Carbon is amazing. Where would life be without it? It forms short molecules, including the one essential to (previously) intelligent life, ethanol. It forms long linear molecules (aliphatics), round molecules (aromatics) and even spherical molecules (fullerenes) or cylinders (nanotubes). As a solid it forms thin, strong sheets (graphite). If such a sheet is composed of a single molecule, one layer thick, it becomes the wonder material graphene, useful for just about anything from protecting mobile phone screens to distilling whiskey. Graphite is black, but graphene is transparent. And finally, it can form crystals. These are the diamonds, beautifully transparent and the hardest material known on Earth. Diamonds set pulses racing. No matter how useful they might be for their hardness, the beauty wins out. Rarity has something to do with this. There are not many places where they are found, and the market is tightly regulated by the de Beers company, Cecil Rhodes’ final legacy. Wars have been fought over them. Diamonds even have their own measure of weight: one carat is 0.2 grams. Its origin is the carob seed which is remarkably uniform in size and weight.
Before the great African finds, the only major source for diamonds, and in particular large diamonds, was India and the kingdom of Golconda. As long ago as the middle of the 17th Century, 20,000 men and women were employed at the main mines and in all more than 50,000 people were employed in their extraction, according to the French traveller and explorer Jean-Baptiste Tavernier, Baron d’Aubonne, who made six long voyages to the East between 1630 and 1668. Almost all of the famous, historical diamonds hail from this locality and were described first by Tavernier: the Koh-I-Nur (Mountain of Light), thought to have been discovered 5,000 years ago, the rose-pink Nur-al-Ain (Light of the Eye), the inscribed Shah, the large, flawless pink tablet Darya-I-Nur (Sea of Light), the Taj-I-Mah (Crown of the Moon). The mere names fascinate and spellbind our imagination.
Originally, diamonds were found in alluvial deposits along major rivers. The discovery of their real origin was made in South Africa. The Colesberg used to be a flat-topped hill, on the farm of the de Beers brothers. Diamonds were found near it, far from any river, and in 1871 digging started at what is now called the Kimberley mine. At one time, 50,000 people were involved with the manual digging. By 1914, further digging finally became too dangerous and unproductive. The ‘hill’ was now 240 meter deep and had become (and still is) the deepest hand-dug hole in the world. All in all, 2700 kg of diamonds were retrieved. For some time South Africa accounted for 90% of the world’s diamond finds. Further finds were made elsewhere in southern Africa, including the coastal desert sands of Namibia, in Botswana, Angola, and the Congo. The largest mine is the Yubileyny diamond mine in Siberia, or Argyle in Western Australia, depending on how reserves are counted. South Africa now only accounts for 15% of diamond discoveries.
Diamond mines still look like the Kimberley: a deep hole extending vertically into the ground, with a small footprint for its depth. How do these diamonds form and how do they end up in these narrow pipes?
Tales from the deep
Surprisingly, and in spite of their extreme hardness, diamonds are not fully stable. If heated to 900C, a diamond will convert to graphite, and it is not possible to form diamond at normal temperature and pressure. Diamonds form only under specific conditions: high pressure (45-60 kilobar) and moderately high temperatures (900-1300 C). These pressures are reached in the lithosphere at 150-300 kilometer depth, but temperatures here are often a bit high, especially underneath oceanic plates. Temperatures in the lithosphere underneath thick continental plates are a bit lower, and diamonds can form and survive here. It has recently been argued that sea water, taken down into the mantle by subduction, can allow diamonds to form at less extreme depth. Diamonds can form in the mantle below the lithosphere, as long as enough carbon is available. A few diamonds come from the crust, but the large majority formed in the mantle.
The mantle is a complex region. The lower mantle sits above the Earth’s core, between 600 and 2900 kilometer deep. Above this, between 400 and 600 kilometer deep, is a transition zone, and above the transition zone is the upper mantle. Part of the upper mantle can attach itself to a continent, forming a mantle keel. Diamonds form mainly in the upper mantle and transition zone, and from there find their way to the mantle keel.
Diamond come in two different types. Eclogitic diamonds are associated with basaltic magmas, and formed in or from subducted oceanic plates. Peridotitic diamonds are found in peridotites, rocks with more than 40% olivine, and are associated with the mantle. Olivine is a magnesium-iron-silicate (ultra-mafic), ranging from fayalite (almost no magnesium) to forsterite (almost no iron). The two types of diamonds can be found together and can form underneath the same region, perhaps at different times and different depths.
For eclogitic diamonds, the carbon from which the diamonds are made must have comes from the ocean, brought into the mantle by subduction. The carbon of peridotitic diamonds may come from the mantle itself, but this is an open question as the original mantle material is rather carbon poor. The two types differ in the fraction of C-13 (the rare, heavy isotope of carbon) compared to C-12 (the normal, light isotope), suggesting there is some difference in the source of their carbon. The Mir diamond mine in Russia contains eclogitic diamonds with two different isotopic ratios, showing that the source of the carbon could change over time even in one location. One diamond even has both compositions, one in the core and another in the rim: the change of carbon source happened while this diamond grew. The growth of the diamond must have been on a geological time scale, as slow as the dance of the continents.
