Kimberlite



Kimberlite is a type of igneous rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5 carat diamond in 1871 spawned a diamond rush, eventually creating the Big Hole.

Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today. The general consensus reached on kimberlites is that they are formed deep within the mantle, at between 150 and 450 kilometres depth, from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.

Kimberlite has in many ways attracted more attention than its relative volume might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Furthermore, its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide valuable information on the composition of the deep mantle, and melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.

Morphology and volcanology
Kimberlites occur as carrot shaped, vertical intrusions termed dykes or diatremes. This classic carrot shape is due to a large proportion of both CO2 and H2O in the system which produces a deep explosive boiling stage and produces a significant amount of vertical flaring (Bergman, 1987). Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement and Skinner 1985, and Clement, 1982).

The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within 1.5-2 km of the surface the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer.

Two Jurassic kimberlite dikes exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners.

Petrology
Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source.

Historically, kimberlites have been subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who re-named these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that Group II kimberlites actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion.

Group I kimberlites
Group I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromian pyrope, almandine-pyrope, chromian diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture cause by macrocrystic (0.5-10 mm) to megacrystic (10-200 mm) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite in a fine to medium grained groundmass.

The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.

Group II kimberlites
Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.

Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite); Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite); potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite.

Kimberlitic indicator minerals
Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (a pyroxene), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.

These indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence, when found, may be indicative of the presence of a kimberlite within the erosional watershed which has produced the alluvium.

Geochemistry
The geochemistry of Kimberlites is defined by the following parameters;
 * Ultramafic; MgO >12% and generally >15%
 * Ultrapotassic; Molar K2O/Al2O3 >3
 * Near-primitive Ni (>400 ppm), Cr (>1000 ppm), Co (>150 ppm)
 * REE-enrichment
 * Moderate to high LILE enrichment; ΣLILE = >1,000 ppm
 * High H2O and CO2

Economic importance
Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. However, only about 1 in 200 kimberlite pipes contain gem-quality diamonds. The deposits occurring at Kimberley, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathered kimberlite which was colored yellow by limonite, and so was called yellow ground. Deeper workings encountered less altered rock, serpentinized kimberlite, which miners call blue ground.

See also Udachnaya pipe.

Related rock types

 * Lamproite
 * Lamprophyre
 * Nepheline syenite
 * Ultrapotassic igneous rocks
 * Kalsititic rocks