Kimberlites

Kimberlite are highly alkaline (usually K-rich) ultramafic rock that 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 SiO2 content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important.

The first Kimberlites were described by Vanuxen in 1837 from Ludlowiville near Ithaca, New York state; however, the term Kimberlite was introduced by Lewis (1887) to describe the diamond-bearing, porphyritic mica peridotites of the Kimberley area of South Africa. As originally defined, the term described an idea, rather a fixed composition in a systematic classification of igneous rocks; and the idea was that Kimberlites are rare ultramafic rocks that contain diamond, together with variable amounts of xenocrysts and xenolith.

The petrography of Kimberlites is both unusual, and complex, because:

1) They are hybrid rocks the contain minerals, rock fragments and congealed magmatic materials that formed in diverse physical and chemical environments.
2) They vary greatly in modal compositions: Olivine, serpentine group minerals, phlogopite, garnet (usually pyrope), pyroxenes, carbonate minerals, monticellite, ilmenite, chromite and Perovskite are the minerals usually found in Kimberlites.

However the relative abundance of these minerals in different rocks varies greatly, and their bulk chemical composition also vary in response to the modal differences. There are thus no simple modal or chemical criteria for defining the term Kimberlite.

Mitchell (1970) defined Kimberlites as:

A porphyritic, alkali peridotite, containing rounded and corroded phenocrystals of olivine (serpentinized, carbonatized or fresch), phlogopite (fresh or chloritized), magnesian ilmenite, pyrope and Cr-pyrope set in a fine grained groundmass composed of second generation olivine and phlogopite together with calcite, serpentine, magnetite, apatite, Perovskite. Diamond and garnet peridotite xenoliths may or may not occur.

More recently Clement (1984) defined Kimberlite (sensu stricto) as:

A volatile-rich, potassic, ultrabasic, igneous rock which occur as small volcanic pipes, dykes and sill. It has distinctively inequigranular texrure resulting from the presence of macrocrysts set in a fine-grained matrix. This matrix contains, as prominent primary phenocrystal and/or groundmass constituent, olivine and several of the following minerals: phlogopite, carbonate, clinopyroxene, monticellite, apatite, spinel perovskite and ilmenite. The macrocryst are anhedral, mantle derived, ferromagnesian mineral which include olivine, phlogopite, ilmenite, pyroxenes, spinel, garnet. Olivine is extremely abundant relative to the other macrocrysts all of which are not necessarily present.

As said previously the modal composition of Kimberlites vary greatly. Olivine is usually the most abundant mineral, but it may be partly, or completely, replaced by secondary minerals. Some Kimberlitic rocks contain three generation of olivine: Large rounded olivine megacrysts (> 3-4mm) with composition in the range Fo84-86; Medium-sized olivine with composition that are often more magnesium-rich than Fo90 and small groundmass olivines with composition that are intermediate between the other two type. The abundance of phlogopite and carbonate minerals are also highly variable; for this reason Kimberlites can be divided into 3 type:

Kimberlites (sensu stricto); Micaceous Kimberlites; Calcareous Kimberlites

Based on studies of numerous kimberlite deposits, geologists have divided kimberlites (fig.1) into 3 distinct units based on their morphology and petrology. These units are:

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

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Fig.1: Kimberlite Pipe. Modified after Mitchell, 1986



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.

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Fig.2: Crater Facies Kimberlite. Modified after Mitchell 1986



A) Pyroclastic Rocks:
These rocks are found preserved in tuff rings around the crater and within the crater. Tuff rings have small height: Igwissi Hills in Tanzania and Kasami in Mali are the only pipes with well-preserved tuff rings. Heights range from 1-4 to 15-50 metres. Deposits are commonly bedded, vesicular and carbonatised.

