These rocks are probably the last remains of Earth’s early crust

Just over 4 billion years ago, magma from Earth’s mantle infiltrated a fracture in the young planet’s primordial crust. Over the following aeons, nearly all of the planet’s early crust melted back into the mantle except for a small area around this fracture, which survives today.

At least, that is the story according to the latest analysis of radioactive isotopes in this rock, which is still accessible on the surface as part of the Nuvvuagittuq Greenstone Belt, a formation on the shore of Hudson Bay in Canada. This potential sample of Earth’s early crust is the subject of a long-standing debate among geologists: is it the world’s oldest rock, or merely extremely old?

Jonathan O’Neil at the University of Ottawa and his colleagues kicked off the debate in a 2008 study that estimated the rocks surrounding the intrusion are about 4.3 billion years old, which would make them the world’s oldest. With that age, they would have formed during the Hadean Aeon just a few hundred million years after the planet itself.

While a few mineral grains have been found that are older than that, complete Hadean rocks would offer a new window on this early period of Earth’s history, perhaps shedding light on geological mysteries like the start of plate tectonics and the make-up of the first oceans.

However, the method the researchers used to date the rocks made the 4.3-billion-year-old age controversial. Ideally, very old rocks can be dated using a hardy mineral called zircon, which maintains its original chemical make-up over billions of years. But these volcanic rocks didn’t contain zircon. “We can’t date these rocks using that technique that everybody loves,” says O’Neil.

Instead, they measured the atomic weight of neodymium and samarium in the rock. As samarium decays, it produces different isotopes of neodymium at known rates. The ratio of neodymium and samarium isotopes remaining in the rocks can thus serve as a “clock” counting up from the time the rock crystallised from magma. In fact, two isotopes of samarium decay at different rates, allowing them to serve as two parallel clocks. The trouble was, the two clocks didn’t agree on the age of the rock, leading researchers to dispute that it actually was Hadean.

“I don’t think a majority of the early-Earth-studying community was convinced,” says Richard Walker at the University of Maryland.

Now, O’Neil and his colleagues have counted neodymium and samarium isotopes in rocks that intrude into the layer they think is 4.3 billion years old. By definition, such intrusions are younger than the strata that surround them. Therefore, dating the intrusion would set a minimum age for the surrounding rock.

In the intrusion, unlike the older rock that surrounds it, the two clocks tell the same story: the rock is about 4.16 billion years old. “Both clocks are giving the exact same age,” says O’Neil. This supports the idea that the surrounding rock formed well within the Hadean Aeon, which would make it the only known remnant of Earth’s early crust.

“I think they make as good a case as you can make,” says Graham Pearson at the University of Alberta in Canada.

“The simplest explanation for this data is that these are the oldest rocks in the world,” says Jesse Reimink at the Pennsylvania State University. However, this is unlikely to be the last word on the matter, he says. “When dealing with the oldest rocks and minerals, there’s no such thing as settled.”