Diamonds aren’t always pristine. In fact, most of the time they contain inclusions – trapped chemical compounds they’ve picked up along the way – which can give them a welcome or unwanted hue. Take blue diamonds, for example: although rare, it’s well known that it’s the element boron that gives them their famous shade.
As explained by a new Nature study, precisely where this boron comes from has been something of a mystery for some time now. Fortunately, after tinkering with the geochemistry of dozens of these blue diamonds, the Gemological Institute of America (GIA) has provided us with an answer.
Boron is near-ubiquitous in the crust, both oceanic and continental. Clearly, this is getting sucked down deep below to depths where diamond-forming fluid accumulates, but it’s not been clear how it got there.
Taking a closer look at the mineral inclusions in these diamonds – many of which hadn’t been seen before – using X-rays and lasers, the team matched them with mineral ancestors, geochemical precursors if you like, found in oceanic crust segments.
Specifically, these were segments that trapped seawater and dragged it into the mantle – the superheated, colossal, solid layer that hides beneath the crust. As these broken crustal giants headed through this layer, the seawater-enriched rocks were “cooked”, and boron-rich fluids were released.
The physical structure of these mineral inclusions also betray just how deep these blue diamonds form: in the lowest parts of the mantle, at depths of at least 660 kilometers (410 miles). According to the study, this makes them “among the deepest diamonds ever found.”
This research, above all else, adds another chapter to an already insane story about the journey they go on before we dig them up. It’s more surprising than you probably realize, because diamonds aren’t what you think they are: compressed coal. So where do they come from?
The vast majority of Earth’s diamonds were forged somewhere in the mantle. Most diamonds formed between 1 and 3 billion years ago, at depths of around 150 kilometers (93 miles) beneath the continental crust, or 200 kilometers (124 miles) beneath the oceanic crust.
It’s likely that carbon dioxide, a major component of magmatic bodies, is buried at extreme depth, where the high temperature and very high pressures there compress it into a diamond. It seems that they can only form in a very narrow sliver of the upper mantle.
There’s no way we’d be able to mine diamonds at their formation depth, though, so how the hell do they get to the shallow crust where we can dig them out? That took the most bonkers form of volcanic process known to science: kimberlite eruptions.
Kimberlites are essentially cone-shaped pipes, and they only seem to appear in ancient continental crust. Their magma source was once exceedingly hot, very runny, and full of dissolved gases.
When the internal pressure got a bit too much for the surrounding crust, it rushed up through the crust. There was likely a froth of carbon dioxide at the front, which was so buoyant that it moved at speeds of 1,000 kilometers (621 miles) per hour until it reached the surface.
These eruptions no longer happen. It’s thought that they could only take place many millions of years ago, when Earth’s innards were far hotter.
It’s a good thing they did, though. Their deep-seated magma often included “alien rocks” from the mantle, named xenoliths – and those xenoliths frequently contained those elusive diamonds. The speed of the eruptions was key: if these eruptions were any slower, the diamonds would transform into other, more commonplace forms of carbon.
It seems that blue diamonds formed in the lower mantle, and then got to the surface through the very same rocket ride. Not only is that pretty damn cool, but it’s clear evidence that doomed crust that once sat at the world’s surface is being recycled at truly hellish depths.