Tuff deposits preserved within the crater are also rare, however, the Igwissi Hills pipes (Fig.3) in Tanzania have been examined and revealed three distinct units. From top to bottom, they are: 1. well-stratified tuffs - layers defined by lapilli and ash size particles. Graded bedding and depositional features appear absent. Believed to be products of air fall and possibly settling through water.
2. poorly stratified coarse pyroclastics - recognized by deposits of complex folding and slumping. Shards of glass, scoriaceous materials, cauliflower bombs and pelletal lapilli were not observed.
3. basal breccia



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Fig.3: Tuff-ring of the Igwissi Hills pipes in Tanzania



B) Epiclastic Rocks:
These sediments represent fluvial reworking of pyroclastic material from the tuff ring in the crater lake formed on top of the diatreme. They are complex and resemble a series of overlapping alluvial fans mixed in with lacustrine deposits. They coarsen with distance from the wall rock and become better sorted towards the center. Fossils may be found in these sediments. Some epiclastic deposits have been replaced with chalcedony, evidence for late-stage volcanic hot-spring activity.

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:

Country rock fragments-angular; Cognate fragments (juvenile)-rounded to angular; Country rock xenoliths found 1000 meters below depositional unit; Local stratigraphy is crudely preserved by floating reefs in diatreme; Pelletal lapilli appear to have formed by the rapid crystallization of a volatile poor magma containing phenocrysts. They are characterised by a crystal nucleus surrounded by microphenocrysts which align themselves tangentially to the central crystal; Matrix composed almost entirely of fine-grained diopside, serpentine and phlogopite; Crystallisation in diatreme occurs at low temperatures based on the lack of thermal effects seen in intruded limestones; Contact metasomatic/metamorphic effects with the country rock are few; Upwarping and fractures associated with the intrusive body are absent.

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.

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Fig.4: Hypabyssal facies kimberlite.The white matrix material is calcite while the green material is serpentine. Rounded fragments are unknown. From the university of british columbia



Kimberlite Emplacement Models

Various models of kimbcrlite pipe emplacement have been proposed over the years. These include: 1) The explosive boring theory, 2) the magmatic theory and 3) the hydrovaolcanic theory.

The explosive boring theory
This theory involves the pooling of kimberlite magma at shallow depths and the subsequent build-up of volatiles. When the pressure within this pocket, termed an intermediate chamber, is sufficient to overcome the load of rock above, an eruption follows. The epicentre of the eruption was believed to be at the hypabyssal/diatreme facies contact.
Through extensive mining it is clear that this theory is untenable. No intermediate chamber has been found at depth. Also, the dip angle of the vast majority of kimberlite pipes is too high (80-85 degrees) to have been formed from such depths that is, the surface radii to depth ratio is too small. Diatreme facies/hypabyssal facies transitions are generally 2 km deep while craters are generally 1 km wide thus producing a ratio of 1:2. Studies of buried point source explosions have revealed that the ratio should be closer to 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).

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Fig.5: Magmatic (Fluidization) Theory. From Mitchell 1986



According to this theory, 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 1km 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.

Recent discoveries of kimberlite pipes in Canada have prompted a re-evaluation of the magmatic theory. Field and Scott Smith do not deny that water may play a role in the wide array of kimberlite pipe morphologies seen. They believe that in some cases kimberlite magmas may come in contact with aquifers, in which case the resultant kimberlite pipe morphology will be significantly different from pipes found elsewhere, particularly in South Africa. They consider the geologic setting in which the kimberlite is emplaced to play a significant role in the kimberlite morphology. Well consolidated rocks that are poor aquifers, such as the flood basalts which cover most of South Africa, promote the formation of steep sided pipes with 3 distinct kimberlite facies. Poorly consolidated sediments act as excellent aquifers and may promote the formation of gently dipping pipes which are infilled with crater facies kimberlite while diatreme facies kimberlite is absent.

Hydrovolcanic (Phreatomagmatic) Theory
The main proponent of this theory is Lorenz (1999), who has pushed the hydrovolcanic model for 3 decades. According to this theory, 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.

Problems with this theory include:

i) it doesn't explain why every kimberlite eruption must come in contact with water. Surely some eruptions would have occurred in water-poor regions?
ii) the complex network of pipes found at the hypabyssal facies/diatreme facies transition is not explained.
iii) the absence of subsidence features throughout the pipe
iv) the absence of upwarping associated with kimberlite pipes

Petrogenesis

The rocks of the Kimberlite kindred are a paradox, as they generally have major element composition the are similar to primitive pricrites; yet they are also enriched in the incompatible elements, and enrichment of this type is characteristically found in more differentiated rocks. Many authors have proposed that kimberlitic magmas are generated in a source region that is relatively deep within the mantle, and the magmas are the product of a low degree of partial melting.

kimberlite magmas are sourced from deeper in the Earth’ s mantle (Fig.6) than any other magma type, this is clear not only from the occurrence of diamond in many (a property shared with some lamproites), but also from analysis of the garnet lherzolite and harzburgite xenoliths that kimberlites typically contain. Such xenoliths are fragments of conduit wall-rock detached by kimberlite magma during its rapid ascent through the lithosphere, and they place useful constraints on where and under what conditions the kimberlite melt formed.

