Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Mitchell Paige
Diamonds Formed in Nature
Diamonds have been growing naturally under unbearable temperatures from 1 to 3.3
billion years. Having the hardest hardness of any other mineral it cannot be cut
by anything other than diamond.
Diamonds are formed over 111 miles below the earth’s crust mostly made up of molten rock and metals, with temperatures of up to 2,462 degrees Fahrenheit. Carbonates work in this heat to form an unstable crystalline structure of the perfect hardness. These carbonate melts, among other things, form when limestone enter the Earth’s mantle. This occurs as a result of subduction, when old oceanic crust plunges down below an adjoining crustal plate. By means of high-pressure experiments in the laboratory, postdoc Arno Rohrbach and ETH Zurich Professor Max Schmidt from the Institute of Geochemistry and Petrology of ETH Zurich studied how the oxidized carbon contained in the carbonates behaves in the Earth’s mantle, and in so doing they made an exciting discovery: at a depth of more than 200 kilometres, the submerged carbonates of the oceanic crust reach their melting point, which is 300 to 400 degrees lower than that of silicates. A carbonatite melt is formed and migrates from the subducted oceanic crust into the surrounding mantle, where it causes partial melting therein. However, because the Earth’s mantle is strongly chemically reducing and contains elemental metallic iron, the CO2 in the carbonate melt reacts with the elemental iron. This causes diamonds to form.
However, as a result of convection in the Earth’s mantle over hundreds of millions of years, the “agglomerates” of diamonds, which cover a distance of between one and ten centimeters per year, can be transported into the lower mantle down to a depth well in excess of 2000 kilometers, and can rise up again from there. During the up rise, the diamonds remain stable until they pass back through the transition zone from the lower to the upper mantle of the Earth, in which both the chemical equilibrium and the pressure and temperature conditions change again: here the minerals containing trivalent iron – perovskite and garnet – become unstable and release trivalent iron which then reacts with the diamonds. The carbon is re-oxidized in this process, the diamonds are destroyed and the trivalent iron is reduced to divalent iron. Carbonate melts can form again due to the presence of carbon dioxide, and the divalent iron is incorporated into the minerals olivine and pyroxene, which make up the majority of the upper mantle.
This enabled the researchers to show not only how and where carbonate melts are formed in the Earth’s mantle. They also demonstrated how the carbon cycle from the Earth’s surface down to its interior functions: from carbon dioxide transported with carbonates into the interior, reduced to pure carbon and finally oxidized back to carbon dioxide as it rises. The resulting carbonate melt, most of which is normally dissolved in a silicate melt at crustal pressures, ultimately brings the carbon dioxide back to the surface of the Earth through active volcanoes such as the Ol Doinyo Lengai
Diamonds are held as a symbol of love being a common wedding ring; they make beautiful jewelry and show high value. Diamonds are weighed in carats with one carat weighing about the same as a paper clip, or 0.2 grams. Just as a dollar is divided into 100 pennies, a carat is divided into 100 points which means that a diamond of 50 points weighs 0.50 carats. But two diamonds of equal weight can have very different values depending on their clarity, color, and cut. Carat weight is the most intuitive of the 4Cs – you expect a larger diamond to be worth more.