Jeweller - November 2021

SCIENTIFIC REPORT
Super-Deep Diamonds
Uncovering secrets of the
SUPER-DEEP DIAMONDS
per-Deep Diamonds
CIENTIFIC REPORT
New research from Curtin University has revealed clues about Earth’s ancient
history – all locked within ultra-rare diamonds, writes DR LUC DOUCET.
Diamonds are said to be forever, and it seems
the secrets they contain about the history of
the Earth are also endless.
However, not all diamonds are alike; there are three main
types – lithospheric, oceanic, and super-deep continental.
Most diamonds formed in the most ancient continental
lithosphere – the outermost, rigid layer of Earth – between
150–300km from the surface.
These are known as lithospheric diamonds, and they form in
relatively stable environments, harvesting ambient carbon.
But not all diamonds form in this way. A relatively tiny portion
of all mined diamonds – approximately 1 per cent – originate
from the deep mantle of the Earth, located down to 1,000km
below the surface.
Recent research conducted by the Earth Dynamics Research
Group at Curtin University in Western Australia, published
by the journal Nature, discovered that diamonds found in
oceanic rocks and super-deep continental diamonds share
a common origin.
They are made of organic carbon – that is, former living
organisms that have effectively been ‘recycled’ – deep within
the Earth’s mantle.
Bringing new meaning to the old ‘trash-to-treasure’ adage,
this research discovered that the Earth’s ‘engine’ turns
organic carbon into diamonds many hundreds of kilometres
below the surface.
The Cullinan
The Great Star of Africa
530.20-carats
DI A MOND FIGURES
Going Deep
1,000km
depth at which
super-deep
diamonds form
3,106
carat weight of
the super-deep
Cullinan diamond
0.9%
proportion
of Earth's
diamonds that
are super-deep
Super-deep diamond formation is linked to the plate tectonic
processes that make our planet unique in the Solar System.
Carbon capture
The term ‘plate tectonics’ may be familiar to some from high
school science classes; essentially, the lithosphere is made
up of moving plates that shift and collide against each other.
Because of the tremendous force of mantle convection,
oceanic tectonic plates sink back into Earth’s mantle
underneath another plate in what we call the ‘subduction
zone’. When a plate is subducted, it carries with it all the
material needed to form diamonds.
This includes carbon from the rocks and from formerly living
things – the organic matter. At around 400km below the
surface, the subducted tectonic plate will enter the Earth’s
mantle’s ‘transition zone’.
In this place, pressure and temperature conditions allow
the formation of metallic melt, from which the super-deep
diamonds will form.
It is also a mystery as to why diamonds formed in the
transition zone utilise only recycled organic carbon – this
may be linked to the physical and chemical environment.
So, how do we discover them?
Both super-deep diamonds and lithospheric diamonds
are brought up to the surface during small but very
powerful eruptions.
Ballooning hot rocks, known as mantle plumes, rise from
deep in the mantle, transporting the diamonds to the surface
48 | November 2021