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What Is a Kimberlite Pipe?

Biot Report #238: July 17, 2005

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One of the questions that intrigued the late astrophysicist Thomas Gold was, “Where did all the carbon, including diamonds, on the Earth’s surface come from?” The answer to the question lies with the story of the Kimberlite pipes, the transporters of diamonds from deep in the Earth to its surface.*

Carbon forms a large part of the sedimentary rocks, the Earth’s crust and the biosphere (us). Yet erosion of the rocks that led to the production of the sediments contributed only a minor proportion of all the carbon in the sediments. In addition, the proportion of carbon that comprises the sediments as calcium carbonate (limestone) adds up only to a tiny proportion of the total carbon we know exists in sedimentary rocks, the Earth’s crust, and the biosphere. (Recall that calcium carbonate (limestone) originates in the combination of carbon dioxide in the atmosphere (air) dissolved in water (the oceans), and precipitated by reaction with calcium (and magnesium).) So where did all the “excess carbon” come from?

Thomas Gold asked rhetorically, “What is the explanation usually offered for the origin of this carbon?” (Gold, p. 24) The answer usually given is the “outgassing” of carbon dioxide from an unknown source material at great depth. The reason for this conventional assumption is that volcanoes emit large amounts of carbon dioxide, and so it seemed straightforward that its source lies deep below. “Balderdash!,” Gold said (not really). His idea was that a deep source of methane (natural gas) would equally result in carbon dioxide as the dominant carbon gas from volcanoes, because “in bubbling through most lavas at a high temperature and low pressure near the surface the methane would be largely converted to carbon dioxide.” (p. 24)

So now the question becomes, does the primary material that outgasses methane (in the form of carbon dioxide) contribute directly to the carbon found in crustal rocks? Thomas Gold thinks, yes, and bases his idea on his knowledge and understanding of the Solar System. Carbon, after all, is the fourth most abundant element in the Solar mix, after hydrogen, helium and oxygen, and many planets have lots of unoxidized carbon (methane) in the form of hydrocarbons (methane) in their atmospheres.

Gold carefully broaches this idea: Did the Earth form from two distinct source materials, one that formed the Earth’s mantle from a high-temperature condensate similar in composition to some iron-containing meteorites, and one that formed the Earth’s crust from carbonaceous chondrites, another type of meteorite? Meteorites are extremely helpful because they provide much evidence about the conditions in the early Solar System. They represent a debris left over from the many and varied processes that took place around the time of formation of the planets.

Indeed, the early Earth’s mantle of condensed material is likely to have been bombarded with diverse materials, including chunks of carbonaceous chondrites stuff, which contain up to 5% pure carbon. “Major impacts would have made a patchwork out of any neatly layered structure,” writes Gold, ending up with a “chemically and structurally uneven outer mantle, full of craters of all sizes and looking much like the lunar surface. There is no considerable evidence that shows the chemical inhomogeneity of the outer mantle, and even the frequent occurrence of circular features underneath the crust.” (p. 33)

The next question is, “Could the hydrocarbon materials [in carbonaceous chondrites] survive the accretion process onto the Earth?” (p. 35) Wouldn’t incoming material be heated so much falling into the already very massive Earth that all these hydrocarbon molecules would be dissociated? The answer is, no, according to Gold. Most of the accretion occurred in the form of small grains falling in, gently, without heating them very much, and packing down in layers in the forming Earth. Occasionally, and here is the zinger, occasional major pieces of material would strike the Earth leaving evidence, such as “island arcs, lines of volcanoes, or certain fluid-derived deposits.” (p. 35) When the material has been covered over, and is at a depth of more than a hundred kilometers, it is heated, predominantly by the effect of the radioactive minerals in the rocks.

What hydrocarbon could survive the intense heat that deep in the Earth? It turns out that the combination of high temperature and high pressure stabilizes carbon molecules at great depth. Methane, for example is largely stable at these temperatures and pressures. Gold did the calculations to determine whether the Earth could contain enough of this deep cooked but stable carbonaceous chondrite material to account for the amounts of carbon we see in the sedimentary rocks, the Earth’s crust and the biosphere (us). At this stage, there are “no quantitative problems” with this idea. (p. 37)

Now we must ask, “Can methane from such deep levels reach the surface and what chemical changes might it go through on the way up?” Are there any clues that we can find to support that such processes really do occur? “In this the remarkable story of the diamonds helps, because they area direct product of carbon, treated with temperatures and pressures that exist at 150 to 300 kilometers. Even though only small quantities of diamonds are found near the surface, the fact that they exist at all tells us that high concentrations of carbon occur at these deep levels. Had the Earth not accreted a material rich in unoxidized carbon, it would be most unlikely that centimeter-sized diamonds could have formed anywhere on it.” (p. 37)

Diamonds from a Kimberlite pipe.
Source: Dr. ML Bevier, Department of Earth and Ocean Sciences, University of British Columbia, 2002, at: http://www.eos.ubc.ca/courses/
eosc221/ign.kimb.ss/pages/mlb444_jpg.htm, accessed July 17, 2005.

