A deeper look at the geology of diamonds

Posted on 19 May, 2013 by Metageologist

The geology of diamonds is fascinating in itself, but they also give insights into wider geological processes and history. Up until 1725, diamonds were only known from India. That all changed when Brazilians panning river sediments for gold, instead found diamonds. Recent studies of inclusions in Brazilian diamonds give insights into what was going on deep under Brazil back when it was part of Gondwanaland.

Most diamonds form at relatively shallow depths in the mantle (a ‘mere’ 150km or so) – we know this from studying little pieces of mineral (“inclusions”) found within them. The Juina kimberlite province in Brazil is notable as it contains inclusions formed at great depths in the earth. These diamonds formed below the earth’s surface layer (the lithosphere) in a region called the aesthenosphere. This portion of the earth, between the lithosphere and the metallic core, forms the majority of the volume of the earth. Rocks within the aesthenosphere are constantly flowing in massive convection currents. The convection patterns periodically cause large upwellings of material called mantle plumes. We can never reach the aesthenosphere, but diamond inclusions give us ways of understanding it better.

As described by Ben Harte of the University of Edinburgh and Steve Richardson of Cape Town the Juina suite of diamonds contain three sets of inclusions. The ultrabasic set of inclusions contains MgSi-perovskite and ferropericlase exotic minerals that are the high-pressure equivalents of minerals like olivine and pyroxene. These minerals are thought to have formed at depths of around 660km.

Image courtesy of Ben Harte, University of Edinburgh

‘Eclogitic’ diamond inclusion from Brazil, showing garnet with exsolved pyroxene from majoritic garnet. Image courtesy of Ben Harte, University of Edinburgh

The second suite of inclusions contains a special sort of garnet, called majorite. In ‘normal’ low-pressure garnet, silica is in ‘4-fold coordination’ – it is surrounded by 4 oxygen. In majoritic garnet silica is in both 4 and 6-fold coordination. This change is caused by extreme pressures that favour more compact crystal structures – majoritic garnet is therefore indicative of extreme depths. The change in coordination changes the mineral composition – the garnet forms a ‘solid-solution’ with pyroxene. For the sample pictured above, after the majorite was formed, at some point on its journey back to the surface it changed back to normal garnet and pyroxene, a process called exsolution that leaves the two minerals intimately intertwined.

The third suite consists of a set of Ca-rich minerals with names like Ca-perovskite, titanite 1, wahlstromite and ‘phase Egg’2.

Using a variety of evidence, Harte & Richardson identify where each suite of diamonds formed. The majorite suite formed at depths of 250-450km. Their chemistry shows they didn’t form from the mantle itself, but from oceanic crust (basalt or gabbro) which suggests they formed in a subducting slab. The ultrabasic suite formed from more typical mantle material, but the authors believe it formed from the mantle part of a subducting slab. They link diamond formation with reactions that occur only in hydrated peridotite around the upper-lower mantle boundary at 660km depth. Hydrated peridotite is only found in oceanic slabs, where sea-water enters them along fractures.

The paper paints a picture of diamonds forming in a sinking slab. Isotope evidence fits this too. The majoritic diamonds contain ‘light carbon’ that has passed through the process of photosynthesis. The Ca-rich inclusions give us an insight into the processes that brought the diamonds back to the surface. They formed from carbonated rocks in the slab, perhaps calcareous oozes. Trace element evidence suggests these diamonds formed from carbonatitic magmas formed from the melting of these carbonated rocks. These diamonds formed at depths of 300 to 600 km. The melting of the carbonated rocks and the process that mixed the 3 suites of diamonds and brought them to the surface are all linked: a mantle plume.

A plume of hotter mantle from greater depths passed through the subducted slab, captured diamonds as it went. Once it reached the lithosphere beneath the Amazonian craton it initiated the production of kimberlite magmas which took some diamonds on the final journey to the surface. In a post written with Nicola Cayzer (also at Edinburgh), Ben Harte took a closer look at some of the eclogitic inclusions that were originally majoritic garnet and now a mixture of pyroxene and garnet. Assuming that patterns of composition are controlled by diffusion means that information on the speed of processes can be deduced (geospeedometry). They estimate the diamonds rose at a rate of 1.3m a year through the upper mantle. Within 2 orders of magnitude – the likely error of the estimates – this matches theoretical estimates of mantle flow rate (1-100 cm a year).

