Kimberlites

Kimberlite is a variety of ultramafic rock that is fine- to medium-grained, with a dull gray-green to bluish color. Often referred to as a mica peridotite, kimberlite originates in the upper mantle under high temperature and pressure conditions. It frequently occurs at the earth’s surface as old volcanic diatremes and dikes. Kimberlite can be the source rock for diamonds.

Kimberlite Composition and Emplacement

The rock known as kimberlite is a variety of mica-bearing peridotite and is characterized by the minerals olivine, phlogopite (mica), and the accessory minerals pyroxene, apatite, perovskite, and various opaque oxides such as chromite and ilmenite. Chemically, kimberlite is recognized by its extraordinarily low silicon dioxide content (25-30 percent), high magnesia content (30-35 percent), high titanium dioxide content (3-4 percent), and the presence of up to 10 percent carbon dioxide. It is a dark, heavy rock that often exhibits numerous crystals of olivine within a serpentinized groundmass. Upon weathering, kimberlite is commonly altered to a mixture of chlorite, talc, and various carbonates and is known as blue ground by diamond miners. Occasionally, kimberlites contain large quantities of diamonds, which makes them important economically.

“Kimberlite” was first used in 1887 by Carvill Lewis to describe the diamond-bearing rock found at Kimberly, South Africa. There, it primarily occurs as a breccia found in several deeply eroded volcanic pipes and also in an occasional dike. Unfortunately, the kimberlite is so thoroughly brecciated and chemically altered that it does not lend itself to detailed petrographic study. Instead, the kimberlite of Kimberly, like kimberlite elsewhere, is more often noted for the varied assortment of exotic xenoliths (inclusions of other rocks) and megacrysts (large crystals) it contains. The explosive nature of a kimberlite pipe leads to the removal of country rock (rock surrounding an igneous intrusion) as the magma passes through the crust and thus can provide scientists with samples of material that originated at great depths. Among the many xenoliths found in the kimberlite breccia pipes are ultramafic rocks such as garnet-peridotites and eclogites, along with a variety of high-grade metamorphic rocks. These specimens provide scientists with an excellent vertical profile of the rock strata at various locations, serve to construct a model of the earth’s crust at various depths, and provide information on the chemical variations in magma.

The emplacement of kimberlite is believed to be due to the eruption of a gas-rich magma that has intruded rapidly up a network of deep-seated fractures and probably breached the surface as a volcanic eruption. The magma’s rate of upward mobility must have been rapid, as evidenced by the occurrence of high-density xenoliths such as eclogite and peridotite within the kimberlite pipes. The final breakthrough of the kimberlite magma may have taken place at a depth of 2-3 kilometers, where contact with groundwater contributed to its propulsion and explosive nature. Brecciation rapidly followed, along with vent enlargement by hydrologic fracturing of the country rock. Fragments of deep-seated rock, along with other country rocks, were then incorporated within the kimberlite magma.

Kimberlite and Texture

As a rock, kimberlite is very complex. Not only does it contain its own principal mineral phases, but it also has multicrystalline fragments or single crystals derived from the various fragmented xenoliths that it collected along the way. These fragments represent upper mantle and deep crustal origins and are further complicated by the intermixing with the mineralogy of a highly volatile fluid. As a result, no two kimberlite pipes have the same mineralogy. The continued alteration of the high-temperature phases after crystallization can produce a third mineralogy that can affect the interpretation of a particular kimberlite’s occurrence.

The characteristic texture of kimberlite is inequigranular because of the presence of xenoliths and megacrysts within an otherwise fine-grained matrix. In relation to kimberlite, the term “megacryst” refers to both large xenocrysts and phenocrysts, with no genetic distinctions. Among the more common megacrysts present are olivine (often altered to serpentine), picro-ilmenite, mica (commonly phlogopite), pyroxene, and garnet. These megacrysts are usually contained in a finer-grained matrix of carbonate and serpentine-group minerals that crystallized at considerably lower temperatures. Among the more common matrix minerals are phlogopite, perovskite, apatite, calcite, and a very characteristic spinel. Found within the textures of these matrix minerals are examples of both rapid and protracted cooling, with the latter evidenced by zoning. Zoning indicates that the matrix liquid cooled after emplacement and that there was sufficient time for crystals present to react with the remaining liquid, which is common to the megacrysts as well. In addition, the megacrysts exhibit an unusual, generally rounded shape that is believed to be a result of their rapid transport to the surface during the eruptive phase.