Graphite specimen from a different kind of mine than a
Kimberlitepipe mine.
Source: http://www.auburn.edu/~leeming/graphite.jpg. Accessed July 17, 2005.

“Diamonds are a form of very pure carbon, assembled in the most tightly bound crystalline configuration known. Chemical theory and the experience in making artificial diamonds show that high pressures of the order of 45 kilobars are needed to produce this dense crystal. Such pressures are found in the Earth only at a depth of 150 kilometers or more, and it is somewhere at such depths that natural diamonds must have been formed. The temperature there exceeds 1000 degrees Centigrade.

“If a diamond were slowly brought up from the high-pressure region in which it originated, so that at each level it adopted the temperature that is normally there, it would NOT survive the trip to the surface, but instead it would turn to graphite [in your pencil] on the way...How then can a diamond ever get to the surface? It can only survive if it is cooled so fast that there is not enough time for the crystal structure to come apart. At the surface temperature the diamond is metastable, meaning that this is not its equilibrium configuration, but that it would take a very long time – certainly more than 100 million years – for it to revert to the equilibrium, which of course is graphite.

“The only way in which sufficiently rapid cooling could have been accomplished is if the diamond was brought up by a gas at a very high speed and therefore cooled in the expanding gas. But what kind of eruption would that be? Obviously we are not dealing with the ordinarily known volcanic processes, for they deliver hot magmas at the surface, in which a diamond could not survive.” (p. 39)

Diamond mine and top of Kimberlite pipe, looking down it.
Source: Dr. ML Bevier, Department of Earth and Ocean Sciences, University of British Columbia, 2002, at: http://www.eos.ubc.ca/courses/
eosc221/ign.kimb.ss/pages/mlb445_jpg.htm,
accessed July 17, 2005.

Diamond mine and top of Kimberlite pipe, looking down it.
Source: http://www.gannon.edu/resource/dept/enviro/
EnvGeo/projects2003/kimberlite/kimberlites3.htm. Accessed July 17, 2005.

Diamonds are found concentrated in rare structures called “Kimberlite pipes”, named after the first such pipe discovered near the South African town of Kimberley. Most of the known Kimberlite pipes are in South Africa and Siberia, but there are also many in North America, Australia, Brazil, and elsewhere. A Kimberlite pipe is a “funnel-shaped structure, a few hundred meters in diameter. It narrows with depth, becoming a fissure that presumably extends all the way through the crust into the upper mantle, to the level where the diamonds were formed.” (p. 39) According to two observers, “The diamond pipes serve as window that gives us a look into the Earth. There is probably no other group of rocks that originated from even remotely as great a depth as have these.” (pp. 39, 41) Kimberlite pipes were produced by an eruption driven essentially by gas, which was so violent that it carried up fragments from the mantle fast enough that rapid cooling preserved the diamonds. The youngest known Kimberlite pipes are several tens of millions of years old.

Kimberlite pipe boundary (light rock) with surrounding rock (red rock).
Source: Source: Dr. ML Bevier, Department of Earth and Ocean Sciences, University of British Columbia, 2002, at:http://www.eos.ubc.ca/courses/eosc221/
ign.kimb.ss/pages/mlb503_jpg.htm.
Accessed July 17, 2005.

Diagram of Kimberlite pipe.
Source: Thomas Gold: “Power from the Earth,” p. 40.

The mere existence of the diamonds, concludes Gold, proves that unoxidized carbon exists in the outer mantle and can be brought up without becoming oxidized. High pressure gases, such as methane, force their way violently through all the overlying rocks carrying with them their diamond inclusions. “This clearly shows that the Earth has an unmixed, inhomogeneous mantle, and there is a high concentration of carbonaceous material in many areas of the globe.” (p. 44) Thus, Gold answers his original question, where did all the carbon on Earth come from?, with, “It came from outgassing of methane in the upper mantle.”

Editor’s Note: Thomas Gold strikes gold again in the area of rich imagination. Time will prove whether his intriguing and persuasive ideas are correct.

Sources:

*Thomas Gold: “Power from the Earth: Deep Earth Gas-Energy for the Future.” J.M. Dent & Sons, London. 1987, chapters 2 and 3: “Where did the carbon come from?” and “The origin of diamonds.” The book is out of print and difficult to come by.