Kimberlite

Kimberlite is an igneous rock that major source of diamonds. Kimberlite is a variety of peridotite.  It is rich in mica minerals content and often in form of crystals of phlogopite. Other containt abundant minerals are chrome-diopside, olivine, and chromium- and pyrope-rich garnet.  Kimberlite is typically found in pipes – structures with vertical edges that are roughly circular in cross-section. The rock may have been injected into the areas of weakness in the mantle. Parts of the mantle rocks are often brought to the surface in kimberlites, making them a valuable source of information about the inner world.

Despite its relative rarity, kimberlite has attracted attention because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth’s surface. 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 information about 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.

Name origin: The rock kimberlite was named after Kimberley, South Africa, where it was first recognized. Kimberley diamonds were originally found in weathered kimberlite, which was colored yellow by limonite, and was therefore called yellow ground. Deeper workings produced less altered rock, serpentinized kimberlite, which miners call blue ground.

Kimberlite Classification

Based on studies on a large number of kimberlite deposits, geologists divided the kimberlites into 3 separate units based on their morphology and petrology.

These units are:

1. Crater Facies Kimberlite
2. Diatreme Facies Kimberlite
3. Hypabyssal Facies Kimberlite

1) Crater Facies Kimberlite

The surface morphology of an unweathered kimberlite is characterised by a crater, up to 2 kilometers in diameter, whose floor may be several hundred meters below ground level. The crater is generally deepest in the middle. Around the crater is a tuff ring which is relatively small, generally less than 30 meters, when compared to the diameter of the crater. Two main categories of rocks are found in crater facies kimberlite: pyroclastic, those deposited by eruptive forces; and epiclastic, which are rocks reworked by water.

2) Diatreme Facies Kimberlite

Kimberlite diatremes are 1-2 kilometer deep, generally carrot-shaped bodies which are circular to elliptical at surface and taper with depth. The dip contact with the host rocks is usually 80-85 degrees. The zone is characterized by fragmented volcanoclastic kimberlitic material and xenoliths plucked from various levels in the Earth’s crust during the kimberlites journey to surface. Some Textural features of Diatreme Facies Kimberlite:

3) Hypabyssal Facies Kimberlite

These rocks are formed by the crystallization of hot, volatile-rich kimberlite magma. Generally, they lack fragmentation features and appear igneous. Some Textural features: Calcite-serpentine segregations in matrix; Globular segregations of kimberlite in a carbonate-rich matrix; Rock fragments have been metamorphosed or exhibit concentric zoning; Inequigranular texture creates a pseudoporphyritic texture.

Carbon and Kimberlite

Carbon is one of the most common elements in the world and is one of the four essentials for the existence of life. Humans are more than 18 percent carbon. The air we breathe contains traces of carbon. When occurring in nature, carbon exists in three basic forms:

Diamond – an extremely hard, clear crystal

Diamonds form about 100 miles (161 km) below the Earth’s surface, in the molten rock of the Earth’s mantle, which provides the right amounts of pressure and heat to transform carbon into diamond. In order for a diamond to be created, carbon must be placed under at least 435,113 pounds per square inch (psi or 30 kilobars) of pressure at a temperature of at least 752 degrees Fahrenheit (400 Celsius). If conditions drop below either of these two points, graphite will be created. At depths of 93 miles (150 km) or more, pressure builds to about 725,189 psi (50 kilobars) and heat can exceed 2,192 F (1,200 C). Most diamonds that we see today were formed millions (if not billions) of years ago. Powerful magma eruptions brought the diamonds to the surface, creating kimberlite pipes.

Kimberlite pipes are created as magma flows through deep fractures in the Earth. The magma inside the kimberlite pipes acts like an elevator, pushing the diamonds and other rocks and minerals through the mantle and crust in just a few hours. These eruptions were short, but many times more powerful than volcanic eruptions that happen today. The magma in these eruptions originated at depths three times deeper than the magma source for volcanoes like Mount St. Helens, according to the American Museum of Natural History.

The magma eventually cooled inside these kimberlite pipes, leaving behind conical veins of kimberlite rock that contain diamonds. Kimberlite is a bluish rock that diamond miners look for when seeking out new diamond deposits. The surface area of diamond-bearing kimberlite pipes ranges from 2 to 146 hectares (5 to 361 acres).

Diamonds may also be found in river beds, which are called alluvial diamond sites. These are diamonds that originate in kimberlite pipes, but get moved by geological activity. Glaciers and water can also move diamonds thousands of miles from their original location. Today, most diamonds are found in Australia, Borneo, Brazil, Russia and several African countries, including South Africa and Zaire.

Kimberlite Emplacement Models

Mitchell (1986) consider several theories and presents a more comprehensive critique of each emplacement theory.

1. Explosive volcanism theory
2. Magmatic (fluidization) theory
3. Hydrovolcanic theory

1. Explosive Volcanism Theory

This theory involves the pooling of kimberlite magma at shallow depths and the subsequent build-ıp of volatiles. When the pressure within this pocket, termed an intermediate chamber, is sufficient to overcome the load of rocks above, an eruption follows. The epicenter of the eruption was believed to be at the diatreme facies contact.

Through extensive mining it is clear that this theory is untenable. No intermediate chamber has been found at depth.

