In geology, kimberlite is described as a magmatic or igneous rock with a high magnesium content (MgO greater than 25% by weight), therefore it is considered a variety of rock belonging to the peridotite group. Thus, kimberlites are ultramafic intrusive igneous rocks.
The first kimberlites discovered were described in 1837 by Vanuxen from Ludlowiville near Ithaca, in New York state. However, you should know that the term Kimberlite was introduced by Lewis in 1887 and referred to a variety of peridotites with a porphyritic texture rich in mica minerals and that could contain diamonds, all in the Kimberley area, South Africa.
This rock is very important because it is considered the main source of diamonds in the world. In addition, due to the fact that its components originate even in areas of the mantle, they represent a valuable source of information about the layers of the earth and especially the mantle.
Properties and characteristics of kimberlite
Rock chemistry
Chemically, a kimberlite is an ultrabasic rock, therefore it is rich in magnesium, in fact MgO represents more than 25% of the weight of the rock. In addition, its composition in silica (SiO2) is low, being less than 43% in total weight of the rock.
It is also important to mention that kimberlites are rich in volatiles (water, carbon dioxide (CO2), and fluorine), they also contain anomalies of K, Na, Ba, Sr, and rare earth elements such as Ti, Zr, Nb, and P.
Therefore a kimberlite is a hybrid rock between potassic and ultrabasic rocks rich in volatiles, mainly CO2, which presents an uneven texture, resulting from the presence of mega crystals in a fine-grained texture. Some of these mega crystals may be diamonds.
Main minerals and accessories
Kimberlite rock is rich in mica minerals, typically phlogopite. In addition, other main minerals are diopside rich in chrome, olivine, chrome garnets and pyrope. Regarding accessory minerals, rutile, perovskite, ilmenite, magnetite, calcite, serpentine, pyroxene and Diamond.
Kimberlite becomes especially interesting when it contains diamonds as accessory minerals.
Kimberlite texture
Kimberlite is a rock that has a mixed texture. Samples have been found that are typically coarse-grained, that is, they are coarse-grained phaneritic. However, because these rocks are ejected from the mantle by structures called diatremes, they can undergo sudden cooling, which means that their mineral components may not crystallize at the same time as the others. This causes the kimberlites to also have a porphyritic texture, equally coarse-grained.
It is also worth mentioning that some of its mineral components such as olivine or diamond can appear as mega crystals on a finer crystal texture.
Origin and formation of kimberlites
Geologists and geoscientists have come to the agreement that kimberlites are formed by the partial melting and crystallization of magmatic materials that come from the upper mantle and in fact it is said that they are the rocks that are formed at greater depths.
These rocks are said to originate at depths between 150 and 450 kilometers, an ideal geological environment for diamond formation.
The original magma explodes rapidly and violently towards the surface through weak zones of the continental crust, known as diatremes or deep volcanic conduits, often these explosions include large amounts of (CO2) and other volatile components. These deep explosions expel the kimberlites towards the surface.
Diatremes are characterized by having an inverted conical shape, and at surface their morphological expression is rarely preserved, but they have been found to be similar to a volcanic maar.
Many kimberlite pipes are thought to have formed between 70 and 150 million years ago, but in southern Africa, there are several that formed between 60 and 1.6 billion years ago (Mitchell, 1995, p. 16).
Despite this, the origin and formation of kimberlites remains an enigma, perhaps in the future it will be known exactly how these rocks are formed and where they come from.
Importance of kimberlites
Currently it is known that a kimberlite is capable of hosting diamonds as accessory minerals in its mineral composition. For this reason, kimberlites are considered the main source or deposit of natural diamonds in the world.
Some 6,400 kimberlite pipes or diatremes are known to have been discovered in the world, yet only 900 of them have been classified as diamondiferous kimberlites and only 30 of them have passed economic evaluation and have been shown to be profitable.
Deposits and distribution of kimberlites
These rocks can be found on all the continents of the world, however, geological studies have verified that economically profitable kimberlites to extract diamonds occur in patterns associated with Precambrian cratons, and of Archean age (about 2.5 G years), after this pattern was called Cliffords rule. Therefore, there are currently no known diamond deposits located in the crust with an age of less than 1.6 G years.
This peculiar association suggests a link between the presence of diamonds and the age of the subcontinental lithosphere, and Clifford’s rule has long been considered a valuable selection criteria in diamond exploration programs.
It is worth noting here that diamondiferous kimberlites are usually young compared to the age of the lithosphere into which they have been introduced. Many (including most of the South African examples) are from the Cretaceous.
Kimberlite uses
- The main economic use of kimberlite is associated with diamonds. This rock is considered as the main source of natural diamonds in the world.
- Another use of this rock is associated with the study of the composition of the earth’s mantle. Kimberlites are known to come from the melting of the rocks of the upper mantle. For this reason, earth scientists study these rocks in order to understand the geological environment of the upper mantle of the earth.
- Another use is related to collecting. Many rock collectors are willing to pay large sums of money for exotic kimberlite samples, even better if they have diamonds in their composition.