The Earth's mantle is so deep down, we've never been able to drill through the crust to sample it. We have only indirect ways of learning about it. This is a very different kind of geology than most peopleeven most geologistsknow about. It's like studying a car engine without being able to open the hood. But we do have some actual samples from down there . . . you may have one on your hand or your ear. I'm talking about diamonds, what else?
You know that a diamond is a hard, dense form of pure carbon. Physically there is no harder substance, but chemically speaking, diamonds are pretty fragile. More precisely, diamond is a metastable mineral at surface conditions. Experiment shows us that it cannot form except under conditions found at least 150 kilometers deep in the mantle beneath ancient continents. Take them a little above those depths, and diamonds swiftly turn to graphite. At the surface they can endure in our gentle environment, but not anywhere between here and their deep birthplace.
Well, the reason we have diamonds is that they cross that distance quickly, in just a day or so, in very peculiar eruptions. Aside from impacts from outer space, these eruptions are probably the most unexpected occurrences on Earth. Have you seen footage, or just a cartoon, of an oil gusher? That's how these work. Certain magmas at extreme depths find an opening and rush upward, burrowing through various rocksincluding diamond-bearing zonesas they go. Carbon dioxide gas comes out of solution as the magma rises, exactly like soda fizzing, and when the magma finishes puncturing the crust, it explodes into the air at several hundred meters per second. (One proposal is that it's supercritical CO2.)
We've never witnessed a diamond eruption; the most recent one, in the Ellendale Diamond Field, seems to have been in Australia in the Miocene, some 20 million years ago. Geologically speaking, that's just last week. But they have been very rare since about a billion years ago. We know about them from the bottomless plugs of solidified mantle rock that they leave behind, called kimberlites and lamproites, or just "diamond pipes." Some of these are found in Arkansas, in Wisconsin, and in Wyoming, among other places around the world with very old continental crust.
Inclusions and Xenoliths
A diamond with a speck inside it, worthless to the jeweler, is treasure to the geologist. That speck, an inclusion, is often a pristine specimen of the mantle, and our tools are good enough to extract lots of data from it. Some kimberlites, we have learned in the last two decades, deliver diamonds that appear to have come from 700 kilometers and deeper, below the upper mantle entirely. The evidence lies in the inclusions, where minerals are preserved that can only form at these unheard-of depths.
Also, along with diamonds come other exotic chunks of mantle rock. These rocks are called xenoliths, a great Scrabble word that means "stranger-stone" in scientific Greek.
What xenolith studies tell us, briefly, is that kimberlites and lamproites come from very old seafloor. Pieces of ocean crust from 2 and 3 billion years ago, pulled beneath the continents of the time by subduction, have sat down there for over a billion years. That crust and its water and sediments and carbon have simmered into a high-pressure stew, a red-hot broth that, in diamond pipes, burps back up to the surface like the taste of last night's tamales.
There's another conclusion to make from this knowledge. Seafloor has been subducting beneath the continents for almost as far back in time as we can tell, but diamond pipes are so rare, it must be that almost all subducted crust is digested in the mantle.
If the crust is mixing back into the mantle like this, then how deep does that mixing go? How has the process changed over the 4 billion years of Earth history? And does this knowledge shed light on other deep-seated mysteries that plate tectonics doesn't explain? These are the frontier questions explored later in this series.
PS: If it weren't for the high value of diamonds, we wouldn't have spent so much effort learning all this. And pretty soon, within our lifetimes, artificial diamonds will destroy the market and the mining industry and maybe even the romance. Heck, right now eleventh-grade kids are making diamonds in high school.