Diamond History Have you ever wondered, how old really, are the diamonds we see around us? Diamonds are aptly called as messengers from the deep earth. Scientists having been studying them closely to understand what goes on into the otherwise inaccessible depths from which they come. But the messages are often hard to read.
Recently, a team has come up with a way to determine the age of diamonds with inclusions, and the chemistry of their parent material. The research has allowed them to map out geologic events going back more than a billion years – which is of huge significance not only in the study of diamonds, but of planetary evolution.
Nature of Diamonds
Gem-quality diamonds are grid-like structures of nearly pure carbon. This purity (being made of just one element) gives them them their brilliance; but it also means they carry very little information about their age and origin.
However, some lower-grade diamonds contain imperfections in the form of tiny pockets of liquid – essentially what’s left of the more complex fluids from which the diamond crystals evolved.
By analyzing these fluid filled imperfections, the scientists through a new study worked out the times when different diamonds formed, and the shifting chemical conditions around them.
What Do We Know So Far On Diamonds?
Most diamonds are believed to have formed some 150 to 200 kilometers under the surface, in relatively cool masses of rock beneath the continents. This may have happened as far back as 3.5 billion years, and probably continues even today at some rate. Occasionally, these diamonds in the making or completed are carried upward by powerful, volcanic eruptions called kimberlites.
Much of what we know about diamonds comes from lab experiments, and studies of other minerals and rocks that come up with the diamonds, or are sometimes even encased within them. Fibrous, dirty-looking diamond pieces containing solid or liquid impurities would disqualify them as jewelry, but carry potentially valuable chemical information.
Up to now, most researchers have concentrated on solid inclusions, such as tiny bits of garnet, to determine the ages of diamonds. But the ages that solid inclusions indicate can be debatable, because 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 stuff from which the diamond itself formed.
Method Of Diamond Fluid research
What the scientist did different this time was find a way to date the fluids. They did this by measuring traces of radioactive thorium and uranium, and their ratios to helium-4, a rare isotope that results from their decay.
The scientists also figured out the maximum rate at which the nimble little helium molecules can leak out of the diamond – without which data, conclusions about ages based on the abundance of the isotope could be thrown far off. (As it turns out, diamonds are very good at containing helium.)
The Result
The team identified three distinct periods of diamond formation. These all took place within separate rock masses that eventually combined into present-day Africa.
- The oldest took place between 2.6 billion and 700 million years ago. Fluid inclusions from that time show a distinct composition, extremely rich in carbonate minerals. The period also coincided with the buildup of great mountain ranges on the surface, apparently from the collisions and squishing together of the rocks. These collisions may have had something to do with production of the carbonate-rich fluids below, although exactly how is vague, the researchers say.
- The next diamond-formation phase spanned a possible time frame of 550 million to 300 million years ago, as the proto-African continent continued to rearrange itself. At this time, the liquid inclusions show, the fluids were high in silica minerals, indicating a shift in subterranean 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 fluid composition switched: Now, it was high in saline compounds containing sodium and potassium. This suggests that the carbon from which these diamonds formed did not come directly from the deep earth, but rather from an ocean floor that was dragged under a continental mass by subduction. This idea, that some diamonds’ carbon may be recycled from the surface, was once considered improbable, but recent research b has increased its likelihood.
- One interesting find: At least one diamond encapsulated fluid from both the oldest and youngest eras. The shows that new layers can be added to old crystals, allowing individual diamonds to evolve over vast periods of time.
It was at the end of this most recent period, when Africa had largely assumed its current shape, that a great bloom of kimberlite eruptions carried all the diamonds the scientists studied to the surface. The solidified remains of these eruptions were discovered in the 1870s, and became the famous De Beers mines. Exactly what caused them to erupt is still part of the puzzle.
The tiny diamond-encased droplets provide a rare way to link events that took place long ago on the surface with what was going on at the same time far below.
When quizzed whether the findings could help geologists find new diamond deposits, the research team relied “Probably not,” . But, added, the method could be applied to other diamond-producing areas of the world, including Australia, Brazil, and northern Canada and Russia, to disentangle the deep histories of those regions, and develop new insights into how continents evolve.
