Diamonds found in Johannesburg more than 80 years ago reveal how the ancient Earth was shaped

[Jim__]

From phys.org:

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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 amongst Geoscientists 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.

By using 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, Smart and her team found that plate tectonics was likely in operation on Earth as early as 3.5 billion years ago.

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"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."

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[DhhD]

1. The discovery of Earth's oldest rocks. This reports is from 2009.

The discovery of the Earth's oldest rocks
http://rsnr.royalsocietypublishing.org/content/63/4/381


Science from the past.
Footnote 8
8 Radiogenic isotope ratios are used to date and to identify individual episodes of mantle differentiation and continental crust formation throughout geological time. The isotope ratios referred to here are 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 143Nd/144Nd; that is, the radiogenic isotope divided by the non-radiogenic isotope (see note 7). Full descriptions of dating techniques and associated isotopic methods can be found in A. P. Dickin, Radiogenic isotope geology (Cambridge University Press, 1995) and G. Faure and T. M. Mensing, Isotopes: principles and applications, 3rd edn (John Wiley & Sons, New York, 2005).

[DhhD]

2. Interesting study on the growth and dating of Earth's crust and its oceans.

http://www.geo.cornell.edu/geology/classes/Geo656/656notes13/IsotopeGeochemistryChapter7.pdf