December Editorial      


When is a diamond not a diamond?
The 'oldest diamonds' found in Jack Hills zircons may be no such thing


In our 2007 September editorial we covered the topic of diamond inclusions in zircons. A research group from Westfali William University in Germany had discovered tiny diamonds within zircons (namely zirconium silicate (ZrSiO4) which were believed to be 4.25 billion years old. The paper, which was was published in the journal Nature, showed what were thought to be the oldest diamonds ever discovered. But those results have now been challenged.

In a recent publication of Earth and Planetary Science Letters researchers from the Department of Earth Sciences at the University of California, working jointlyt with the GFZ German Research Center for Geosciences, analyzed the Jack Hills zircons with electron microscopy. They discovered that the diamonds within these zircons may be merely an artifact created during the preparation of zircon for the original research.

It is well established that the Earth is about 4.6 billion years old. The earliest period in that 4.6 billion years is the Hadean era between 4.6 and 4 billion years ago. Unfortunately, we know of no rocks which have survived from that distant era through to the present. In fact detrital zircons ('detrial' means the detrius of loose grains of minerals or rocks that have been eroded from other rocks or earth materials ) are the only known surviving fragments of the Earth’s crust from this period. Some detrital zircons can be dated as far back as 4.4 billion years ago. Not surprisingly they have been studied extensively in the last few years. Zircons form at temperatures between 1,112 degrees and 1,652 degrees Fahrenheit (600-900 degrees Celsius) from granitic melts at shallow depths. Once crystallized, a zircon's chemical make-up and structure doesn't change, making it easy to establish its age through a process similar to carbon dating. So the assumption that one can find other geological markers as inclusions inside the detrital zircons is not unreasonable. But diamonds? As Harry Green (a professor of the Graduate Division at UC Riverside and co-author of the latest research points out: ‘Diamonds are not indigenous to the zircons’ . To complicate the matter even more, after the earlier diamond discovery, another study surveyed a very large sample the inclusion suite (>400) of Jack Hills zircons (JHZs) and this survey found abundant inclusions consistent with the previous studies but no diamonds in those inclusions.

The lead author of the latest research, Larissa Dobrzhinetskaya, with her colleagues decided to revisit the issue of diamonds inside zircons once again. They used a high-resolution electron microscope which would allow them to see the diamond inclusions in more detail. ‘It occurred to us that a long-term history of diamond recycling with intermittent trapping into zircons would likely leave some sort of microstructural record at the interface between the diamonds and zircon’ Larissa Dobrzhinetskaya said. The study did indeed confirm the presence of diamonds inside the zircon inclusions. But those diamonds were much less than 4 billion years old. They were in fact diamond fragments mechanically injected when the zircon was polished with diamond paste. In other words: ‘They are contamination. This, combined with the lack of diamonds in any other samples of Jack Hills zircons, strongly suggests that there are no indigenous diamonds in the Jack Hills zircons’ says Harry Green.

You can read more about this research in the article below.


  1. Larissa Dobrzhinetskaya, Richard Wirth, Harry Green. Diamonds in Earth's oldest zircons from Jack Hills conglomerate, Australia, are contamination. Earth and Planetary Science Letters, (2014); 387: pp212.


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