In the last 20 years scientists have discovered new sources of diamond. Continental collisions -- a result of plate tectonics -- can subject slices of a crust to immense burial and uplift. In Kazakhstan, for example, diamonds formed in buried crust that returned to Earth's surface. Meteor impacts produce immense pressures, and diamonds can be formed and sprayed among the impact debris.

Meteorites also experience impacts themselves and can contain diamonds. And the most ancient meteorite material contains star dust, the remnants of the death of stars. Some of this star dust is extremely tiny bits of diamond, just big enough to be crystals and older than the solar system itself.

Very small "microdiamonds," averaging only 12 micrometers across, were discovered during diamond exploration in a region called the Kokchetav Massif, in northern Kazakhstan, in large slices of metamorphic rock that must have been pushed at least 120 kilometers deep into Earth and returned. Discovery of this process, termed ultrahigh pressure (UHP) metamorphism, has revolutionized ideas about and interest in what can happen to Earth's crust. Recently scientists have found traces of diamond around meteor impacts.

At the 35-million-year-old Popigai crater in Siberia, graphite transformed into microdiamond aggregates up to 1 centimeter across. It is now suspected that diamonds form in most major impacts, becoming a new indicator of ancient cosmic collisions. In 1987, microscopically small fragments of diamond, called "nanodiamonds," were recovered from meteorites that predate the solar system. New studies indicate that they formed more than 5 billion years ago in flashes of radiation from dying red-giant stars into surrounding clouds of methane-rich gas. The process is essentially the same as the new process for growing synthetic diamond called CVD -- chemical vapor deposition.

The diagrams above illustrate the formation of a UHP terrane that can yield diamonds. At top, the down-going subducted ocean crust (green) has a thin covering of sediment (gray) that is sheared off and driven upward (inset), apparently caused by the continental collision (middle) that squeezes the diamond-bearing metamorphic rocks back into the crust (bottom).

Diamonds as star dust

Diamonds..

.... star dust

In the last 20 years scientists have discovered new sources of diamond. Continental collisions -- a result of plate tectonics -- can subject slices of a crust to immense burial and uplift. In Kazakhstan, for example, diamonds formed in buried crust that returned to Earth's surface. Meteor impacts produce immense pressures, and diamonds can be formed and sprayed among the impact debris.

Meteorites also experience impacts themselves and can contain diamonds. And the most ancient meteorite material contains star dust, the remnants of the death of stars. Some of this star dust is extremely tiny bits of diamond, just big enough to be crystals and older than the solar system itself.

Very small "microdiamonds," averaging only 12 micrometers across, were discovered during diamond exploration in a region called the Kokchetav Massif, in northern Kazakhstan, in large slices of metamorphic rock that must have been pushed at least 120 kilometers deep into Earth and returned. Discovery of this process, termed ultrahigh pressure (UHP) metamorphism, has revolutionized ideas about and interest in what can happen to Earth's crust. Recently scientists have found traces of diamond around meteor impacts.

At the 35-million-year-old Popigai crater in Siberia, graphite transformed into microdiamond aggregates up to 1 centimeter across. It is now suspected that diamonds form in most major impacts, becoming a new indicator of ancient cosmic collisions. In 1987, microscopically small fragments of diamond, called "nanodiamonds," were recovered from meteorites that predate the solar system. New studies indicate that they formed more than 5 billion years ago in flashes of radiation from dying red-giant stars into surrounding clouds of methane-rich gas. The process is essentially the same as the new process for growing synthetic diamond called CVD -- chemical vapor deposition.

The diagrams above illustrate the formation of a UHP terrane that can yield diamonds. At top, the down-going subducted ocean crust (green) has a thin covering of sediment (gray) that is sheared off and driven upward (inset), apparently caused by the continental collision (middle) that squeezes the diamond-bearing metamorphic rocks back into the crust (bottom).

Inclusions of Diamonds.

Diamonds...
...& Inclusions

Diamonds with inclusions are like little space capsules from the mantle: pristine mineral samples are protected by the diamond's indomitable embrace and transported to the surface by a volcanic rocket. Inclusions capture a picture of the rock and environment in which diamonds grow and indicate that garnet harzburgite (a type of peridotite) and eclogite are the most common rocks in which diamonds have grown.

A single mineral inclusion rarely defines a specific rock, but two or more minerals may enable interpretation of rock associations and origin. Some inclusion minerals are virtually unique to diamond sources and are thus sought in the exploration for diamonds.

