The sparkle of a diamond hides an extraordinary journey from the depths of the Earth, a voyage that science is only beginning to understand. These precious stones, so prized for their beauty, owe their existence to rare and violent geological phenomena that tear them from the bowels of our planet and bring them to us.
Most natural diamonds come from particular volcanic formations called kimberlites. These carrot-shaped conduits plunge more than 150 kilometers (about 93 miles) below the Earth's surface, offering geologists a unique window into the deep layers of our planet. The magma that composes them rises at impressive speeds, reaching up to 130 kilometers per hour (about 80 miles per hour), carrying with it fragments of rock and minerals captured during its rapid ascent.
Unsplash illustration image Researchers from the University of Oslo recently published a study in
Geology that sheds light on the mechanisms behind these eruptions. Using computer models, the team analyzed how certain volatile substances alter the ability of magmas to rise to the surface. Their work makes it possible for the first time to precisely quantify the conditions necessary to trigger a kimberlite eruption, partially solving a geological mystery that has persisted for several decades.
The speed of the ascent is essential for preserving diamonds, as it prevents their transformation into graphite, a more stable form of carbon near the surface. However, the exact composition of the original magma and the reasons for its exceptional speed remained poorly understood. Scientists cannot directly observe these primitive magmas, forcing them to use indirect methods to reconstruct their properties.
The team focused on the Jericho kimberlite, located in northwestern Canada, to chemically model different original mixtures. Their approach involved simulating the evolution of a kimberlite magma at different depths, varying the proportions of carbon dioxide and water. These atomic simulations made it possible to determine how the density of the magma changed according to conditions, and whether it remained light enough to continue its ascent.
Diamond mine.
Credit: A. Anzulović
The results show that carbon dioxide plays a determining role in the success of the eruption. For the Jericho kimberlite, at least 8.2% of this volatile substance is needed to ensure the ascent. Without this critical amount, the magma would be too dense to pass through the Earth's crust, leaving the diamonds trapped in the depths. Water, on the other hand, maintains the fluidity of the magma, facilitating its upward movement.
This research demonstrates how small chemical variations can influence large-scale geological processes. The precise modeling of the behavior of magmas rich in volatile substances opens new perspectives for understanding the formation of diamond deposits and the deep dynamics of our planet.
The formation and preservation of natural diamonds
Diamonds form under extreme conditions, at depths where the pressure exceeds 45,000 times that of the atmosphere and where the temperature approaches 1,000 degrees Celsius (about 1,832 degrees Fahrenheit). These pure carbon crystals slowly crystallize over millions of years, trapped in rocks of the Earth's mantle. Their stability under these deep conditions contrasts with their fragility near the surface.
The transformation of diamonds into graphite represents the main danger during their ascent to the surface. This alteration occurs when the crystals remain exposed for too long to intermediate temperatures and pressures during their ascent. The atomic structure of the diamond, so hard and transparent, then reorganizes into graphite, which is much softer and opaque.
The exceptional speed of kimberlite eruptions is the key to preservation. Rising at more than 100 kilometers per hour (about 62 miles per hour), diamonds quickly pass through the dangerous zones where transformation could occur. This express journey brings them to the surface before their crystalline structure has time to change.
Kimberlites act as ultra-fast natural elevators, capturing diamonds in their mad rush to the surface. Without this accelerated transport mechanism, none of these precious stones would ever reach the Earth's crust where we can discover them.