A reservoir of rock that remained intact for nearly the entire history of Earth could tell us about how our planet was built. Its chemistry hints that Earth’s building blocks may have had a rough time of it, losing their skins before they could unite.
The rocks were thrown up by volcanoes in the Arctic wastes of Baffin Island and Greenland only 62 million years ago, but it seems they came from a store of rock in the mantle that formed 4.5 billion years ago – just after Earth formed.
Because the mantle slowly churns and chemically alters, this comes as a surprise. “We had almost given up hope of finding anything that had survived so long,” says Matt Jackson of Boston University. But Jackson and colleagues found that their samples carry isotopes of helium, hafnium and lead in ratios that would only be found in such ancient rocks.
Rasmus Andreasen of Imperial College London is cautious about the claimed age, because radioactive decay deep in the Earth can throw results off. “The isotope evidence is intriguing, but not conclusive,” he says.
If the dates are right, however, these rocks were undisturbed for almost all of Earth’s history, and so hold chemical clues to our planet’s origin. It is widely thought that Earth was assembled from material similar to that found in meteorites called chondrites. But Baffin’s ancient rocks are different: they contain less of certain heavy elements than chondrites do.
Richard Carlson of the Carnegie Institution of Washington in Washington DC, who was also involved in the research, offers two possible explanations. One is that the elements missing from the Baffin rocks are dense, so they may simply have sunk deep into the Earth where we can’t see them.
Alternatively, these heavy elements may have been lost before our planet came together. In the young solar system, meteoroids first gathered to form bodies called planetesimals, many of which were big enough to have volcanic activity. The missing heavy elements dissolve well in molten magma, so eruptions would have brought them to the surface. Glancing collisions between the planetesimals would then have scraped some of their crusts off, and Earth formed from what was left behind.
Meteoroids that did not coalesce into planetesimals, on the other hand, kept their heavy metals, and these were the rocks that later fell to Earth as chondrites.
Journal reference: Nature, DOI: 10.1038/nature09287