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Fig.6: Cartoon from Mitchell (2005) linking the origins of kimberlite (K), melilitite (M) and nephelinite (N) to lithosphere thickness and plume input. LAB, lithosphere – asthenosphere boundary; MN, melilite nephelinite.



The Al2O content of orthopyroxene crystals coexisting with garnet is known, from laboratory experiments, to vary with the pressure at which the equilibrium was established (MacGregor, 1974 ). Potentially therefore electron microprobe analysis of Al2O3 in natural orthopyroxenes in garnet peridotite xenoliths allows their depths of residence in the mantle (i.e. depth of origin of the xenolith) to be estimated. The ambient temperature experienced by the xenolith immediately prior to its entrainment by kimberlite magma can likewise be estimated by geothermometry based on the CaO content of clinopyroxene coexisting with orthopyroxene.

Experimental studies by Willie and Huang (1975), and others, have shown the kimberlitic magmas are most likely to be generated by the partial melting of suitable peridotitic materials when CO2 is present in the source region. The experimental studies indicate that, at pressure of between 5.0 – 6.0 GPa, the initial partial melt, of phlogopite-bearing garnet lherzolite source rock in the presence of CO2 and H2O, is likely to be kimberlitic in composition. They also suggest that at pressure greater than 5.0GPa, kimberlitic liquid might be relatively common within the mantle and that the rarity of kimberlites as rocks may be attribuited to the rarity of tectonic setting conductive to the ascent of appropriate magmas.

In order to explain why kimberlitic rocks contain high incompatible element abundance, and also why they normally contain megacrysts and xenolith that equilibrated at high pressures, Harris and Middlemost (1970) proposed that kimberlitic magmas are generated in two-stage processes. In the first stage a tenuous magma, enriched in volatile components (mainly CO2 and H2O), generated by volatiles degassing from deep mantle, rise from a deep approximately of 600 Km. at higher levels in the upper mantle (260Km), the relatively hot, volatile and incompatible element-rich tenuous magma induces partial melting to occur in the garnet Iherzolite mantle rock. The new magma, which is in equilibrium with the solid phase present at this deep in the upper mantle, is picritic in major element composition, but significantly enriched in the incompatible elements. Under ideal condition, such a kimberlitic magma rises rapidly (12m/s) towards the surface from a depth of at least 200Km. at 200 Km, the kimberlite material is essentially a magma, but as it rises to higher levels it become a mechanical mixture of liquid magma, phenocrysts, xenoliths, together with large volume of a separate low-viscosity fluid phase.

As this mixture-magma is propelled upward through a variety of different physical and chemical environments, changes occur as the many phases of which it is composed attempt to adjust to the changing physical environments. The first batch of this magma-mixture that bursts explosively through to the surface is likely to produce a maar crater, or a low-relief coneless crater, that is surrounded by a crater-ring of kimberlitic pyroclastic materials.

With the arrival of more batches of kimberlitic magma, the materials in the surface vents and contiguous feeder-dyke are entrained and mixture; and the solids are abraided in a vigorous-active fluidized system. Eventually the fluidized system collapses, and the different materials coalesce, and the typical rocks of the kimberlite kindred form as the result of this process, assisted by crystallization and growth of a variety of low-temperature and low-pressure secondary minerals.



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kimberlite. Note the large green Cr-diopside megacrysts



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kimberlite, Sloan Ranch, Colorado. Note the large pyrope megacryst



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kimberlite breccia with macrocrysts of serpentinized olivine.






Bibliography



•E.M.W.Skinner (2009): Devolopments in Kimberlite emplacement theory. The SOllthern African Institute of Mining and Metallurgy Diamonds-Source to Use 2009