Kimberlite Occurrence

The geological occurrence of kimberlite, clearly volcanic in origin, takes the form of diatremes, dikes, and sills of relatively small size. In shape, kimberlite diatremes usually have a rounded or oval appearance but can occur in a variety of forms. Quite often, diatremes occur in clusters or as individuals scattered along an elongated zone. They rarely attain a surface area greater than a kilometer but in profile will resemble an inverted cone that descends to a great depth. When it occurs as a dike, kimberlite is quite small—often not more than a few meters wide. It may occur as a simple ring dike or in swarms of parallel dikes. Kimberlite’s occurrence as a sill is quite rare and will have a wide variability in thickness.

As compared with other types of igneous rocks, the occurrence of kimberlite is considered to be quite rare. Based on factors such as specific matrix color, density, mineral content, and xenolith size, shape, and number, connections between a certain kimberlite and a specific occurrence can be made. Aside from their scientific value, most kimberlites are economically worthless, except when they contain diamonds.

Diamond-Bearing Kimberlite

Of the many minerals that constitute kimberlite, diamond is the most noteworthy because of its great economic value. Not all kimberlite contains diamonds, and when diamonds are present, they are not always of gem quality. Even in the most diamondiferous kimberlites, the crystals and cleavage fragments are rare and highly dispersed. Diamond mining from kimberlite requires the removal and processing of enormous amounts of rock to produce relatively few diamonds. This method is expensive and can be dangerous. A much better rate of return can be gained from the placer mining of riverbeds and shorelines where nature has weathered out the diamonds and concentrated them in more readily accessible areas. The discovery of a large diamond in a riverbed in 1866 led to the eventual search for the source rock that produced diamonds. Up to that time, diamonds were recovered only from alluvial deposits and not from the host rock. In 1872, as miners were removing a diamond-bearing gravel, they uncovered a hard bluish-green rock that also contained diamonds. Further mining revealed the now familiar circular structure of the diatreme that continued downward to an undetermined depth. In 1887, the first petrographic description was made of the rock now known as kimberlite, and it was then recognized to be similar to other volcanic breccias from around the world.

Diamond-bearing kimberlite locations can be found around the world, the most famous being in South Africa. The diamond pipes of South Africa have been the leading producer of diamonds for well over a hundred years and are still unrivaled in terms of world production. Other locations of kimberlites that produce diamonds include the pipes of Siberia, western Australia, and Wyoming. For many years, Arkansas had the only commercial diamond mine in the United States, but the site is no longer active and is now preserved as a state park where visitors can hunt for diamonds.

Kimberlite Origin Hypotheses

The chemistry and mineralogy characteristic of kimberlite indicate a very complex set of conditions that existed during their emplacement and subsequent crystallization. This fact, combined with kimberlite’s limited occurrence and the altered nature of its matrix, makes kimberlite a puzzle to scientists. Three hypotheses have been proposed to explain its origin. A zone-refining hypothesis describes a liquid generated at great depth (600 kilometers) that is dynamically unstable and, as a result, rises toward the surface. As it reaches specific lower pressures, its composition is altered through partial crystallization and fractionation. A second hypothesis envisions a residual process: Partial melting of a garnet-peridotite parent, at depths of 80-100 kilometers, produces fractional crystallization of a picritic basalt (olivine-rich, or more than 50 percent olivine) at high pressure, which could lead to the formation of eclogite and a residual liquid of kimberlite composition. The third hypothesis describes kimberlite as either the residual end product of a long fractionation process or the product of a limited amount of partial melting. Each of the three hypotheses has merit but falls short of providing a definitive answer, partly because the near-surface environment where kimberlites are found is one of complex chemical reactions, which makes interpretation difficult. Adding to the problem is that no kimberlite eruption has ever been observed, so the exact processes are still somewhat speculative.

Although kimberlites tantalize scientists with their complexity, the xenoliths and megacrysts that arise with the kimberlites provide substantial data on the relationship between the lower crust and the upper mantle. Kimberlites show the earth’s upper mantle to be very complex in terms of petrography, and they define both large- and small-scale areas of chemical and textural heterogeneities. Pressure and temperature conditions have been accurately established for depths down to 200 kilometers through studies based on the specimens brought up during kimberlite eruptions.