Super deep diamonds are a rare group of diamonds which contain inclusions of minerals which only form under extreme pressure, in the lower mantle. They are known mainly from Juina State, Brazil. The inclusions formed at around 650 kilometer depth, at the transition between the upper and lower mantle. A very deep subducted slab reaching down to the lower mantle may have provided the rare conditions to form diamonds at this extreme depth. Plumes then brought the diamonds to the bottom of the continental craton.
There is one other way to make diamonds: major meteoric impacts. The pressures from a big impact (we are talking dinosaur extinction size here) is high enough to flash carbon into diamonds. They are tiny, and not useful for fashion purposes. Microdiamonds also exist in space, and it is possible that some of these diamonds came with the impact: alien diamonds, too small to see, but perhaps older than the Earth.
Diamonds take forever
All diamonds are ancient. The volcanoes which brought them up from the deep are typically 100 million years old, but the diamonds found in them have ages from 1 to over 3 billion years. In Canada, diamonds have been found as old as 3.5 billion years. Eclogitic diamonds are younger than 3 billion years; peridotitic diamonds are often older. The oldest diamonds formed when the Earth was a very young, and very different, planet!
Volcanoes with kimberlite magmas are found both on ancient cratons and in younger locations. But only those volcanoes on the cratons contain diamonds; the kimberley magmas in younger locations are diamond free. The cratons are the ancient cores of the continents, such as in South Africa, Sierra Leone, India, Brazil, Canada and Siberia. Only they are old enough to have protected the diamonds stored underneath in their mantle keels. The ancients still have it. Perhaps diamonds only formed in the distant past and do so no longer, or they took forever to form. Protection against mantle heat was necessary: otherwise the diamond would ‘graphitise’. The remnants of graphitised diamonds have been found in pyroxenite bands in Morocco, which come from hot peridotite. Not all diamonds have survived!
The South Africa craton formed from a western part and an eastern part, brought together in one of the oldest continental collisions known. The diamonds found in the western part of the craton are 3.2 billion years old, and the eastern ones are 1-1.5 billion years. Presumably the ancient subduction zone, where diamonds formed, extended underneath the western part. The eastern diamonds formed during a rather more more recent subduction event.
As with any rule, there are exceptions. The Argyle diamond mine in Australia is located off-craton.
Life and Diamonds
The age difference between eclogitic and peridotitic diamonds deserves attention. Eglotitic diamonds form from carbon in the sediment at the bottom of the ocean, which is subducted into the upper mantle. Much of the carbon is deposited as carbonates by living organisms. Life favours the lighter isotope C-12 over C-13, and indeed eclogitic diamonds tend to have lower C-13. Their carbon comes from life. The peridotic diamonds lack this signature of life.
Life is believed to have started 3.8 billion years ago (give or take a few hundred million years), but there is little fossil evidence older than 2.5 billion years and it is possible life had a slow start. When did life really colonize the deep oceans and when did life start to rule the Earth? The eglotitic diamonds are less than 3 billion years old: the isotopic ratios show that life was abundant by that time. The peridotitic ones are mostly more than 3.2 billion years old. Perhaps this was at a time when carbon was still deposited on the ocean floor through normal chemistry, not involving life? If the peridotic diamonds also get their carbon from subduction, perhaps they come from a time when life wasn’t as important. Diamonds are old enough that the Earth where they formed was different from the planet we know.
But not completely different. The eclogitic diamonds tell us that subduction existed 3 billion years ago, and therefore plate tectonics is at least that old.
Getting to the diamonds to the surface requires volcanism of a different dimension. Basaltic volcanism, which brings mantle material to the surface, tends to occur in spreading centres where the mantle is warm – far too warm for diamonds. Continental volcanism operates from magma chambers typically 10-20 kilometer deep, not 200! Neither is likely to bring up diamonds. Indeed, the volcanoes of the diamond excavations are unlike any other.
The diamonds are found with a solidified magma called (of course) kimberlite. It is a rare, highly basic rock with a lot of greenish olivine, various minerals such as mica, serpentine, and calcite, but lacking quartz or feldspar. The olivine comes straight from the mantle, from a coolish melt which formed at 200 kilometer depth which is somewhat deeper than the location of the diamonds. The diamonds are hitchhikers, picked up on the way. How the magma gets the buoyancy to break through 200 kilometer of rock, while carrying a load of perhaps 25% of various minerals and diamonds, is hotly (no pun intended) discussed. Volatiles are needed, but these are lacking in the mantle. Lionel Wilson, and more recent Donald Dingwell, have argued that the driving force is CO2. The ultra-basic mantle magma is rich in carbonates, which dissolve as CO2 into the magma. While traveling up through the rocky base of the continent, the magma picks up silicates which melt into the magma and make it more silicic. This acidifies the magma, and makes the CO2 much less soluble. Eventually the CO2 becomes saturated, and comes out of the solution to form a foamy gas, which breaks the crusts and creates an open path for the ascending magma. It behaves like a shaken bottle of fizzy drink.
