Scientists play 'geological genealogy'

Oldest rock: A team of scientists has tracked the history of a rock back to the earliest times on Earth using an innovative technique.

Scientists have tracked the ‘family history’ of a rock back to some of the earliest times on Earth. Researchers analysed the concentration and distribution of particular types of atoms in the granite to show that it must have been recycled from something that existed 4.2 billion years ago. This ‘parent rock’ was very probably basalt of the sort produced on the ocean floor, they say. The team reported its work recently in Science Magazine.

With most rock on Earth being broken down by weathering or taken back into the planet’s interior just a few hundred million years after production, there is very little, truly ancient material to look at. Geological genealogy offers a very useful alternative in the circumstances, says Dr Jonathan O’Neil from the University of Ottawa, Canada. “To put everything into perspective, Earth is about 4.6 billion years old, and I would say that rocks that are 3.8 billion years and older — we can count them on the fingers of our hands. We have very limited places to learn something about the first billion years of Earth history. That’s our challenge,” he said.

Relatively young
Pursuing this type of research delves into questions about when tectonics — the formation and movement of continents — got started on the planet; when oceans would have first covered its surface; and, of course, when conditions might have become suitable for life. Dr Jonathan and colleagues examined samples of rock from the ‘Canadian Shield’, a swathe of territory moving east of Hudson Bay in Canada.

The specific pieces they took into the lab were relatively young — ‘only’ 2.7 billion years old. But by looking at an atomic tracer in the minerals, the team could determine the parentage. This tracer takes the form of a radioactive decay chain centred on versions, or isotopes, of the chemical elements samarium and neodymium. The isotope of samarium with an atomic mass of 146 is unstable and decays very rapidly into the isotope of neodymium that has a mass of 142.

In fact, the decay is so rapid that any samarium-146 that was present on Earth at its formation 4.6 billion years ago is all but gone within 500 million years. But even though ‘the clock’ stops running, it leaves the mark of its existence in the concentration and distribution of the neodymium. When the scientists take into account the known age of their granite samples, the time it would take for these granites to form from a recycled parent rock, and the neodymium isotope record present in the samples — they conclude the pre-cursor material must be older than 4.2 billion years.

“We can also say something about (the parent rock’s) nature,” said Dr Jonathan. “Were they granites or basalts? Were they continental crust or were they oceanic crust? We think they were oceanic crust. So, the earliest crust on Earth — at least the evidence we have in our samples — was oceanic crust at approximately 4.3 billion years old.” Does any of this original material still exist? Dr Jonathan points to a nearby sliver of rock on the shore of the Hudson Bay called the Nuvvuagittuq Supracrustal Belt (NSB).

Recycled crust
This is presumed to be oceanic volcanic rock and has been dated to be between 3.7 and 4.3 billion years old. It is an age bracket that also relies on a neodymium analysis. “The younger rocks that we have analysed are around (the NSB), so it makes sense that this is the piece of crust that has been recycled to form our continents,” said Dr Jonathan. “It fits the bill. It has exactly the right composition and the right mineralogy to be it.” The Nuvvuagittuq Supracrustal Belt was reported recently by a team led from University College London (UCL), UK, to contain the fossilised remains of tiny lifeforms. However, it has to be said that there is significant debate over the exact age of the NSB, which may not quite fit with this genealogy story.

“The neodymium isotope compositions in the volcanic rocks of the NSB do indeed yield an age of 4.28 billion years, but this has been interpreted both as the true age of this rock and alternatively as an older signature inherited from the early mantle of Earth in these younger rocks, that have a minimum age of 3.77 billion years,” explained UCL’s Dr Dominic Papineau. “This debate can possibly be settled using new approaches and techniques, as well as using careful microscopy-based observations of rocks from key geological relationships in the field,” he said.