Diamonds used to `probe` ancient Earth

Johannesburg: Diamonds dug up from ancient rock formations in South Africa - between 1890 and 1930 – have unveiled secrets of how the Earth was shaped more than 3.5 billion years ago, a new study has found.

The three diamonds extracted from Witwatersrand Supergroup - the rock formation that is host to the famous Johannesburg gold mines - were investigated by researchers from the University of Johannesburg, Wits University in South Africa and University of Alberta in Canada to study when modern-style plate tectonics began to operate on planet Earth.

The Earth is approximately 4.5 billion years old, and while a rock record exists from about 4 billion years ago, the complex preservational history of the most ancient rocks exposed on Earth's surface has led to a heated debate among scientists on when plate tectonics began operating on Earth.

Many researchers believe plate tectonics began in the Archaean (the Eon that took place from 4 to 2.5 billion years ago), although the exact timing is highly contested.

While the diamonds of this study were found in 3 billion-year-old sedimentary rocks, diamond formation occurred much deeper, within Earth's mantle.

Additionally, based on the nitrogen characteristics of the diamonds, they also formed much earlier, around 3.5 billion years ago.

Transport of the diamonds to the surface of the Earth by kimberlite-like volcanism, followed by their voyage across the ancient Earth surface and into the Witwatersrand basin, occurred between 3.5 and 3 billion years ago.

Researchers used an ion probe to analyse the carbon and nitrogen isotope compositions of the Witwatersrand diamonds, which have been pristinely preserved for more than three
billion years.

"We can use the carbon and nitrogen isotope compositions of the diamonds to tell us where the source material involved in the formation of the Witwatersrand diamonds over 3 billion years ago came from," said Katie Smart from Wits University.

"The nitrogen isotope composition of the Witwatersrand diamonds indicated a sedimentary source (nitrogen derived from the Earth's surface) and this tells us that the nitrogen incorporated in the Witwatersrand diamonds did not come from the Earth's mantle, but that it was rather transported from Earth's surface into the upper mantle through plate tectonics," she said.

"This is important because the nitrogen trapped in the Witwatersrand diamonds indicates that plate tectonics, as we recognise it today, was operating on ancient Archaean Earth, and actively transported material at Earth's surface deep into the mantle," Katie added.

The plate tectonic process is vital for shaping the Earth as we know it, as the activity of plate tectonics causes earthquakes, volcanic eruptions, and is responsible for constructing Earth's landscapes, such as deep sea trenches and building of mountains on the continents.

The findings were published in the journal Nature Geoscience.