Diamonds are sometimes described as messengers from the deep earth; scientists study them closely to gain insight into the otherwise inaccessible depths from which they originate. But the messages are often difficult to read. Now a team has come up with a way to solve two long-standing puzzles: the age of diamonds containing individual fluids, and the chemistry of their original material. The research allowed them to sketch geological events dating back over a billion years – a potential breakthrough not only in the study of diamonds, but of planetary evolution.
Gem quality diamonds are almost pure carbon lattices. This elementary purity gives them their brilliance; but it also means that they carry very little information about their age and origins. However, some inferior specimens show imperfections in the form of tiny pockets of fluid – remnants of the more complex fluids from which the crystals evolved. By analyzing these fluids, scientists in the new study determined when different diamonds formed and the changing chemical conditions around them.
“It opens a window – say, even a door – to some of the very big questions” on the evolution of the deep earth and continents, said lead author Yaakov Weiss, assistant scientist at Columbia University in Lamont. – Doherty Earth Observatory, where the analyzes were carried out, and lecturer at the Hebrew University of Jerusalem. “This is the first time that we can get reliable ages for these fluids.” The study was published this week in the journal Nature communications.
Most diamonds are thought to form 150 to 200 kilometers below the surface, in relatively cold rock masses below the continents. The process can be traced back as far as 3.5 billion years and probably continues today. Sometimes they are carried upwards by powerful, deep volcanic eruptions called kimberlites. (Don’t expect to see one today; the youngest known kimberlite deposits are tens of millions of years old.)
Much of what we know about diamonds comes from laboratory experiments and studies of other minerals and rocks that produce diamonds, or are sometimes even locked inside. The 10 diamonds the team studied came from mines founded by the De Beers company in and around Kimberley, South Africa. “We love the ones that no one else really wants,” Weiss said – fibrous, dirty specimens with solid or liquid impurities that disqualify them as jewelry, but carry potentially valuable chemical information. So far, most researchers have focused on solid inclusions, such as tiny pieces of garnet, to determine the age of diamonds. But the ages that the solid inclusions indicate can be questionable, as the inclusions may or may not have formed at the same time as the diamond itself. Encapsulated fluids, on the other hand, are the real thing, the substance from which the diamond itself was formed.
What Weiss and his colleagues did was find a way to date fluids. They did this by measuring traces of radioactive thorium and uranium, as well as their ratios to helium-4, a rare isotope that results from their decay. Scientists also determined the maximum speed at which agile small helium molecules can escape from the diamond – without which data, conclusions about ages based on the abundance of the isotope could be dismissed far and wide. (It turns out that diamonds are very good at containing helium.)
The team identified three distinct periods of diamond formation. All of this took place in separate rock masses that eventually merged with present-day Africa. The oldest occurred between 2.6 billion and 700 million years ago. The fluid inclusions of this period show a distinct composition, extremely rich in carbonate minerals. The period also coincided with the accumulation of large mountain ranges on the surface, apparently as a result of collisions and crushing of rocks. These collisions may have something to do with the production of the carbonate-rich fluids below, although the exact way is unclear, the researchers say.
The next phase of diamond formation spanned a possible period of 550 to 300 million years, as the Proto-African continent continued to reorganize. At this point, the liquid inclusions show that the fluids were rich in silica minerals, indicating a change in underground conditions. The period also coincided with another major mountain building episode.
The most recent known phase took place between 130 million years and 85 million years ago. Again, the composition of the fluid changed: now it was rich in saline compounds containing sodium and potassium. This suggests that the carbon these diamonds formed from did not come directly from the deep earth, but rather from an ocean floor that was dragged under a landmass by subduction. This idea, that some diamonds’ carbon can be recycled from the surface, was once considered unlikely, but recent research by Weiss and others has increased its value.
An intriguing find: at least one diamond-encapsulated fluid from the oldest and youngest eras. The shows that new layers can be added to old crystals, allowing individual diamonds to evolve over long periods of time.
It was at the end of this most recent period, when Africa had largely taken its present form, that a large bloom of kimberlite eruptions brought to the surface all of the diamonds studied by the team. The solidified remains of these eruptions were discovered in the 1870s and have become the famous mines of De Beers. The exact cause of their eruption is still part of the puzzle.
The tiny droplets surrounded by diamonds offer a rare way to connect events that took place long ago on the surface with what was happening at the same time well below, scientists say. “What’s fascinating is that you can coerce all of these different episodes from fluids,” said Cornelia Class, a geochemist at Lamont-Doherty and co-author of the article. “Southern Africa is one of the best-studied places in the world, but we have very rarely been able to see beyond indirect indications of what has happened there in the past.”
When asked if the finds could help geologists find new diamond deposits, Weiss just laughed. “Probably not,” he said. But, he said, the method could be applied to other diamond-producing regions of the world, including Australia, Brazil, northern Canada and Russia, to unravel the deep histories of those regions and develop new knowledge about how the continents evolve.
“These are really big questions, and it will take a long time for people to answer them,” he said. “I will be retiring, and I still haven’t finished this walk. But at least it gives us some new ideas on how to find out how things work.”
The other authors of the study are Yael Kiro of the Weizmann Institute of Science in Israel; Gisela Winckler and Steven Goldstein of Lamont-Doherty; and Jeff Harris from the University of Glasgow in Scotland.