	A purple pyrope garnet, an indicator of garnet harzburgite, in a brownish diamond octahedron from the Udachnaya pipe, Sakha Republic, Russia (about 0.8 mm across).
	Orange "G5" garnet, typical of diamond eclogite, showing the conspicuous octahedral shape imposed by the enclosing diamond (about 0.5 mm across).
	Red chromian pyrope and green chromian diopside, indicators of a peridotite, in a diamond octahedron from the Mir pipe, Sakha Republic, Russia (each about 0.2 mm across)...

What 'Xenoliths' is to Diamond

Diamonds...
... from Xenoliths

Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's mantle to the surface. Xenoliths are geologists' only samples from the deep Earth, and carry information about diamond growth conditions. The 2 most common types of xenoliths are peridotites and eclogites. Peridotite is the main constituent of the mantle beneath the crust and consists primarily of olivine -- the gem variety is peridot.

Eclogite, consisting primarily of garnet and a green pyroxene, is formed by plate tectonics when basalt of the ocean crust founders into the mantle.

Certain kinds of xenoliths contain diamonds.

These diagrams show the compositions of mantle xenoliths. Shown above is Lherzolite, a variety of peridotite thought to form most of the upper mantle.

Harzburgite is another kind of peridotite with less clinopyroxene. Garnet harzburgites contain red garnet and, occasionally, diamonds.

Eclogite, a very different rock, consists of garnet and sodium-rich pyroxene; some also contains diamonds...

How old - Diamonds & Kimberlites

Age Of Diamonds...
..... & Kimberlites

Kimberlites are generally much younger than the diamonds they bring to Earth's surface. Kimberlites and lamproites have been dated between 50 and 1,600 million years old. Diamonds associated with harzburgites are about 3.3 billion years old -- more than two thirds the age of Earth itself, and those from eclogites generally range from 3 billion to less than 1 billion years old. These age differences help clarify a picture of diamonds having crystallized and been stored beneath the ancient continental cratons and only later being lifted to Earth's surface by kimberlites.

Since inclusion minerals crystallized simultaneously with their diamond host, the age of the inclusions gives the age of the diamond. The ancient age of peridotite diamonds suggests that the formation of ancient Archean continental cores (archons) included diamond crystallization in the underlying mantle lithosphere. A relatively cool, rigid, deep keel beneath these continental nuclei provided a stable environment in which diamonds crystallized and were stored.

Subsequently, oceanic crust diving into the mantle was metamorphosed into eclogite and pasted onto this keel. Much later passage of kimberlite magmas through the keel dislodged diamonds from both peridotite and eclogite and sent them to Earth's surface.

This cross-section of continental crust shows the 200-km-thick cool keel (part of the mantle lithosphere) that provided a stable environment for diamond crystallization and preservation. Kimberlites centered over the keel are likely to yield harzburgite-hosted diamonds from the storage zone (marked with diamonds).

Kimberlites near the edge of the keel are more likely to contain eclogite-hosted diamonds, while those off the keel are likely to be barren of diamonds...

Diamond Mine Tunnel

Diamond...
.... mine tunnel.

The best way to see a kimberlite pipe is first hand, like a miner or geologist, in the tunnels that provide access to the pipe in an underground mine. The tunnel recreated in the exhibition goes from the local bedrock, through a boundary zone that is highly fragmented, and into the kimberlite, with its inclusions of mantle rocks and diamonds.

In a tunnel that contained a kimberlite pipe, you might be able to see the following:

Shattering of local bedrock and mixing with kimberlite in the boundary zone: This shows that the eruption was extremely violent, explosive in its early stages.

Mica: In addition to mica, the tunnel contains calcite, which is invisible. Large proportions of these two minerals are unusual in an igneous rock. There is water in mica and carbon dioxide in calcite, representing abundant gas (steam and carbon dioxide) dissolved in the magma. The gases bubbled out of the magma and propelled the violent eruption -- like uncorking a bottle of hot champagne.

Round and angular pieces of rocks in the kimberlite: These are xenoliths, literally foreign rocks in the kimberlite. They were dislodged from Earth's mantle by the rising magma. These rare samples are valuable to geologists studying Earth's interior and the origin of diamonds. You can learn more about this in the room off of the tunnel.

Red garnets and a diamond: Both minerals are found in kimberlites and in some of the xenoliths. Certain garnets may indicate the presence of diamonds.

What Diamond is to kimberlite and Lamproite

Diamond is....
... to Kimberlite & Lamproite

The complex volcanic magmas that solidify into kimberlite and lamproite are not the source of diamonds, only the elevators that bring them with other minerals and mantle rocks to Earth's surface.

Although rising from much greater depths than other magmas, these pipes and volcanic cones are relatively small and rare, but they erupt in extraordinary supersonic explosions.

