Earth Planet. Sci. Lett. 377–378, 324–335 (2013)

Primordial material left over from Earth's formation about 4.5 billion years ago has mostly been stirred into the mantle by convection, but the rate of mantle mixing is unclear. Geochemical analyses of ancient rocks from Greenland indicate that Earth's mantle became largely homogenized between 3.4 and 3.3 billion years ago.

Hanika Rizo at Clermont Université, Université Blaise Pascal, France, and colleagues analysed the geochemistry of rocks sampled from the Isua Supracrustal Belt in southwest Greenland. The rocks formed from a source in Earth's mantle between about 3.8 and 3.3 billion years ago. The researchers found that the oldest rocks contained an excess amount of the isotope 142Nd that is not observed in mantle-derived rocks today, and is therefore thought to be the fingerprint of primordial material. This fingerprint was detected previously in rocks formed 3.7 and 3.4 billion years ago. However, it was absent from the Greenland rocks aged 3.3 billion years. Together, these findings suggest that the window between 3.4 and 3.3 billion years ago marked a period of mantle homogenization.

The results imply that any distinct reservoirs of primordial material were probably largely erased from the mantle within about a billion years of Earth's formation.