Kimberlite is a volatile-rich, potassic ultramafic igneous rock, emplaced from very great depths within the Earth's mantle, and is by a wide margin the most economically important primary host of diamond. The rock takes its name from the town of Kimberley in South Africa's Northern Cape Province, where, between 1869 and 1872, the discovery and progressive deepening of the Big Hole established that diamond was not only an alluvial mineral but the product of an identifiable host rock. From a gemmological standpoint, kimberlite is significant because the diamond crystals it brings to the surface have grown in the lithospheric mantle at depths of 150 to 200 kilometres, and the kimberlite eruption is the violent transit that delivers them to the surface fast enough to preserve the diamond rather than allow it to revert to graphite.

Mineralogy and texture

Kimberlite is classified as an ultramafic rock dominated by olivine, with subsidiary phlogopite mica, monticellite, perovskite, spinel, calcite and serpentine. Important accessory minerals, used as indicator phases by exploration geologists, include pyrope garnet (often the chrome-rich purple-red variety), chromite, picroilmenite (magnesian ilmenite), and chrome-diopside. The rock typically shows a distinctive texture of large crystals (macrocrysts and megacrysts) of olivine and other mantle-derived minerals set in a finer-grained groundmass. Two principal facies are recognised: hypabyssal kimberlite, formed at depth where magma cooled slowly; and volcaniclastic kimberlite, including pyroclastic and resedimented varieties, which formed at and near the eruption surface.

The kimberlite pipe

A kimberlite pipe is the surface-and-near-surface form of the eruption. It consists of a deep, narrow root zone passing upward through a diatreme zone (typically several hundred metres in diameter and one to two kilometres deep) into a crater zone at the surface. The classic carrot-shaped, tapering form is conventional but not universal: shapes vary considerably with country-rock geology and eruption dynamics. The Big Hole at Kimberley, the Mir pipe in Yakutia, the Premier (Cullinan) pipe near Pretoria, and the Diavik and Ekati pipes in the Northwest Territories of Canada are among the most famous examples, each having yielded historically significant diamonds.

Diamond and the cratonic association

Diamond-bearing kimberlites are not randomly distributed across the continents. They occur preferentially in cratonic interiors, that is, on the most ancient and tectonically stable regions of the continental lithosphere, typically more than 2.5 billion years old. This is the so-called Clifford's Rule, formulated in the 1960s, and it remains a useful first-order exploration heuristic. The reason is that diamonds are stable only at the high pressures and relatively low temperatures found in thick cratonic mantle roots, where the lithosphere extends to 200 kilometres or more. Younger and thinner mantle does not provide the diamond stability field, and kimberlites that erupt through such crust may be barren of diamond or carry only graphitised xenocrysts.

Lamproite and other related hosts

Kimberlite is not the only primary host. Lamproite, a related but chemically distinct potassic ultramafic rock, is the host of the Argyle deposit in Western Australia, which from 1983 to 2020 was the world's largest single diamond mine by volume and the principal source of pink and red diamonds. Other rare hosts include certain ultramafic massive flows and minette intrusions, although these are economically minor compared with the kimberlite-and-lamproite pair.

Exploration significance

For modern diamond exploration, the indicator-mineral chemistry of kimberlite is the central tool. G10 chrome-rich pyrope garnets, picroilmenites of appropriate magnesian composition, and chromites of mantle origin can be recovered from streams and tills hundreds of kilometres from a parent pipe and used as a vector to a discovery. The discovery of the Canadian diamond fields in the 1990s, opening Ekati (1998) and Diavik (2003), proceeded by exactly this method: indicator-mineral trains in glacial till were tracked to their source pipes beneath the lakes of the Slave Craton.