Kimberlite and lamproite are similar mixtures of rock material. Their important constituents include fragments of rock from Earth's mantle, large crystals, and the crystallized magma that glues the mixture together.

The magmas are very rich in magnesium and volatile compounds such as water and carbon dioxide. As the volatiles dissolved in the magma change to gas near Earth's surface, explosive eruptions create the characteristic carrot- or bowl-shaped pipes.

Kimberlite magma rises through Earth's crust in networks of cracks or dikes. The pipes only form near Earth's surface. This cross-section of a kimberlite pipe shows the carrot-shaped profile produced by explosive eruption.

The root zone starts in fissures, where gases are released from the rising magma and drive the eruption; they blow out the fragment-laden kimberlite to form the volcano's tuff ring and fill the pipe. Depth measurements show the level of erosion for various kimberlite pipes in South Africa. Adapted from Hawthorne (1975).

These drawings illustrate the formation and filling of the typical champagne-glass shape of a lamproite pipe.

The initial stage of the eruption, powered by gases either from the lamproite magma or from boiling ground water, corrodes the hosting rock to form the champagne-glass shape (top).

The eruption then produces particles of ash, lapilli, and pumice that partially fill the crater and form a tuff ring (middle).

Finally, the crater fills with a lava pond from the degassed lamproite magma (bottom).

Adapted from a sketch by Barbara Scott-Smith..

Diamonds as Mineral Indicators

Diamonds as...
... Indicators.

Certain minerals are present in the rocks from the upper mantle that occur with diamonds in kimberlite and lamproite pipes, as seen in nearby cases of xenoliths and diamond inclusions. Some of these minerals, being resistant to weathering and denser than quartz sand, concentrate in channel bottoms.

Because they occur in far greater abundance than diamond, exploration geologists look for these "indicators" among the gravel of regions they suspect may host diamond-bearing pipes.

Indicator minerals for diamond include, in order of decreasing significance: garnet, chromite, ilmenite, clinopyroxene, olivine, and zircon. But the order of persistence in streams is zircon, ilmenite, chromite, garnet, chromian diopside, and olivine.

Diamond itself is obviously a most important indicator.

Most indicator minerals have a distinctive color. Seen here are red pyrope garnets, green chromian clinopyroxene, black ilmenite and chromite, and yellowish-green olivine...

Where on Earth Diamonds can be found.

Where Diamonds...

.... can be found on Earth

The search for diamonds has determined that most are derived from kimberlite pipes in the oldest, nuclear portions of the continents, where the basement rocks are older than 1.5 billion years.

The oldest parts of continents are called cratons, and can be divided into two terranes: Archean-age archons, which are older than 2,500 million years, and Proterozoic-age protons, which are 1,600 -- 2,500 million years old. The distribution of these terranes is shown on the map. Kimberlite pipes occur in many parts of the continental crust, but most diamond-rich ones are found in archons.

This fact suggests that most diamonds were formed and stored deep below the cratons, in the area shown in the lower figure, and were later transported to the surface by kimberlite and lamproite magmas that extracted them and other samples from the mantle.

Diamond surf to the surface

How Diamonds Move....

..... to the Surface of Earth

Diamonds ascend to the Earth's surface in rare molten rock, or magma, that originates at great depths. Carrying diamonds and other samples from Earth's mantle, this magma rises and erupts in small but violent volcanoes. Just beneath such volcanoes is a carrot-shaped "pipe" filled with volcanic rock, mantle fragments, and some embedded diamonds.

The rock is called after the city of kimberlite, Kimberley, South Africa, where the pipes were first discovered in the 1870s. Another rock that provides diamonds is lamproite.

The volcano that carries diamond to the surface emanates from deep cracks and fissures called dikes. It develops its carrot shape near the surface, when gases separate from the magma, perhaps accompanied by the boiling of ground water, and a violent supersonic eruption follows. The volcanic cone formed above the kimberlite pipe is very small in comparison with volcanoes like Mount St. Helens, but the magma originates at depths at least 3 times as great.

These deep roots enable kimberlite to tap the source of diamonds. Magmas are the elevators that bring diamonds to Earth's surface.

100 Million Years Ago

Sections of the Earth's Interior (center to surface):
Inner Core, Outer Core, Lower Mantle, Upper Mantle, Crust

Diamonds are Formed in the Upper Mantle
Kimberlite magma rings move up from the Mantle
Diamonds are carried upward by the magma
Magma and gases explode to the surface
Falling ash and rocks form a cinder cone

Millions of Years Later
Only a slight depression remains

Today
A South African Diamond Mine