Focussing on a single suite of diamonds allows the authors to make links with regional geological history. The sinking slab in which the first diamonds formed was created 200-180 million years ago. It sank along a subduction zone along the edge of Gondwanaland. This zone is still active along the Pacific margin of South America. The Ca-rich inclusions formed at 101Ma, as the plume punched through the subducted slab. Finally the kimberlite erupted 93 million years ago.

What of the slab in which the diamonds formed? It is still down there in the mantle. Since we know plate movements since that time, we can guess where it is – the South Atlantic. Oceanic basalts in the south Atlantic have an unusual isotopic and trace element composition, known as the DUPAL anomaly. The presence of this slab, containing sedimentary rocks, may explain the geochemical patterns of lavas erupting today.

References

Harte, B., & Richardson, S. (2012). Mineral inclusions in diamonds track the evolution of a Mesozoic subducted slab beneath West Gondwanaland Gondwana Research, 21 (1), 236-245 DOI: 10.1016/j.gr.2011.07.001

Harte, B., & Cayzer, N. (2007). Decompression and unmixing of crystals included in diamonds from the mantle transition zone Physics and Chemistry of Minerals, 34 (9), 647-656 DOI: 10.1007/s00269-007-0178-2

Some facets of the Geology of Diamonds

Posted on 12 May, 2013 by Metageologist

Originally published on the Scientific American guest blog.

Geoscientists can’t say if diamonds are forever, but they can say that some are already billions of years old. They form in a place we’ll never reach: the deep earth, hundreds of kilometres under our feet. Diamonds tell us much about this hidden world and how it is linked to the surface – and life – in surprising ways.

Diamonds are made of carbon atoms which are densely packed into a structure that is extremely strong. On earth they form only under extreme pressures – under conditions very unfamiliar to us surface-dwellers. Some form in the sudden shock-waves created when material from space hits the earth. The global impact layer found suspiciously close in time to the extinction of the dinosaurs contains countless tiny diamonds. Impact diamonds are rare. Most diamonds, certainly any big enough to put in an engagement ring, form slowly within the deep earth.

Imagine a slab of concrete – about 5cm thick – resting on your chest. The pressure is small, but tangible. The pressure found in the deepest ocean is equivalent to some 80,000 of such slabs. Diamonds form at pressures that are at least 45 times greater still, equivalent to millions of slabs or hundreds of kilometres of rock. The earth’s deep interior is a place where even rocks are transformed by the massive pressure.

Natural diamonds don’t form, Superman-style, by the application of pressure directly to other solid forms of carbon (such as coal). They grow by the interaction between a carbon bearing fluid and rock – typically involving redox reactions such as the breakdown of CO2 or methane. Diamonds show complex patterns that suggest they grow gradually. Studies of diamonds from a single area often show a wide distribution of ages, from over 3 billion years old to a few hundred million.

The ‘Picasso diamond’ shows complex growth patterns highlighted by cathodoluminescence. Image with permission of University of Edinburgh

Diamonds form within the earth’s mantle, the thick layer between the thin crust and earth’s metal core. They are particularly associated with parts of the mantle that are stuck to the bottom of long-lived continental crust. Here the mantle forms stable ‘keels’ and doesn’t take part in the convection-driven movements that happen lower down. The portions of stable crust with keels are called cratons – the largest are found in North America, Africa and Australia -all areas rich in diamond mines.

Cratonic keels are very stable, but are not totally insulated from the dramatic events in the rest of the dynamic earth. Subduction at the edge of cratonic plates allows oceanic crust to sink deep into the mantle underneath the craton. Carbon-bearing fluids from the sinking oceanic crust rise into the cratonic keel and may cause a phase of diamond formation. Mantle plumes, columns of hotter rock rising from the base of the mantle can do likewise.

In contrast to how they form, the way diamonds reach the surface involves one of the quickest and dramatic geological events we know. Most diamonds reach the surface brought up within an odd type of molten rock called Kimberlite. This magma forms at great depth in cratonic keels and is rich in volatile elements such as CO2 which makes it highly pressured. If it is able, it will rise to the surface extremely quickly through vertical fractures. At the surface it forms a carrot-shaped pipe which nowadays is often the site of a large circular diamond mine.

