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.
1) Explosive Volcanism
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
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)
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
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
3) Hydrovolcanic (Phreatomagmatic)
The main proponent of this theory is Lorenz (1999),
who has pushed the hydrovolcanic model for 3 decades. An article by
refuting the magmatic theory in favour of hydrovolcanism can be found
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
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 pipes
The 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
Schematic diamgrams of: a maar-diatreme
produced by hydrovolcanism
on the left (Lorenz, 1986); and a kimberlite
on the right (Mitchell, 1986).
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
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
Mitchell, R. H. (1986). Kimberlites:
mineralogy, geochemistry and petrology. New York, Plenum Press.