Kimberlite

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region of Northern Ontario, Canada. Green olivine grains and purplish red garnet are visible. The sample is 13 cm (5 inches) long.

Kimberlite is a type of rock (geology) best known for sometimes containing diamonds. It is named after the town of Kimberley, Northern Cape in South Africa, where the finding of a large kimberlite pipe in the 1870s spawned a diamond rush.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 (geology) 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 asthenosphere mantle.Kimberlite occurs in the Earth's crust (geology) 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 volatiles components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.

Morphology and volcanology Kimberlites are found as dike (geology) and volcanic pipes which underlie and are the source for rare and relatively small explosive volcanoes (maars). Kimberlites in the Guyana Shield, in Venezuela and French Guyana, form thin, tabular dipping sill (geology)s.

Kimberlite pipes are the result of explosive Volcanic pipe volcanism from very deep Earth's 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.

Many kimberlite pipes are believed to have formed about 70 to 150 million years ago, but in Southern Africa, there are several formed between 60 to 1600 million years agoRoger Howard Mitchell - Kimberlites, Orangeites, and Related Rocks page 16.

Two Jurassic kimberlite Dike (geology) exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette County, Pennsylvania and Greene County, Pennsylvania Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, Pennsylvania County, does not outcrop at the surface and was discovered by miners.Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers, (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.

Petrographic Characters Kimberlites are a clan (geology) of volatile-rich (dominantly carbon dioxide) potassic ultramafic rocks. Commonly, they exhibit a distinctive inequigranular texture resulting from the presence of rounded, anhedral and fragmented macrocrysts (0.5-10 mm) and in some instances megacrysts (10-200 mm) set in a fine grained matrix (geology). The megacryst/macrocryst assemblage consists of rounded anhedral crystals of magnesium ilmenite, chromium-poor titanian pyrope, olivine, Cr-poor clinopyroxene, phlogopite, enstatite and titanium-poor chromite. Olivine is the dominant member of the macrocryst assemblage. The matrix minerals may include: second generation euhedral primary olivine and/or phlogopite, together with perovskite, spinel (Titanium magnesian aluminous chromite, titanian chromite, members of the magnesian ulvospinel-ulvospinel-magnetite series), diopside (Al- and Ti- poor), monticellite, apatite, calcite, and primary late-stage serpentine (commonly Fe rich). Some kimberlites contain late-stage poikilitic eastonite phlogopites. Nickeliferous sulphides and rutile are common accessory minerals. The replacement of early-formed olivine, phlogopite, monticellite, and apatite by deuteric serpentine and calcite is common. Evolved members of the clan may be devoid of, or poor in, macrocrysts, and composed essentially of calcite, serpentine, and magnetite together with minor phlogopite, apatite and perovskite.

Petrogenesis Since the discovery of diamonds in kimberlite many different theories regarding the processes involved in kimberlite formation have been put forward Here we will examine 2 theories: 1) the magmatic theory and 2) the hydrovolcanic theory.1) Magmatic (Fluidization) Theory:The original proponent of this theory was Dawson (1962, 1971). It was subsequently built upon by Clement (1982) and is presently being pushed by Field and Scott Smith (1999).A brief outline of the magmatic/fluidization theory is as follows. Kimberlite magma rises from depth with different pulses building what are termed 'embryonic pipes' (Mitchell, 1986) on top of each other. The result is a complex network of overlapping embryonic pipes of hypabyssal facies kimberlite. The surface is not breached and the volatiles do not escape. At some point the embryonic pipes reach a shallow enough depth (~500 meters) whereby the pressure of the volatiles is able to overcome the load of the overlying rock and the volatiles escape. As the volatiles are escaping, a brief period of fluidization ensues. This involves the upward movement of volatiles which are sufficiently fast to 'fluidize' the kimberlite and fragmented host rock so that particles are entrained in a gas-solid-liquid medium. Fragments of country rock found in this fluidized system may sink depending on their density. The fluidized front moves downwards from the initial depth. Fluidization is believed to be short lived as fragments are commonly angular.

This theory is suppose explains features seen in kimberlite pipes such as:i) fragments of country rock (geology) found as much as 1 km below their lithostratigraphy level through fluidization.ii) steep-sided pipes with angles ~80-85 degrees. As the initial explosion is at a relatively shallow depth (~500m) the surface radii to depth ratio will be closer to 1.iii) complex network of pipes of hypabyssal facies found at depth.iv) the transition from hypabyssal facies to diatreme facies.

