Kimberlite

Kimberlite is a type of rock best known for sometimes containing diamonds. It is named after the town of Kimberley 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 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. 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.

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Morphology and volcanology

Kimberlites are found as dikes 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 sills.

Kimberlite pipes are 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.

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 ago^[1].

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.^[1]

Petrographic characters

Kimberlites are a clan 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. The megacryst/macrocryst assemblage consists of rounded anhedral crystals of magnesian 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 (titaniferous 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 found as much as 1 km below their stratigraphic 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 (Phreatomagmatic) 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 characteristics

Kimberlites and orangeites are complex hybrids in which an underminable quantity of foreign and cumulate material has been integrated, disaggregated, and variably absorbed into the liquid. It is thus difficult, if not impossible, to determine what constituents are components of the original melt, and what has been incorporated en route to the surface. Thus the nature of the primitive liquids is largely unknown. This hybrid nature presents an obvious problem when it comes to interpreting the geochemistry. Differentiation process lead to the concentration of macrocryst + phenocryst phases and evolved liquids, eventually resulting in the evolution of carbonate- rich residua. Major element concentrations thus vary widely as a result of both contamination, accumulation and fractionation processes. Variation in phenocryst composition suggests that Kimberlites are commonly mixtures that result from the coalescence of smaller magma batches as they rise.

Major Oxides Composition of Kimberlite includes following oxides:- (a) SiO2 (b) TiO2 (c) Al2O3 (d) FeO (e) MnO (f) MgO (g) CaO (h) Na2O (i) K2O (j) P2O5 Late 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 Elements Trace elements associated with Kimberlites are:- Sc, V, Cr, Ni, Co, Cu, Zn, Ba, Sr, Zr, Hf, Nb, Ta, Th, U, La and Yb Due to the mantle source of Kimberlite high levels of compatible trace elements (Ni, Cr, Sc, V, Co, Cu, Zn) are present.

Average Analysis and Compositional Ranges of Kimberlites and Orangeites.

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

1. Macrocrysts of olivine,
2. Picroilmenite,
3. Cr-diopside,
4. Pyrope garnet;
5. Phenocrysts of olivine and
6. Microphenocrysts of monticellite, perovskite, kinoshitalite mica
7. Spinel in a calcite + serpentine matrix.

Common accessory minerals

1. Nickeliferous sulphides
2. Rutile

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.

Related rock types

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

References

• Kimberlite
• Kimberlite hosted diamonds