Diamonds are pure carbon structures that form under immense pressure deep within the Earth’s mantle. Magma is the molten rock beneath the surface, often reaching temperatures well over 1000°C. Magma does not melt a diamond in the traditional sense of turning it into a liquid. Instead, the real danger to a diamond traveling through magma is a chemical process known as dissolution, which effectively unbuilds the crystal structure from the outside in.
The Nature of Diamonds and Magma
A diamond’s extraordinary stability comes from its internal atomic arrangement, a crystal structure known as the diamond cubic lattice. In this structure, each carbon atom is covalently bonded to four neighbors in a rigid, three-dimensional network, making it the hardest natural substance. Diamonds are formed and stabilized at depths of 150 kilometers or more, where the pressure is extreme.
Magma, the substance that transports diamonds to the surface, is a complex molten silicate material. Its temperature typically ranges between 700°C and 1300°C, though some ultramafic magmas can exceed 1500°C. Magma contains a significant percentage of volatile compounds, including water vapor, sulfur gases, and carbon dioxide, which play a direct role in diamond destruction.
The Role of Temperature and Pressure
The concept of a diamond melting is a question of its thermodynamic stability under various conditions. At the low pressures found near the Earth’s surface, a diamond does not possess a true melting point. Instead, if heated to an extremely high temperature, approximately 3550°C to 4000°C, it would skip the liquid phase entirely and sublime directly into a gas.
Magma’s temperature, even at its hottest, is far below the threshold required to physically sublime a diamond. The maximum temperatures of common magmas fall short by several thousand degrees Celsius. This temperature difference explains why a diamond, if simply heated in an inert environment, would survive the thermal exposure of any common magma.
The physical stability of a diamond is defined by pressure, as seen on the carbon phase diagram. Diamonds are only the stable form of carbon at the immense pressures of the deep mantle. If a diamond is exposed to high temperatures but low pressures for a prolonged period, it will revert to graphite, which is the thermodynamically stable form of carbon under surface conditions.
Dissolution: The True Threat
Magma destroys a diamond through a chemical attack called dissolution or resorption. The magma acts as a corrosive solvent that slowly breaks down the diamond’s carbon structure, driven by the volatile components dissolved within the molten rock.
Oxidizing agents like oxygen and carbon dioxide within the magma react with the pure carbon of the diamond. This chemical reaction converts the solid carbon atoms into gaseous carbon compounds, such as carbon monoxide or carbon dioxide, which then dissolve into the surrounding melt. The rate of this destructive process depends on the magma’s specific chemical composition.
Magmas rich in oxidizing volatiles are the most destructive to diamonds. If a diamond remains in contact with this corrosive melt for too long, its sharp crystalline edges become rounded or etched, showing clear signs of resorption. This chemical dissolution explains why diamonds recovered from volcanic pipes often exhibit distinctive surface features.
Diamond Survival and Transport
Diamonds still make it to the surface because they are transported by a unique, high-speed delivery system. Diamond-bearing magmas, known as kimberlites and lamproites, originate deep in the mantle and erupt through narrow, vertical conduits called volcanic pipes. This specific type of magma is rich in the volatile compounds that both destroy and ultimately propel the diamonds.
The ascent of this specialized magma must be extremely rapid to minimize the destructive contact time between the diamond and the corrosive melt. Estimates suggest the magma travels from depths of 150 to 300 kilometers to the surface in a geological instant, possibly taking only hours or a few days. This speed prevents the diamond from converting to graphite or being completely dissolved by the hot magma.
The speed is achieved because volatile compounds, particularly carbon dioxide, exsolve from the melt as the pressure drops during the ascent. This process creates a foaming action that supercharges the magma, propelling it upward in an explosive eruption. This rapid, volatile-driven transport is why diamonds survive their journey through the hostile environment of the magma.