2) Hydrovolcanic (Phreatic eruption) Theory:The main proponent of this theory is Lorenz (1999), who has pushed the hydrovolcanic model for 3 decades.Kimberlite magmas rise from depth through narrow ~1m thick fissures. Either the kimberlite magma is focused along structural faults which act as focuses for waters, or, the resultant brecciation due to volatile exsolution from the rising kimberlite may act as a focus for water. In any case, the near surface environment is rich in water and the interaction of the rising hot magma with the cool water produces a pheatomagmatic explosion. The explosion is short lived. The brecciated rock becomes recharged with groundwater. Another pulse of kimberlite magma follows the same structural weaknesses in the rock to surface and again comes in contact with water producing another explosion. Subsequent pulses react with water in the same way while the contact front moves downwards to the average depth of hypabyssal facies/diatreme facies transitions.

Chemical Characterstics Kimberlites and orangeites are complex hybrids in which anunderminable quantity of foreign and cumulate material has beenintegrated, disaggregated, and variably absorbed into the liquid. Itis thus difficult, if not impossible, to determine what constituentsare components of the original melt, and what has beenincorporated en route to the surface. Thus the nature of theprimitive liquids is largely unknown. This hybrid nature presentsan obvious problem when it comes to interpreting thegeochemistry.Differentiation process lead to the concentration of macrocryst +phenocryst phases and evolved liquids, eventually resulting in theevolution of carbonate- rich residua. Major element concentrationsthus vary widely as a result of both contamination, accumulationand fractionation processes.Variation in phenocryst composition suggests that Kimberlites arecommonly mixtures that result from the coalescence of smallermagma batches as they rise.

Major OxidesComposition of Kimberlite includes following oxides:-(a) SiO2(b) TiO2(c) Al2O3(d) FeO(e) MnO(f) MgO(g) CaO(h) Na2O(i) K2O(j) P2O5Late stages of intrusion commonly take the form of crystal-liquid slurry in which the relative proportions of the constituent minerals might easily vary. Therefore there is wide variation in SiO2 ,CaO ,MgO, CO2 and H2O. There is a low concentration of Al2O3 and Na2O

Trace ElementsTrace elements associated with Kimberlites are :-Sc, V, Cr, Ni, Co, Cu, Zn, Ba, Sr, Zr, Hf, Nb, Ta, Th, U, La and YbDue to the mantle source of Kimberlite high levels of compatible trace elements(Ni, Cr, Sc, V, Co, Cu, Zn) are present.

Averege Analysis and Compositional Ranges of Kimberlites And Orangeites.

Kimberlite Orangeite SiO2 33.0 27.8-37.5 35.0 27.6-41.9TiO2 1.3 0.4-2.8 1.1 0.4-2.5Al2O3 2.0 1.0-5.1 2.9 0.9-6.0FeO* 7.6 5.9-12.2 7.1 4.6-9.3MnO 0.14 0.1-0.17 0.19 0.1-0.6MgO 34.0 17.0-38.6 27. 10.4-39.8CaO 6.7 2.1-21.3 7.5 2.9-24.5Na2O 0.12 0.03-0.48 0.17 0.01-0.7K2O 0.8 0.4-2.1 3.0 0.5-6.7P2O5 1.3 0.5-1.9 1.0 0.1-3.3LOI 10.9 7.4-13.9 11.7 5.2-21.5Sc 14 20V 100 95Cr 893 1722Ni 965 1227Co 65 77Cu 93 28Zn 69 65Ba 885 3164Sr 847 1263Zr 263 268Hf 5 7Nb 171 120Ta 12 9Th 20 28U 4 5La 150 186Yb 1 1

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 alluvium. Their presence, when found, may be indicative of the presence of a kimberlite within the erosional watershed which has produced the alluvium.

