetar y accretion (and perhaps a stage involving a magma ocean) were overshadowed by fractionations imposed by modern-style geodynamics. Because of the lack of samples older than 4 Gyr, this early dynamic regime of the Earth is poo rly understood. Here we compare published Hf-Nd isotope data on supracrusta ls from Isua, Greenland, with similar data on lunar rocks and the SNC (mart ian) meteorites, and show that, about 3.8 Gyr ago, the geochemical signatur e of the Archaean mantle was partly inherited from the initial differentiat ion of the Earth. The observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat. The su rvival of remnants from an early layering in the modern deep mantle may acc ount for some unexplained seismological, thermal and geochemical characteri stics of the Earth as observed today.