Diamonds and other deep minerals are brought to the surface as fragments within the kimberlite magma. Diamonds are able to survive the rough-and-tumble of the eruption very well, but it helps that the eruption events are very quick. Not just geologist-quick, but normal-folk quick. Estimates are that diamonds travel to the surface in at most months but maybe as quick as a few hours. Diamonds are only stable under surface conditions because they are too cold to change their structure. The speed with which they reach the surface and cool down keeps them beautiful and prevents them from turning into worthless graphite on the way up.

Some diamonds are not conventionally beautiful. They contain blemishes, tiny blebs of fluid or inclusions of other minerals that dim their brilliance. But to geologists these are the most attractive diamonds of all. Listen to them carefully and they will whisper secrets about a place we’ll never reach – the deep earth.

The deep earth is only a few hundred kilometres below your feet, but is completely inaccessible. The deepest hole ever drilled is a puny 12.2 kilometers. At diamond depths the rocks are at temperatures over 1000°C – few man-made materials can survive such conditions.

Fortunately we can tell a lot remotely. Seismologists gather information on the way waves created by earthquakes pass through the earth and they can dimly make out structures at great depths. This ‘seismic tomography’ applies the same principles that PET or MRI scanners use to study a human body. Such tools are useful, but in medicine as in geology, sometimes direct sampling of the interior is required: kimberlites act like biopsies, making samples of the interior available for detailed study.

A tremendous range of experimental techniques have been used to study diamonds and their inclusions. Some have poetic-sounding names (“Raman spectroscopy”) but many do not (“combustion analysis”, “laser ablation ICPMS”). Most are used to measure the elemental composition of the minerals or the isotopic makeup of those elements. These data are not just of interest to chemists.

The chemistry of mineral inclusions can yield information about the pressures and temperatures at which they (and the diamond) formed. Radioactive isotopes can be used to estimate the age of formation.

Stable isotopes tell some of the most remarkable stories in the earth sciences. Particular processes create distinctive isotopic signatures that may be preserved through a whole range of subsequent events. One isotopic signature only forms when ultraviolet light interacts with sulphur in an oxygen-poor environment. This signature has been found in diamonds, meaning that they contain material that was once at the surface (rock is a very good sun-block, so UV reactions do not occur inside the earth). Also, the sulphur was at the surface very early in Earth history, before photosynthesis caused atmospheric Oxygen levels to rise.

Photosynthesis has its own distinctive isotope signature, affecting carbon. Some diamonds contain this ‘light carbon’, meaning they are formed from life itself. They are the most amazing type of ‘fossil’ imaginable. Some living organism ended its life as a smear of black carbon in a sedimentary rock. It was then buried deep by subduction. Some of its atoms rose up again, first in fluid and then as part of a diamond, suddenly flung to the surface for us to find and marvel at. This deep loop of the carbon cycle is small in terms of volume but conceptually it is enormous. The cycling of carbon between plants, animals and the atmosphere is well know. Uncomfortably, we are becoming more aware of the additional link between buried coal, atmospheric carbon and climate. But the far deeper cycling of carbon into the mantle, demonstrated by diamonds is only recently proven. We can never reach the deep earth, yet it is intimately linked to surface via the subduction of oceanic crust.

Not all diamonds form from surface material. Carbon has been part of the mantle since the formation of the earth and this carbon forms diamonds too. Tracing types of mineral inclusions, it is possible to distinguish diamonds formed from subducted material from other types. This reveals an interesting pattern: diamonds that are older than 3 billion years show no trace of subducted material. This suggests – consistent with other evidence – that plate tectonics as we know it was not active in the very early earth. Subduction may only have started 3 billion years ago.

Inclusions of lower-mantle minerals (ferropericalse and MgSi-perovskite) inside a diamond that formed at >600km depth. Image kindly supplied by Prof. Ben Harte, University of Edinburgh.

Most diamonds form in the upper reaches of the mantle but some come from deeper down. These ‘sub-lithospheric diamonds’ form in the part of the mantle that slowly circulates in convection currents. This lower mantle forms the majority of the earth by volume, yet is poorly understood. At these depths only exotic minerals are stable, traces of which are found as tiny inclusions within diamonds. The only other place we can see these materials is in the laboratory. Here ‘anvils’ are used to squeeze tiny samples to tremendous pressures. The material they are made of is very strong, but also transparent, so that observations can be made and lasers fired through it to heat the samples. What are these special anvils made of? Diamonds, of course. These are precious stones indeed.

