Kimberlite is a rare and scientifically significant rock type, named for the South African town of Kimberley where it was first identified in diamond-bearing formations. Forming at depths far greater than most other volcanic rocks, kimberlite provides a window into the planet’s deep interior. Its connection to the Earth’s mantle and its economic importance in the diamond industry make it a major focus of geological study.
Defining Kimberlite: A Volcanic Classification
Kimberlite is formally classified by geologists as a volatile-rich, potassic, ultramafic igneous rock. The term “igneous” indicates that the rock originated from the cooling and solidification of magma. It is an eruptive rock that ascends from the mantle and forms structures in the Earth’s crust, primarily as vertical conduits.
The “ultramafic” designation describes its chemical composition, specifically its low silica content and very high concentration of magnesium and iron oxides. Magnesium oxide (MgO) levels in kimberlite typically exceed 12%, which is characteristic of rocks derived directly from the deep mantle. This composition places it among the most primitive and least chemically evolved magmas to reach the surface.
The “potassic” or “ultrapotassic” label means kimberlite is chemically enriched with potassium relative to other elements like sodium. This high potassium content suggests a unique source region deep within the mantle. The presence of volatiles, primarily carbon dioxide and water, is a defining feature that drives its explosive eruptive style.
The Unique Mineral Composition
The mineral makeup of kimberlite is distinct from most other volcanic rocks, providing direct evidence of its deep-earth origin. The bulk of the rock is a fine-grained groundmass, or matrix, that is often dominated by secondary minerals like serpentine and carbonate. This matrix material crystallized quickly upon arrival in the crust or was altered soon after emplacement.
Embedded within this matrix are larger crystals and rock fragments. These include macrocrysts of forsteritic olivine, a magnesium-rich variety of the mineral. The presence of mantle xenoliths—fragments of the surrounding mantle rock picked up during ascent—further confirms the extreme depth of its source.
Of particular importance are the kimberlitic indicator minerals, which are remnants of the deep mantle that resist weathering. These minerals, such as pyrope garnet, chrome diopside, and magnesian ilmenite, are used by prospectors to locate kimberlite deposits. For example, the chromium content gives pyrope garnet a distinct purple color, while chrome diopside appears emerald green.
Formation from the Deep Mantle
Kimberlite magma is generated at extreme depths in the Earth’s mantle, likely between 150 and 450 kilometers below the surface. This is significantly deeper than the source regions for most other volcanic rocks, which typically form in the upper mantle or crust. The formation requires a low degree of partial melting of an unusually enriched mantle source, often beneath the ancient, stable continental cores known as cratons.
The ascent of this magma to the surface is a rapid and explosive process driven by its rich volatile content, primarily carbon dioxide and water. As the magma rises and the pressure decreases, these dissolved gases rapidly exsolve, causing a powerful expansion. This process is thought to be the most rapid and violent type of volcanic eruption on Earth.
This explosive ascent forms characteristic vertical, carrot-shaped structures known as kimberlite pipes or diatremes. The rapid travel time is crucial for the preservation of the deep-earth materials it carries. The eruption fractures the surrounding crustal rock, creating a wide, conical structure near the surface that tapers into a narrower feeder dike at depth.
Kimberlite’s Role in Diamond Delivery
Kimberlite is recognized globally as the primary source rock for most of the world’s commercial diamonds. It serves as the transportation system for diamonds, which form much deeper than the kimberlite melt itself. Diamonds typically crystallize from carbon at depths greater than 150 kilometers, where high-pressure and high-temperature conditions exist.
The kimberlite magma originates nearby and acts as an elevator, picking up already-formed diamonds as it passes through the diamond stability field in the mantle. The extremely rapid ascent is the factor that ensures the diamonds survive the journey to the surface. If the transport were slow, the diamonds would convert back into graphite, which is the stable form of carbon at the lower pressures closer to the surface.
The resulting kimberlite pipes are the targets of diamond exploration. Only a small fraction of discovered kimberlite pipes actually contain economically viable quantities of diamonds. However, the presence of specific indicator minerals and the geological structure of the pipe link this rare volcanic rock to the world’s most desired gemstone.