More than just beautiful, coveted stones, diamonds have another kind of wealth: fragments of the deep history of the Earth.
From shortcomings in the almost perfect lattice of the mineral, scientists have just developed how to extract long-hidden records of our planet’s past.
“We like the ones no one else really wants,” said Columbia University geochemist Jacob Weiss, referring to diamonds full of impurities that don’t look as clear and shiny as those desired for jewelry.
These fibrous, dirty-looking gems are where small information repositories are located, full of messages from the inner depths of the Earth. The carbon structure of the perfect diamond does not contain enough radioisotopes to help researchers date it, but the microinclusions found in its flaws can.
These flaws can form small pockets that can encapsulate the chemicals from which diamonds are born.
Sometimes these capsules contain other solids such as strange forms of ice, usually inaccessible minerals from the bowels of our world or even another diamond. These hard messages can be difficult to interpret, as the inclusions may have formed at different times from the diamond capsule in which they now rest.
However, other cavities in the diamond’s structure have trapped liquids that once penetrated the continental lithosphere mantle. This layer of the Earth is the uppermost part of the mantle (located between the earth’s crust and the outer core), 150 to 200 kilometers (90 to 120 miles) below the surface, and diamonds are “born” there.
Here, the extreme heat and lubricating pressure of the entire scale can force carbon atoms into the well-ordered structure of the diamond. In fact, the liquids that seep from above provide the carbon from which diamonds are formed.
Now a new technique has allowed researchers to finally determine the fluids in diamonds found in South Africa.
“For the first time, we can get reliable ages for these fluids,” Weiss said.
The diamond structure appears to prevent helium leakage, which allows Weiss and colleagues to age these rocks using the rare isotope helium-4, the ratio of radioactive atoms in fluid inclusions to the product of their decay.
Using this new method, the team identified three different periods of diamond formation in the underground rock masses that eventually crushed to form the mantle of Africa. Diamond-forming fluids change over the centuries, moving from carbonate-rich to silicone and finally to saline.
The first phase of diamond formation occurred in the Proterozoic, 2.6 billion to 750 million years ago, when these rocks collided in large mountain ranges. Researchers suspect that these collisions have allowed carbonate-rich liquids to sink deep into the Earth, but how exactly is still unknown.
The next phase also coincided with a mountain-forming period, 540 to 300 million years ago during the Paleozoic, producing diamonds with silicone-rich inclusions. At this stage, the beginning of the future African mantle is formed.
Then, 130 to 85 million years ago during the Cretaceous, the liquid was enriched with saline – suggesting that these diamonds formed from the former ocean floor. This was drawn below the now formed continental mass of Africa by subduction, where one continental plate is forced beneath another, where they meet.
All the rocks were then brought closer to the Earth’s surface through deep volcanic activity, like the eruptions of the kimberlites 85 million years ago, where they were recently discovered by miners.
“South Africa is one of the best-studied places in the world, but we have very rarely been able to see beyond the indirect indications of what happened there in the past,” said Columbia University geochemist Cornelia Klass, explaining these minor drops are a rare way to connecting events from the deep earth space with those on the surface.
It is worth noting that today millions of workers rely on diamond mining as a source of income, but the conditions in which they work can be harsh and include human trafficking and child labor. Mines have also polluted the soils and waterways on which entire communities rely.
The company that produced the diamonds in this study, De Beers, one of the world’s two largest diamond producers, often did not disclose which mines the individual diamonds came from.
So while diamonds can reveal a lot about our geological history, extracting them from Earth can also be incredibly expensive.
This study was published in Nature Communications.