Diamonds - how they were formed on Earth?

There are diamonds in space. They can also be formed on Earth as a result of a meteorite impact. But most of the diamonds that end up on ladies' fingers come from the upper mantle of our planet.

The story of diamond formation began over 3 billion years ago in what geologists call the Earth's upper mantle, at depths of between 150 and 200 kilometers beneath the Earth's crust: a layer of the internal structure of our planet that is mainly plastic and sinks to about 700 kilometers below our feet with temperatures of up to 1,500°C and an incredible pressure of up to 75 tons/cm2 prevail. This is more than 70,000 atmospheres or 7 GPa.

Under these extreme conditions, pure carbon crystallizes. It then appears in the form of a surprisingly hard transparent mineral: diamond. But before we can enjoy the beauty of diamonds, we still need the help of a volcano. When it rises to the surface at high speed, it is indeed the molten magma that can carry with it the diamonds buried more than 150 kilometers underground. It then forms rare volcanic rocks containing diamonds: kimberlite, lamproite, or komatiite.

The question of the temperature of the upper mantle

A beautiful story that until recently clashed with the idea that geologists had of the evolution of temperatures inside our Earth over the few billion years since its formation. In the Archean era, the Earth produced much more heat than today. Up to three times more. Thus, researchers naturally assumed that the Earth's mantle must have been much hotter then than today.

But recent work seems to show that the Earth's upper mantle was not warmer than today. This theory is based on the comparative study of ancient rocks and modern lavas, all from the upper mantle. Both have a similar level of magnesium oxide that betrays similar temperatures. Fortunately, because with higher temperatures, the upper mantle would not have transformed carbon into diamond but into sad pieces of graphite.

Interested in similar topics? You may also be interested in this article: Why is the Earth's core still hot and will it ever freeze?