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 mantle except for an ambiguous isotopic signal observed in some rare gases. Although the Earth is expected to have reached a thermal steady state within several hundred million years of accretion, it is not known how and when the initial chemical fractionations left over from planetary 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 poorly understood. Here we compare published Hf-Nd isotope data on supracrustals from Isua, Greenland, with similar data on lunar rocks and the SNC (martian) meteorites, and show that, about 3.8 Gyr ago, the geochemical signature of the Archaean mantle was partly inherited from the initial differentiation 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 survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological, thermal and geochemical characteristics of the Earth as observed today.
Albarede
F
Ecole Normale Superieure de Lyon, France. albarede@ens-lyon.fr
Blichert-Toft
J
Vervoort
JD
Gleason
JD
Rosing
M