The kimberlite travels extremely fast: 20 meter per second in the dyke, reaching explosive speeds of up to 400 meter per second near the surface, close to Mach 1. The ascent from the base of the continent to the surface can happen within days or hours.
The diamonds carried up by the extreme buoyancy do not always survive: kimberlite contains a mix of unbroken and broken diamonds, and diamonds can be destroyed and form CO2 if the magma is oxidizing. The faster the diamonds are carried up, the more likely they are to survive. The high speed rips up pieces from the surrounding rock, and incorporates them in the magma. Kimberlite is quite a mismatch.
The amounts of magma making this long journey up are not large. Kimberlite volcanoes are small or very small, and eruptions do not last long, perhaps only days. The craters are typically only 500 meters across: a small hole at the end of a feeder pipe extending 2-3 kilometer down, where it connects with the dyke coming up from the mantle. Kimberlite sills may form but are rare. The lava that erupts is quite cool, perhaps as low as 600 C. Diamonds are found in the pipes and sills, but rarely in the dykes.
No actual kimberlite eruption has ever been observed. The known ones are almost all ancient, over 30 million years old, and badly eroded. A survivor of the erosion is the Cretaceous Fort a la Corne kimberlites in Canada: pyroclastic cones and volcaniclasts, located on an ancient coast. The Tokapal kimberlite, India, forms a 2-km wide tuff ring around a crater. Angola has a kimberlite volcano which left a layer with a thickness of 130 m, possibly be a solidified lava lake. Overall, these are distinctly unimpressive volcanoes.
Although there are over 6000 kimberlite locations known, less than a thousand contain diamonds and only about 30 locations are actively mined. A related magma type, lamproite, is more widespread than kimberlite. It also formed from mantle melt below 150 kilometer depth, and is enriched in iron, titanium and potassium but depleted in sodium. Lamproite may on rare occasions contain diamonds. The Argyle and Ellendale diamond mines in Australia (unusually, located off-craton) are mining lamproite, containing diamonds which formed 1.5 billion years ago. Finally, there is one location, in Canada, where diamonds are found in lamprophyre, a potassium- and sodium-rich mineral, which like lamproite is from a large depth in the mantle but formed with a lower melt fraction. These diamonds happen to be the oldest ones known, at 3.5 billion years. It is possible that these diamonds formed at a time when the mantle which was hotter and still little affected by subduction of water. Nowadays, almost all of the upper mantle has been affected by subduction of oceanic plates.
Young Kimberlite: The Igwise hills
The youngest kimberlite volcanoes, by a large margin, are found in the Igwisi Hills in Tanzania, 150 kilometer south of Lake Victoria and a similar distance east of Lake Tanganyika. The eruptions are dated to about 12,000 years ago. There are three small volcanoes less than a kilometer apart, with a combined lava volume of 0.0035 km3, and only standing about 20 meters high. The deposits of each of the three volcanoes covers about 300 by 300 meters, and is 50 meter thick. Individual layers of pyroclastic deposits are typically 10 cm thick. The lavas were melted at more than 120 km depth. The hills are within one of the five ancient African cratons; the basement rocks are 2.5 billion years old.
The deposits show three eruption phases: First, an explosive outburst of lithic tuffs and lapillistones; Second, juvenile tuffs and lapillistones from weak eruption plumes 1-4 km high, sheared to the north west by trade winds; Third, effusive lava flows of degassed and partially crystalised magma. The first two phases lasted days, perhaps up to a few months, and the effusive phase may have been as short as a few hours. Eruption rates were of order of 1 cubic meter per second. The final cones were higher to the north west due to the prevailing winds.
The Igwisi Hills show the typical features of a kimberlite eruption: small, explosive, a very deep origin and ability to break through very thick continental crust. The lack of diamonds is also typical as only 15% of kimberlite volcanoes contain diamonds. The Igwisi volcanoes are fairly close to the African Rift Valley but not on the rift itself. Perhaps this is not a coincidence. The Tanzanian craton has a crustal thickness of 36 kilometer, and is hard enough that the African Ridge bends around it. The thickness in the ridge is less, at 25 kilometer, due to crustal extension. (The ridge is trying to become a spreading ridge but so far hasn’t managed that.) The Igwisi Hills are close to where the Rift Valley would have been had it not decided to go around the craton. The rising mantle plume may be sidetracked, but enough heat still collects below the craton to induce some melt.
History on your finger
That diamond you are wearing may be 3 billion years old. It is made of carbon from the oldest oceans, and has seen the first continents on Earth form. After a billion years or more clinging to the bottom of a craton, it traveled through 100 kilometer of rock within hours encased in magma. It is a jewel.