Diamonds are known for their exceptional hardness and brilliance, but they are not formed near the Earth’s surface. Their creation requires carbon atoms to be subjected to extreme conditions found only within the Earth’s mantle, the layer between the crust and the outer core. The formation of the dense, crystalline structure of diamond relies on a specific confluence of raw material, immense pressure, high temperature, and a chemical medium to facilitate growth. These factors explain why this pure form of carbon is so rare and why its origin is tied to the deep geological history of our planet.
The Essential Carbon Source
The carbon atoms that form diamonds originate from two sources. The most common is primordial carbon, incorporated into the mantle during the planet’s formation 4.5 billion years ago. This deep-mantle carbon forms peridotitic diamonds and shows a uniform isotopic signature.
The second source is recycled surface carbon, carried into the mantle through plate subduction. This carbon originates from ancient surface materials, such as marine carbonates, limestone, and organic matter. As oceanic plates sink, they transport this material into the mantle’s high-pressure environment, where it is broken down and reformed. This process creates eclogitic diamonds, which often exhibit distinct, lighter carbon isotope ratios indicating their origin as former surface life.
Extreme Pressure and Temperature Conditions
Diamonds form in the mantle because these depths are the only location where carbon is thermodynamically stable in its dense, crystalline structure. Carbon atoms only adopt the dense, tightly-packed tetrahedral lattice of diamond when subjected to immense pressure. This requirement is known as the diamond stability field, which begins deep beneath the crust.
The pressure needed for formation typically ranges between 4.5 and 6 Gigapascals (GPa), found at depths of roughly 140 to 190 kilometers below the surface. This pressure is equivalent to 45,000 to 60,000 times the atmospheric pressure at sea level. At shallower depths, carbon is stable as graphite, a softer material.
The required temperature window is also restrictive, generally falling between \(900^{\circ}\text{C}\) and \(1300^{\circ}\text{C}\). This heat provides the necessary energy for carbon atoms to overcome kinetic barriers and rearrange into the diamond lattice. Without this high temperature, the atoms would lack the mobility to crystallize, even under sufficient pressure.
These conditions are predominantly met beneath continental cratons, the oldest and most stable parts of the continental crust. These cratons have deep, cold roots, called mantle keels, extending to depths of 200 kilometers or more. The shallow geothermal gradient beneath these keels allows the pressure to reach the diamond stability field before the temperature destabilizes the growth environment.
The Necessity of Chemical Solvents
While pressure and temperature set the stage for diamond formation, the conversion of solid carbon to diamond requires chemical assistance. In the mantle, the direct, solid-state transformation of graphite into diamond is geologically too slow. Instead, the process relies on trace amounts of high-density fluids or melts, which act as solvents or catalysts.
These solvents are typically rich in compounds containing carbon, oxygen, hydrogen, nitrogen, and sulfur (C-O-H-N-S fluids). The fluid dissolves carbon from an existing source material. Once dissolved, the carbon atoms can migrate through the fluid phase and rapidly precipitate as diamond crystals when the conditions are right.
Specific chemical agents that facilitate this growth include carbonated silicate melts and saline fluids rich in elements like potassium, sodium, and chlorine. Metallic melts containing iron and nickel can also act as solvents, particularly in deeper mantle environments. The fluid’s role is to lower the activation energy required for the crystallization reaction, allowing the diamond to grow at a much faster rate than would be possible in a completely solid medium. This metasomatic process is the mechanism by which most natural diamonds are formed over millions to billions of years.
Rapid Ascent and Preservation
After formation, the diamond must survive the journey to the Earth’s surface. Although stable at mantle depths, diamond becomes unstable as pressure decreases closer to the surface. If it remains in the high-temperature upper mantle for too long, it will chemically revert back to graphite.
Preservation requires an extremely rapid transport mechanism to move the diamond out of the high-temperature zone. This is accomplished by rare, explosive volcanic eruptions associated primarily with kimberlite rock, and less commonly, lamproite.
The kimberlite magma, rich in volatiles like carbon dioxide and water, ascends from depths of 150 kilometers or more at blistering speeds. Traveling at rates of one to ten meters per second, the journey to the surface takes less than two days. This ultra-fast ascent rate “freezes” the diamond structure, ensuring the crystal is carried through the graphite stability field before it can dissolve or transform. The resulting vertical geological structures, known as kimberlite pipes, are the primary sources of diamonds accessible to the surface.