Diamond liquid impurities provide new insights into ancient Earth
Geologists have studied tiny pockets of fluids trapped inside diamonds to get a better understanding of how old humanity’s favorite rocks might be. In doing so, they identified three distinct periods of diamond formation in Africa over the past few billion years, with intriguing implications for our planet’s history.
We’re sorry to break it to you, but as pretty as the diamond in your ring or necklace may be, it’s extremely boring from a geological perspective. That’s because the gem-quality diamonds so prized by the jewelry industry have to be as pure as possible, meaning they don’t have any “inclusions” of other elements trapped inside.
But these inclusions are of great interest to geologists. Analyzing these impurities can teach scientists plenty about the age of the diamond, and the environment and conditions under which it formed. Solid inclusions are the most commonly analyzed type, but they may not produce totally accurate dates – these minerals could have formed long before the diamond encased it, or been captured later.
So, for the new study, researchers at Columbia University investigated liquid inclusions, which should be able to provide a more reliable time frame. After all, these little pockets contain the carbon-rich fluid that the diamond itself originally formed from.
The team measured the levels of thorium and uranium in the liquid inclusions of 10 diamonds, and analyzed the ratios of these radioactive elements to helium-4, which is produced as they decay. They also checked how fast helium molecules might be able to leak out of the diamond, which would affect the ratio. Luckily, it seems that they can’t escape very easily.
From their analysis, the researchers identified three separate periods of diamond formation, marked by drastically different chemical compositions. The earliest period dates back between 2.6 billion and 700 million years ago, with the inclusions rich in carbonate minerals. Around this time, huge mountain ranges were apparently forming on the surface, but the researchers aren’t sure whether that’s a coincidence or connected in some way.
The second period spans 550 million to 300 million years ago, with inclusions high in silica minerals. And the third period, between 130 million and 85 million years ago, showed a different composition again – this time, they were high in saline compounds of sodium and potassium. This, the team says, indicates a shift in carbon source, as the diamonds were increasingly being formed from sections of seafloor dragged deep into the Earth.
Finally, the researchers say that it was at the end of this latest period that huge eruptions of kimberlite carried all of these diamonds up towards the surface. This deposit ended up becoming the De Beers diamond mine in South Africa.
Strangely, one of the diamonds that the team studied contained fluids from both the oldest and the youngest of these periods. That suggests that old diamonds can continue to add new layers over huge stretches of time.
The researchers say that the study can improve our understanding not just of the formation of diamonds, but different periods in the history of the Earth itself.
“What is fascinating is, you can constrain all these different episodes from the fluids,” says Cornelia Class, co-author of the study. “Southern Africa is one of the best-studied places in the world, but we’ve very rarely been able to see beyond the indirect indications of what happened there in the past.”
The research was published in the journal Nature Communications.
Source: Columbia University