2. Magmatic Theory

This original proponent of this theory was Dowson (1971). It was subsequntly built upon by Clement (1982) and is pushed by Field and Scott Smith (1999)

Kimberlite magma rises from depth with different pulses building termed as “embryonic pipes”. The surface is not breached and the volatiles do not escape  At some point the embryonic pipes reach a shallow enough depth. Whereby the pressure of the volatiles is able to overcome the load of the overlying rocks. As the volatiles are escaping, a brief period of fluidization ensures. Fluidization is believed to be short lived as fragments are commonly angular.

3. Hydrovolcanic Theory

The main proponent of this theory is Lorenz (1999). Kimberlites magmas rise from depth thorough narrow 1m thick fissures. The kimberlite magma is focused along structural faults which act as focuses for waters or resultant brecciation due to volatile exsolution from the rising kimberlites may act as a focus for water. The brecciated rock becomes recharged with groundwater. Another pulse of kimberlite magma follows the some structural weakness in the rock to surface and again comes in contact with water producing another explosion.

Kimberlite 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;[14]

moderate to high large-ion lithophile element (LILE)[15] enrichment, ΣLILE = >1,000 ppm;

high H2O and CO2.

Kimberlite Composition

Both the location and origin of kimberlitic magmas are subjects of contention. Their extreme enrichment and geochemistry have 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 classified in two different varieties called basaltic” and “micaceous” based on petrographic observations. This was later revised by CB Smith, which renamed “group I” and “group II” of these groups based on the isotopic affinities of these rocks using Nd, Sr and Pb systems. Roger Mitchell later suggested the display of these group I and II kimberlites. These obvious differences may not be as closely related as they once thought. II. The group showed that the kimberlites showed more tendency towards the lampolines than the group I. Therefore, group II reclassified the kimberlites as orange to prevent confusion.

Group I kimberlites

Group-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by primary forsteritic olivine and carbonate minerals, with a trace-mineral assemblage of magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic (0.5–10 mm or 0.020–0.394 in) to megacrystic (10–200 mm or 0.39–7.87 in) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite, and phlogopite, in a fine- to medium-grained groundmass.

Olivine lamproites

Olivine lamproites were previously called group II kimberlite or orangeite in response to the mistaken belief that they only occurred in South Africa. Their occurrence and petrology, however, are identical globally and should not be erroneously referred to as kimberlite.Olivine lamproites are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of olivine lamproites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to “tetraferriphlogopite” (anomalously Al-poor phlogopite requiring Fe to enter the tetrahedral site). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.

Kimberlitic indicator minerals

Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with chemical compositions that indicate they formed under high pressure and temperature within the mantle. These minerals, such as chromium diopside (a pyroxene), chromium spinels, magnesian ilmenite, and pyrope garnets rich in chromium, are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.

Economic importance of Kimberlite

Kimberlites are the most important source of diamonds in the world. About 6,400 kimberlite pipes have been discovered in the world, of those about 900 have been classified as diamondiferous, and of those just over 30 have been economic enough to diamond mine.

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”.

The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 19th century accidentally cut into the blue ground and found gem-quality diamonds in quantity. The economic importance of the time was such that, with a flood of diamonds being found, the miners undercut each other’s prices and eventually decreased the diamonds’ value down to cost in a short time.

Kimberlite Formation

The general consensus is that kimberlites are formed deep within the mantle, at depths between 150 and 450 kilometers, from anomalously enriched exotic mantle compositions. They are erupted rapidly and violently, often with the release of considerable amounts of carbon dioxide (CO2) and volatile components. The violent explosions produce vertical columns of rock—volcanic pipes or kimberlite pipes—that rise from the magma reservoirs. The depth of melting and the process of generation makes kimberlites prone to hosting diamond xenocrysts.

The morphology of kimberlite pipes is varied, but it generally includes a sheeted dike complex of vertically dipping feeder dikes in the root of the pipe, extending down to the mantle. Within 1.5-2 kilometers (km) of the surface, as the magma explodes upward, it expands to form a conical to cylindrical zone called the diatreme, which erupts to the surface.

The surface expression is rarely preserved, but it 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.

Many kimberlite pipes are believed to have formed about 70 to 150 million years ago, but in Southern Africa, there are several that formed between 60 to 1,600 million years ago (Mitchell, 1995, p. 16).

Conclusion

• Kimberlite magmas are rich in carbondioxide and water which brings the magma quickly and violently to the mantle.
• Kimberlite is a gas rich potassic ultramafic igneous rock.
• Auistralia is currently the world’s largest producer of diamonds are low quality and used for industrial purposes.
• The crater facies kimnerlite is recognized by sedimentary features.
• The diatreme facies are recognized by pelletal lapilli.
• The hypabyssal facşes şs commonly recognized by segregationary texture and the presence of abundant cancite.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Kurszlaukis, S., & Fulop, A. (2013). Factors controlling the internal facies architecture of maar-diatreme volcanoes. Bulletin of Volcanology, 75(11), 761.
• Wikipedia contributors. (2019, February 14). Kimberlite. In Wikipedia, The Free Encyclopedia. Retrieved 16:10, May 11, 2019, from https://en.wikipedia.org/w/index.php?title=Kimberlite&oldid=883239063