Kimberlite: the volatile, diamond-bearing igneous rock

Composition and physical characteristics

Kimberlites are typically rich in olivine and other mantle-derived minerals and commonly contain variable amounts of carbonate and hydrous minerals. They are notable for having high contents of volatiles such as carbon dioxide and other volatile components, which drive explosive eruptions. Because of their composition they are classed among ultramafic, potassic magmas and are often altered near the surface to produce distinctive weathered materials historically called "blue ground" and "red ground" by miners.

Shapes, plumbing and eruption style

Kimberlite magma ascends rapidly from great depth and commonly forms vertical, pipe-like structures known as diatremes or "kimberlite pipes"; it also appears as narrow dykes and sheet-like sills. The rapid, gas-charged eruptions that form these features can be highly explosive, producing brecciated material and a chaotic mix of juvenile magma and rock fragments.

Depth of origin and transport of mantle material

Kimberlites originate deep in the Earth, often at depths estimated between roughly 150 and 450 kilometres, in regions of the upper mantle. Because ascent can be very fast, kimberlitic eruptions are able to entrain and bring to the surface diamonds and pieces of the mantle such as garnet peridotite and other xenoliths. This capability makes kimberlites especially valuable to geologists studying the composition and structure of the deep lithosphere beneath ancient, stable continents (the cratonic parts of the crust) and their continental lithosphere roots above the convecting asthenospheric mantle.

Distribution, economic importance and exploration

Kimberlite occurrences are concentrated beneath old continental cratons; prominent diamond-producing regions include parts of southern Africa, central Africa, Siberia, Canada and Australia. Not all kimberlites contain diamonds, and economically valuable deposits are relatively rare. Exploration commonly relies on tracing resistant indicator minerals (for example certain garnets and chromium-bearing diopside) downstream from a source pipe and on geophysical surveys to detect pipe geometry.

History, classification and scientific value

Since their first recognition in the 19th century, kimberlites have been studied both for their economic value and for the mantle information they deliver. Geologists classify kimberlites by petrography and chemistry into variants that reflect different histories of melting and emplacement; terminology and detailed classification have evolved as analytical techniques improved. Beyond diamonds, kimberlites provide rare natural samples of deep Earth material and thus contribute to our understanding of mantle composition, crust–mantle interaction and the thermal and tectonic evolution of continental roots.

• Key functions: diamond transport, mantle sampling, indicator-mineral exploration.
• Typical forms: pipes (diatremes), dykes, sills.
• Scientific uses: study of cratonic lithosphere and deep melting processes.

For further reading on petrology, eruption mechanics and regional occurrences, consult specialized literature and geological surveys (petrology overview, diamond studies, historical accounts).