Since the discovery of diamonds in kimberlite many different theories regarding the processes involved in kimberlite formation have been put forward. Mitchell 1986 covers many of the theories and presents a more comprehensive critique of each emplacement theory. Here we will examine 3 theories: 1) the explosive boring theory, 2) the magmatic theory and 3) the hydrovaolcanic 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. Other faults with this theory are outlined in Mitchell 1986.
2) 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 more complete explanation of the theory can be found in Field and Scott Smith, 1999 or through an article online written by Scott Smith.
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.
From Mitchell 1986
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. A more thorough explanation is found in Field and Scott Smith 1999.
The figure below is taken from Field and Scott Smith 1998. Of particular interest is the pipe morphology of the Fort a la Corne kimberlites in Saskatchewan. Pipe walls have a relatively shallow dip and are infilled with volcaniclastic rocks or crater facies sediments. The local geology is poorly consolidated sediments - which are excellent aquifers. Field and Scott Smith attribute the difference in morphology seen in Saskatchewan pipes to hydrovolcanism.
From Field and Scott Smith 1998
3) Hydrovolcanic (Phreatomagmatic) Theory:
The main proponent of this theory is Lorenz (1999), who has pushed the hydrovolcanic model for 3 decades. An article by Lorenz refuting the magmatic theory in favour of hydrovolcanism can be found online.
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.
From Mitchell 1986
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 pipesThe hydrovolcanic theory does have its merits and has been accepted as the process for the formation of kimberlites found in Saskatchewan, Canada by long-time magmatic/fluidization proponents (Field and Scott Smith, 1999). However, it doesn't appear to explain the features seen in most other kimberlite pipes. Maars are interpreted as being formed by hydrovolcanic explosions and have a distinctly different internal structure to kimberlites. The main features being the internal structure with 'saucer-shaped' subsidence and the ring-faulting and upwarping of the country-rock associated with the explosion. The image below left looks diagramatically similar to the kimberlite diagram, below right. The main difference is the internal structure seen maar diatremes, which is generally absent in the kimberlite diatreme. The relationship between the diatreme and feeder dike is unclear in the maar-diatreme example.
Schematic diamgrams of: a maar-diatreme produced by hydrovolcanism
on the left (Lorenz, 1986); and a kimberlite on the right (Mitchell, 1986).Conclusions:
The debate today is focused on the latter two theories presented here: the hydrovolcanic theory and the magmatic/fluidization theory. For a more detailed explanation of these theories visit the Commission on Explosive Volcanism website. Some papers and books of interest on emplacement theories are:
Clement, C. R. (1982). A comparative geological study of some major kimberlite pipes in the Northern Cape and Orange Free State. Geological Sciences. Cape Town, University of Cape Town: 2 volumes.
Clement, C. R. and A. M. Reid (1989). The origin of kimberlite pipes: An interpretation based on a synthesis of geological features displayed by southern African occurrences. In Ross et al. (1989) q.v., Vol. 1, pp. 632-646.
Field, M. and B. H. Scott Smith (1998). Contrasting Geology and Near-Surface Emplacement of Kimberlite Pipes in Southern Africa and Canada. 7th International Kimberlite Conference, Cape Town, South Africa, Red Roof Designs.
Lorenz, V., B. Zimanowski, et al. (1998). Formation of Kimberlite Diatremes by Explosive Interaction of Kimberlite Magma with Groundwater: Field and Experimental Aspects. 7th International Kimberlite Conference, Cape Town, South Africa, Red Roof Design.
Mitchell, R. H. (1986). Kimberlites: mineralogy, geochemistry and petrology. New York, Plenum Press.
