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 people—even
most geologists—know 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.


Diamond Eruptions



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 rocks—including
diamond-bearing zones—as 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.


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.