A specimen of a Witwatersrand diamond. Credit: Wits University
The 3 diamonds extracted from 3B-yr-old Witwatersrand Supergroup – the rock formation that is host to the famous Johannesburg gold mines to study when modern-style plate tectonics began to operate on planet Earth. “Because diamonds are some of the the hardest, most robust material on Earth, they are perfect little time capsules and have the capacity to tell us what processes were occurring extremely early in Earth’s history,” says Dr Katie Smart.
A cluster of the Witwatersrand diamonds. Credit: Wits University
The Earth is ~4.5B yrs old, and while a rock record exists from about 4B yrs ago, the complex preservational history of the most ancient rocks exposed on Earth’s surface has led to a heated debate amongst Geoscientists on when plate tectonics began operating on Earth. Many researchers believe plate tectonics began in the Archaean (the Eon 4 to 2.5B yrs ago), although 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 nitrogen characteristics of the diamonds, they also formed much earlier, around 3.5B yrs 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 3.5- 3B yrs ago.
By using an ion probe to analyse the C and N isotope compositions of the Witwatersrand diamonds, which have been pristinely preserved for >3B years, Smart and her team found that plate tectonics was likely in operation on Earth as early as 3.5 billion years ago.
A map of the Witwatersrand Basin. Credit: Economic Geology Research Institute (EGRI)
“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. 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.”
Earth as a planet is unique because of the dynamic process of plate tectonics that constantly transports surface material into the Earth’s mantle, which extends between 7 km to over 2800km below Earth’s surface. The process is driven by both convection cells within the Earth’s mantle and the character of crustal plates at Earth’s surface, where newly formed oceanic crustal plates are formed at spreading centres at mid-ocean ridges and then pushed apart. Older, cooler and more dense crust at convergent plate margins is then pulled into, or sinks, into the mantle at subduction zones. The subduction of crustal plates into the mantle can also carry sediments and organic material deep into the Earth’s interior.
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.
“Various researchers have tried to establish when exactly plate tectonics started on Earth, but while there are many investigations of ancient rocks on Earth’s surface – like the 3.5 billion year old Barberton Greenstone Belt here in South Africa, or the 4 billion year old Acasta Gneiss in northwest Canada – we are looking at the problem from a different viewpoint – by investigating minerals derived from Earth’s mantle,” says Smart. “We are not the first research group to study diamonds in order to tell when plate tectonics began, but our study of confirmed Archaean diamonds has suggested that plate tectonics was in operation by at least 3.5 billion years.”
http://phys.org/news/2016-01-diamonds-johannesburg-years-reveal-ancient.htmljCp
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