Main minerals of Kimberlite1) Macrocrysts of olivine,2) Picroilmenite,3) Cr-diopside,4) Pyrope garnet;5) Phenocrysts of olivine and6) Microphenocrysts of monticellite,perovskite, kinoshitalite mica7) Spinel in a calcite + serpentine matrix.Common accessory minerals1) Nickeliferous sulphides2) Rutile

The geochemistry of Kimberlites is defined by the following parameters;a) Ultramafic; MgO >12% and generally >15%b) Ultrapotassic; Molar K2O/Al2O3 >3c) Near-primitive Ni (>400ppm), Cr (>1000ppm), Co (>150ppm)d) REE-enrichmente) Moderate to high LILE enrichment; ΣLILE = >1,000ppmf) 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, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathering kimberlite which was colored yellow by limonite, and so was called yellow ground. Deeper workings encountered less altered rock, serpentinite kimberlite, which miners call blue ground.

Related rock types

Lamproite
Lamprophyre
Nepheline syenite
Ultrapotassic igneous rocks
Kalsititic rocks

References

Kimberlite
Kimberlite hosted diamonds

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region of Northern Ontario, Canada. Green olivine grains and purplish red garnet are visible. The sample is 13 cm (5 inches) long.

Kimberlite is a type of rock (geology) best known for sometimes containing diamonds. It is named after the town of Kimberley, Northern Cape in South Africa, where the finding of a large kimberlite pipe in the 1870s spawned a diamond rush.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 (geology) 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 asthenosphere mantle.Kimberlite occurs in the Earth's crust (geology) 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 volatiles components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.

Morphology and volcanology Kimberlites are found as dike (geology) and volcanic pipes which underlie and are the source for rare and relatively small explosive volcanoes (maars). Kimberlites in the Guyana Shield, in Venezuela and French Guyana, form thin, tabular dipping sill (geology)s.

Kimberlite pipes are the result of explosive Volcanic pipe volcanism from very deep Earth's 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.

Many kimberlite pipes are believed to have formed about 70 to 150 million years ago, but in Southern Africa, there are several formed between 60 to 1600 million years agoRoger Howard Mitchell - Kimberlites, Orangeites, and Related Rocks page 16.

Two Jurassic kimberlite Dike (geology) exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette County, Pennsylvania and Greene County, Pennsylvania Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, Pennsylvania County, does not outcrop at the surface and was discovered by miners.Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers, (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.

Petrographic Characters Kimberlites are a clan (geology) of volatile-rich (dominantly carbon dioxide) potassic ultramafic rocks. Commonly, they exhibit a distinctive inequigranular texture resulting from the presence of rounded, anhedral and fragmented macrocrysts (0.5-10 mm) and in some instances megacrysts (10-200 mm) set in a fine grained matrix (geology). The megacryst/macrocryst assemblage consists of rounded anhedral crystals of magnesium ilmenite, chromium-poor titanian pyrope, olivine, Cr-poor clinopyroxene, phlogopite, enstatite and titanium-poor chromite. Olivine is the dominant member of the macrocryst assemblage. The matrix minerals may include: second generation euhedral primary olivine and/or phlogopite, together with perovskite, spinel (Titanium magnesian aluminous chromite, titanian chromite, members of the magnesian ulvospinel-ulvospinel-magnetite series), diopside (Al- and Ti- poor), monticellite, apatite, calcite, and primary late-stage serpentine (commonly Fe rich). Some kimberlites contain late-stage poikilitic eastonite phlogopites. Nickeliferous sulphides and rutile are common accessory minerals. The replacement of early-formed olivine, phlogopite, monticellite, and apatite by deuteric serpentine and calcite is common. Evolved members of the clan may be devoid of, or poor in, macrocrysts, and composed essentially of calcite, serpentine, and magnetite together with minor phlogopite, apatite and perovskite.

Petrogenesis Since the discovery of diamonds in kimberlite many different theories regarding the processes involved in kimberlite formation have been put forward Here we will examine 2 theories: 1) the magmatic theory and 2) the hydrovolcanic theory.1) Magmatic (Fluidization) Theory:The original proponent of this theory was Dawson (1962, 1971). It was subsequently built upon by Clement (1982) and is presently being pushed by Field and Scott Smith (1999).A brief outline of the magmatic/fluidization theory is as follows. Kimberlite magma rises from depth with different pulses building what are termed 'embryonic pipes' (Mitchell, 1986) on top of each other. The result is a complex network of overlapping embryonic pipes of hypabyssal facies kimberlite. The surface is not breached and the volatiles do not escape. At some point the embryonic pipes reach a shallow enough depth (~500 meters) whereby the pressure of the volatiles is able to overcome the load of the overlying rock and the volatiles escape. As the volatiles are escaping, a brief period of fluidization ensues. This involves the upward movement of volatiles which are sufficiently fast to 'fluidize' the kimberlite and fragmented host rock so that particles are entrained in a gas-solid-liquid medium. Fragments of country rock found in this fluidized system may sink depending on their density. The fluidized front moves downwards from the initial depth. Fluidization is believed to be short lived as fragments are commonly angular.

