A kimberlite pipe is a rare, vertical, carrot-shaped geological structure extending from the Earth’s surface deep into the mantle. Named after Kimberley, South Africa, where they were first studied, these formations are the most important source of naturally mined diamonds globally. Kimberlites are a specific type of igneous rock. The pipes represent the ancient, solidified conduits through which this rock erupted from depths greater than almost any other known magma.
The Unique Structure and Composition of Kimberlite Pipes
The overall structure of a kimberlite pipe is described in three distinct zones that taper downward. The surface zone, if preserved, is the crater zone—a broad, shallow basin formed by the explosive eruption, sometimes resembling a maar volcano. This crater transitions into the deeper diatreme zone, the classic conical or cylindrical “carrot” shape extending downward for up to two kilometers.
The diatreme is filled with fragmented, volatile-rich rock called kimberlite, a hybrid mixture of solidified melt and broken rock fragments. Below the diatreme lies the narrow root zone, which consists of hypabyssal kimberlite. This zone is composed of dikes and sills that represent the deepest, non-explosive feeder channels of the system. Kimberlite rock is ultrabasic, meaning it is low in silica but highly magnesian, containing over 25% magnesium oxide by weight. It is rich in volatile components, primarily carbon dioxide and water, and often contains large crystals of mantle-derived minerals like olivine and phlogopite.
Deep Earth Origins: How Kimberlite Pipes Form
Kimberlite magma originates extremely deep within the Earth’s mantle, at depths ranging from 150 to 450 kilometers. This places its origin within the stable sub-continental lithospheric mantle beneath ancient continental shields, far below typical volcanic source regions. The magma forms through small degrees of partial melting of the mantle rock, resulting in a composition highly enriched in magnesium and volatile compounds.
The presence of substantial carbon dioxide and water drives the unique and violent eruption style of kimberlites. As the melt rises through fractures in the overlying continental crust, decreasing pressure causes these volatiles to rapidly exsolve, or bubble out, leading to massive expansion. This process creates an ultra-fast, explosive ascent that bypasses the slow magma movements seen in typical plate-margin volcanism.
The ascent speed of the kimberlite magma is estimated to be extremely rapid, potentially reaching up to 400 meters per second in the final stages. This speed is necessary because it essentially flash-freezes the deep-earth material. Explosive eruptions near the surface, often triggered by interaction with groundwater, fracture the surrounding country rock, incorporating fragments into the magma and forming the brecciated fill of the diatreme zone.
The Diamond Connection
The profound depth of the kimberlite magma’s origin is directly linked to its ability to carry diamonds. Diamonds are not formed by the magma itself, but crystallize in the surrounding mantle rock under specific conditions. They form only in the diamond stability field, where immense pressure and high temperature—found at depths greater than 150 kilometers—prevent carbon from turning into graphite.
Kimberlite pipes act as rapid, high-speed elevators, plucking these pre-existing diamonds from the mantle rock as they ascend. The astonishing speed of the eruption is the mechanism that preserves the diamonds, preventing them from being exposed to lower pressures near the surface long enough to chemically revert to graphite. While kimberlite pipes are the primary hosts for diamonds, the vast majority are not economically viable for mining. Geologists estimate that only about 1 in every 100 kimberlite bodies contains enough diamonds to justify the cost of extraction. The presence of diamonds depends on the specific part of the mantle the kimberlite tapped and the longevity of the eruption.
Identifying Kimberlites Using Indicator Minerals
Given the ancient age and heavily eroded nature of kimberlite pipes, they are rarely visible as surface volcanoes, making them difficult to locate directly. Exploration geologists search for specific “indicator minerals” that were ejected along with the kimberlite rock. These minerals are dense, chemically distinct, and more resistant to weathering than the surrounding matrix.
The most common kimberlite indicator minerals are:
- Picro-ilmenite (an iron-titanium oxide)
- Chrome-diopside (an emerald-green pyroxene)
- Pyrope garnet (a magnesium-rich garnet)
When a kimberlite pipe weathers away, these heavier minerals are released and dispersed into local stream sediments and glacial till. Geologists trace these mineral grains back up the drainage system to their source. The presence of chrome-diopside is particularly useful because it breaks down quickly in the surface environment, indicating a source pipe is very close by. The chemical composition of these indicator minerals, especially certain types of pyrope garnet, can also provide clues about whether the source kimberlite sampled a diamond-rich portion of the mantle.