Kimberlite Pipes: What They Are and How They’re Formed

How are kimberlite pipes formed?

As mentioned, kimberlite pipes are believed to have been formed by deep-source volcanic eruptions. Jeffrey Post, a diamond expert at the Smithsonian, has described these eruptions as “quite violent.”

“We haven’t seen such eruptions in recent times,” he said in a Q&A on diamond formation.

“They were probably at a time when the earth was hotter, and that’s probably why those eruptions were more deeply rooted. These eruptions then carried the already-formed diamonds from the upper mantle to the surface of the Earth,” added Post.

Indeed, some estimates place the last kimberlite eruption at around 40 million years ago. That has made it tough for experts to determine exactly how they occurred. However, the consensus is that these eruptions happened fairly quickly — their journey from the mantle is estimated to have taken hours, not days or months, as is the case with magma and lava from a volcanic eruption.

“One of the things we know … is that the process of the Kimberlite eruption bringing the diamonds from the upper mantle to the surface of the Earth had to happen very quickly, because if they were traveling too long and too slowly they would have literally turned into graphite along the way. And so by moving quickly they essentially got locked into place into the diamond structure,” said Post.

He added that the pipes may have moved to the surface at a rate of 20 to 30 miles an hour. Some believe that the only way for magma to rise so quickly is if the melt is supercharged with gas; however, nobody has found out where such gas might come from.

What role does carbon play in the formation of kimberlite pipes?

Diamondiferous kimberlites originate from carbon, and experts are also not sure of the source of this carbon. Offering an explanation, Geology.com notes that most likely it was “trapped in the Earth’s interior at the time of the planet’s formation.”

According to Post, in some cases it seems like the carbon originated in the deep mantle, while in others it appears to have come from near the Earth’s surface. In the latter case, the carbon “could have literally been carbon that was part of carbonate sediments or animals, plants, shells, whatever, that was carried down into the upper mantle of the Earth by the plate tectonics mechanism called subduction.”

As is mentioned, the intense heat and pressure of the deep mantle combine to transform this carbon into diamonds. Put very simply, each carbon atom bonds to four other carbon atoms, creating a very strong connection; it’s because of this strength that diamonds are so hard. As more carbon atoms come into the vicinity of the bonded carbon atoms, they too attach.

Eventually, once enough carbon atoms have attached to one another, diamonds are formed. Colored diamonds are created when foreign particulates are trapped during the diamond crystallization process. Traces of different material result in different hues, as well as different hue intensities.

For example, blue diamonds, which are incredibly valuable, are created when boron is introduced in the diamond formation process — it bonds to the carbon atoms, absorbing the red, yellow and green areas of the color spectrum.

Where are kimberlite pipes located?

Kimberlite pipes were first found near Kimberley in South Africa, but since then they have been found on most continents. Of course, as has been noted, not all kimberlite pipes are created equal and only a small percentage of them contain diamonds.

So where should diamond exploration companies look if they want to find kimberlite pipes that contain diamonds? Major diamond mines can be found in South Africa, Botswana, Angola, Russia, Canada and Australia, so starting there is a good bet.

That said, simply showing up in a country that produces diamonds and hoping to find a kimberlite field is not a sound approach. When a company has found a site that it believes may hold diamonds, one thing it may do is look for minerals that are associated with kimberlite pipes. These minerals include garnet, clinopyroxene, orthopyroxene, olivine, spinel, ilmenite, magnetite, rutile and zircon.

A member of the garnet family called pyrope is one of the best when looking for diamonds. A rare form of pyrope garnet called chrome-pyrope forms under the same temperature and pressure as diamonds, meaning that if it is present there is a good chance that a kimberlite pipe with diamonds in it is nearby.

Companies on the hunt for diamonds have other techniques they use to explore, but in general it takes longer to find diamond deposits than it does to find deposits of resources like gold and silver. It is also usually more expensive, though the payoff can certainly be high in the end.