This theory is suppose explains features seen in kimberlite pipes such as:i) fragments of country rock (geology) found as much as 1 km below their lithostratigraphy level through fluidization.ii) steep-sided pipes with angles ~80-85 degrees. As the initial explosion is at a relatively shallow depth (~500m) the surface radii to depth ratio will be closer to 1.iii) complex network of pipes of hypabyssal facies found at depth.iv) the transition from hypabyssal facies to diatreme facies.

2) Hydrovolcanic (Phreatic eruption) Theory:The main proponent of this theory is Lorenz (1999), who has pushed the hydrovolcanic model for 3 decades.Kimberlite magmas rise from depth through narrow ~1m thick fissures. Either the kimberlite magma is focused along structural faults which act as focuses for waters, or, the resultant brecciation due to volatile exsolution from the rising kimberlite may act as a focus for water. In any case, the near surface environment is rich in water and the interaction of the rising hot magma with the cool water produces a pheatomagmatic explosion. The explosion is short lived. The brecciated rock becomes recharged with groundwater. Another pulse of kimberlite magma follows the same structural weaknesses in the rock to surface and again comes in contact with water producing another explosion. Subsequent pulses react with water in the same way while the contact front moves downwards to the average depth of hypabyssal facies/diatreme facies transitions.

Chemical Characterstics Kimberlites and orangeites are complex hybrids in which anunderminable quantity of foreign and cumulate material has beenintegrated, disaggregated, and variably absorbed into the liquid. Itis thus difficult, if not impossible, to determine what constituentsare components of the original melt, and what has beenincorporated en route to the surface. Thus the nature of theprimitive liquids is largely unknown. This hybrid nature presentsan obvious problem when it comes to interpreting thegeochemistry.Differentiation process lead to the concentration of macrocryst +phenocryst phases and evolved liquids, eventually resulting in theevolution of carbonate- rich residua. Major element concentrationsthus vary widely as a result of both contamination, accumulationand fractionation processes.Variation in phenocryst composition suggests that Kimberlites arecommonly mixtures that result from the coalescence of smallermagma batches as they rise.

Major OxidesComposition of Kimberlite includes following oxides:-(a) SiO2(b) TiO2(c) Al2O3(d) FeO(e) MnO(f) MgO(g) CaO(h) Na2O(i) K2O(j) P2O5Late stages of intrusion commonly take the form of crystal-liquid slurry in which the relative proportions of the constituent minerals might easily vary. Therefore there is wide variation in SiO2 ,CaO ,MgO, CO2 and H2O. There is a low concentration of Al2O3 and Na2O

Trace ElementsTrace elements associated with Kimberlites are :-Sc, V, Cr, Ni, Co, Cu, Zn, Ba, Sr, Zr, Hf, Nb, Ta, Th, U, La and YbDue to the mantle source of Kimberlite high levels of compatible trace elements(Ni, Cr, Sc, V, Co, Cu, Zn) are present.

Averege Analysis and Compositional Ranges of Kimberlites And Orangeites.

Lamproite
Lamprophyre
Nepheline syenite
Ultrapotassic igneous rocks
Kalsititic rocks

References

Kimberlite
Kimberlite hosted diamonds

Kimberlite - Wikipedia, the free encyclopedia
Kimberlite is a type of potassic volcanic 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 ...

kimberlite - Hutchinson encyclopedia article about kimberlite
kimberlite. Igneous rock that is ultramafic (containing very little silica); a type of alkaline peridotite with a porphyritic texture (larger crystals in a fine-grained matrix ...

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kimberlite definition of kimberlite in the Free Online Encyclopedia.
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kimberlite
Igneous rock that is ultramafic (containing very little silica); a type of alkaline peridotite with a porphyritic texture (larger crystals in a fine-grained matrix), containing ...

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