Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 35

appear at this time, and change in the world's diamonds also took
place. It's a curious story—but very revealing of just how mysterious
are the ways in which the Earth moves.
Deep down in the mantle, diamonds are now forming, and have
been forming since the Earth began. These highly sought-after crys-
talline cages of carbon need very high pressures to form. On Earth
those kinds of pressures start at around 120 kilometres below the sur-
face. These diamonds must then reach the surface by high-speed
transfer (otherwise they would alter to graphite on the way up). This
transfer is provided by a very special kind of volcano, one that is as
rare as the diamonds themselves. It is called a kimberlite, and kimber-
lite eruptions are so rare that none have been witnessed by humans.
Kimberlite eruptions are deep-rooted and extraordinarily gas-rich,
and, just now and then, over the aeons have punched neat cylindrical
holes—kimberlite pipes—in all of the world's ancient continents, fill-
ing them with minerals (including diamonds) derived from hundreds
of kilometres below the surface.
Not all diamonds are pure and flawless. Many have tiny inclusions
of minerals and rocks that crystallized at those great depths in the
mantle. Trapped and preserved for billions of years within that hard-
est of adamantine shells, those mineral inclusions, just a few thou-
sandths of a millimetre across, can now be analysed, routinely, by the
modern magic of scanning electron microscopes and electron micro-
probes. The story they tell is more valuable than the diamonds
themselves.
Analysis of many diamonds from around the world 32 has shown
that those which are up to 3 billion years old commonly contain tiny
particles of a rock called peridotite—which is basically mantle rock
from which the stuff of ocean crust has been extracted, and also of a
rock called eclogite, which is what subducted ocean crust turns into
once it has slid down to 100 kilometres or so below the Earth's surface.