Earth is the only planet in our solar system with plate tectonics. As a result, Earth’s crust is subject to constant turnover, being created at plate boundaries and subducted at plate margins around the world. It is thanks to plate tectonics that we have a nitrogen-rich atmosphere, and consequently life. Plate tectonics also changed Earth’s interior over time, introducing oxygen and other volatiles to the mantle. The onset of plate tectonics, therefore, is a landmark moment in the history of our planet.
Previous studies suggested that plate tectonics began somewhere between 3.8 and 3.2 billion years ago. However, scientists have found very few unmodified cratonic rocks on Earth older than 3 billion years. Unequivocal evidence for the onset of subduction, therefore, is rare. In a new paper published in Nature Geoscience, Katie Smart (University of the Witwatersrand, South Africa) and colleagues present analyses of three diamonds derived from 3 billion year old sediments from the Kaapvaal craton in South Africa, supporting the hypothesis that plate tectonics began approximately 3.5 billion years ago .
Diamonds, although dominantly composed of carbon with some nitrogen “contamination,” are packed with valuable information. Isotopic ratios of carbon and nitrogen betray the sources and redox character of volatiles involved in diamond growth, as well as the kind of environment in which it formed. Smart and colleagues were able to show, using the nitrogen systematics of the three diamonds they analyzed, that the Wits diamonds likely formed 3.5 billion years ago, approximately 200 – 400 million years before incorporation into the 3.1 – 2.9 billion year old Witwatersrand sedimentary rocks in which the stones were found.
The team’s carbon isotope analyses showed the diamonds began forming in relatively oxidized fluids, but with equivocal primordial mantle carbon or recycled crustal carbon signatures. However, when the authors looked at the nitrogen isotope compositions of the Wits diamonds, they saw signatures indicative of crustal sources.
“What is really interesting about the Witwatersrand diamonds, because they are derived from approximately 3 billion year old sedimentary rocks, is that they are rare robust windows into processes and volatile cycles operating on early Earth,’” said Smart. “The nitrogen results in particular indicate the operation of shallow to deep volatile cycling in the Meso- to Palaeoarchaean, and by inference, the operation of plate tectonics early in Earth’s history.”
These observations imply that the diamonds formed in a region of Earth’s mantle that was influenced by newly subducting crust, and, allowing for a few hundred million years of recycling, that plate tectonics began as early as 3.5 billion years ago.
Image: Enigmatic Witwatersrand diamond (4 mm in diameter). Credit: K. Smart