Kimberlite Specimen Analysis

Analysis of a rock such as kimberlite begins with the collection of a fresh specimen (as unaltered by weathering as possible) and continues with the preparation of a series of thin sections and microprobe sections. In this process, a slice of rock is cut with a diamond saw and glued to a glass plate. A second cut is then made to reduce the specimen’s thickness to nearly 0.03 millimeter. A final polishing will achieve this thickness, which will permit light to pass through the specimen and provide the opportunity for microscopic examination. A similar procedure will produce a microprobe-thin section, but it requires extra polishing to ensure a uniform surface.

Once the thin sections have been prepared, a geologist uses a petrographic microscope, which employs polarized light, to make proper identification of the mineral phases present, along with their specific optical parameters. Opaque minerals require reflective light study. Microscopic examination is usually the first step in classifying a rock, and it may include an actual point count of the minerals present to determine their individual ratios. Afterward, an electron microprobe or electron microscope is used to give specific chemical compositions for individual mineral phases. With these devices, the scientist can analyze mineral grains as small as a few microns with a high degree of accuracy. Mineral grains or crystals that are smaller require an analyzing electron microscope to reveal their composition and fine detail. If the individual minerals are large enough and can be extracted, then X-ray diffraction can be used for a definitive identification of a particular mineral phase.

It is also important to understand the bulk chemistry of a rock specimen. Here, the scientist can select several different methods, including neutron activation analysis (NAA) and atomic absorption spectrometry (AAS). Both techniques provide excellent sensitivity and precision for a wide range of elements. A third method, X-ray fluorescence (XRF), is also commonly used and provides an accurate and quick means for analysis.

In NAA, a specimen is activated by thermal neutrons generated in a nuclear reactor. In this process, radioisotopes formed by neutron bombardment decay with characteristic half-lives and are measured by gamma-ray spectrometry. In AAS, a specimen is first dissolved in solution and then identified element by element. XRF, which provides sensitivity and precision for quantitative determination of a wide range of elements (best for those above atomic number 12), is the principal method used by most geologists to gather element amounts in both rocks and minerals.

After all the bulk chemical analyses have been gathered and run through various computer programs to determine mineral compositions, the data are compared to the microscopy results for evaluation and classification of the rock. This evaluation is compared to similar data from other locations. A final check and comparison to other rock types and strata in the field collection area complete the analysis.

Scientific and Industrial Applications

Although some kimberlites have economic value in their diamond content, most do not. No dramatic breakthroughs from the study of kimberlites will make diamonds more abundant or cheaper in price. The result of these studies is a better understanding of the processes and conditions under which diamonds and semiprecious minerals such as garnet are formed. This knowledge has led to the development of synthetic gems that can be used for industrial applications.

The study of kimberlites also has a direct bearing on scientists’ understanding of volcanic activity. The prediction of eruptions is an aim of geology, as it affects the hundreds of thousands of people who live near potentially dangerous volcanoes. Kimberlite magma, by the nature of its chemical composition, is a very explosive material and can produce violent eruptions. Even so, it is true that most kimberlite magmas do not reach the surface, and when they do, they affect only a small area. Kimberlite magmas are more effective at depth, where rock is being fractured by the rising magma and by the hydraulic pressures exerted by the various gases moving through the magma. By examining the results of the movement of a kimberlite magma, scientists can gain a better understanding of volcanic behavior.

Principal Terms

blue ground: the slaty blue or blue-green kimberlite breccia of the South African diamond pipes

diamond: a high-pressure, high-temperature mineral consisting of the element carbon; it is the hardest naturally occurring substance and is valued for its brilliant luster

diatreme: a volcanic vent or pipe formed as the explosive energy of gas-charged magmas breaks through crustal rocks

dike: a tabular body of igneous rock that intrudes vertically through the structure of the existing rock layers above

magma: a semiliquid, semisolid rock material that exists at high temperatures and pressures and is mobile and capable of intrusion and extrusion; igneous rocks are formed from magma as it cools

mantle: the layer of the earth’s interior that lies between the crust and the core; it is believed to consist of ultramafic material and is the source of magma

peridotite: any of a group of plutonic rocks that essentially consist of olivine and other mafic minerals, such as pyroxenes and amphiboles

ultramafic: a term used to describe certain igneous rocks and most meteorites that contain less than 45 percent silica; they contain virtually no quartz or feldspar and are mainly of ferromagnesian silicates, metallic oxides and sulfides, and native metals

xenocrysts: minerals found as either crystals or fragments in some volcanic rocks; they are foreign to the body of the rock in which they occur

xenoliths: various rock fragments that are foreign to the igneous body in which they are present

Bibliography

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