References

A great open-source review of current knowledge from the Deep Carbon Observatory :
Shirey, S., Cartigny, P., Frost, D., Keshav, S., Nestola, F., Nimis, P., Pearson, D., Sobolev, N., & Walter, M. (2013). Diamonds and the Geology of Mantle Carbon Reviews in Mineralogy and Geochemistry, 75 (1), 355-421 DOI: 10.2138/rmg.2013.75.12

The latest evidence that diamonds are made from life:
Schulze, D., Harte, B., , ., Page, F., Valley, J., Channer, D., & Jaques, A. (2013). Anticorrelation between low 13C of eclogitic diamonds and high 18O of their coesite and garnet inclusions requires a subduction origin Geology, 41 (4), 455-458 DOI: 10.1130/G33839.1

A harder look at the geology of diamonds

Posted on 9 May, 2013 by Metageologist

My recent post about diamonds was a rapid romp through some of the most marvellous things earth scientists have discovered about them. In the interests of keeping the casual reader engaged I left out many things. If this left you with some nagging questions, I hope they’ll be answered here.

How in earth do they know that?

Much of the information we gain from diamonds comes from inclusions within them. The minerals that are included must be at least as old as the diamond – how else could they get there? This means that either they are older and were swallowed up by a growing diamond, or they formed at the same time as the diamond. Some inclusions have flat sides that are oriented parallel to the the crystal structure of the diamond around them, suggesting they grew at the same time. Evidence like this justifies talking about the age of the diamond when in fact we can only directly date the age of the inclusion.

The most dramatic claim for diamonds is that some of them contain carbon that was once part of a living organism. Remarkable claims require remarkable evidence – how can we say such a thing?

Diamonds from life

“Our bodies are startdust; our lives are sunlight”1. All life on earth2 depends on photosynthesis for energy. Photosynthesis is a process that captures energy from sunlight, storing it in the form of carbohydrates. This involves capturing carbon from carbon dioxide (releasing the oxygen into the atmosphere). A key enzyme called rubisco, working deep within the photosynthetic machinery, converts carbon dioxide containing carbon-12 in preference to that containing carbon-13. The carbon that ends up as carbohydrate is then richer in carbon-12. This ‘light carbon’ signature is found in living things and their non-living remains. Organic carbon in sediments has 3 percent more carbon-12 than carbonates (limestones) do.

A set of diamonds, called the eclogite-suite has unusually light carbon isotopes. Showing that this is derived from organic carbon requires us to consider other possibilities. There is a well understood carbon cycle near the earth’s surface – carbon is regularly exchanged between atmosphere, crust and the oceans. This means carbon isotope ratios give a consistent value against which the photosynthetic fractionation can clearly be seen. In the earth’s mantle, the carbon cycle is less well understood. Other processes exist that change isotopic ratios. Also there is no reason to assume that ‘primordial’ carbon, that has always been in the mantle, has a consistent isotopic ratio. Maybe portions of the mantle have always contained extremely light carbon?

A recent study in Geology (see reference below) provides further evidence that light carbon in eclogite-suite diamonds is indeed organic carbon. Looking at diamonds and their inclusions, the paper shows an anti-correlation between low carbon isotope ratios (‘light carbon’) and anomalously high oxygen isotope ratios. The oxygen isotope pattern is interpreted as being caused by alteration of hot oceanic crust (basalt) by sea-water circulating through it. Just as with the carbon, there are other possible explanations for the oxygen signal. Showing a strong association between the two isotopic signals is important as it is exactly what you would expect if the material came from subducted oceanic crust. Other explanations for the isotope patterns wouldn’t predict the correlation between them. That some diamonds made are from subducted critters is not just a beautiful idea: it’s probably true as well.

Ancient sulphur

Another interesting isotopic signature affects sulphur isotopes and indicates they were affected by UV radiation in an oxygen poor atmosphere – conditions that only occurred on the surface of the early earth. This pattern of isotopes is different from ‘light carbon’ as it isn’t related to the mass of the isotopes – it’s know as mass-independent fraction (MIF). As a very recent paper in Nature reveals traces of MIF are found in other material brought up from the deep earth – sulphide grains in lavas produced from a mantle plume. This backs up the diamond evidence in that it shows that ancient crustal material was subducted into the deep earth. It goes further however, as the lava is only 20 million years old, suggesting that some ancient subducted crust is still down there.

Another recent study involving MIF in sulphur adds a twist. The MIF signal has been used to date the ‘great oxygenation event’, an important milestone in earth history when photosynthesising critters finally managed to increase oxygen levels in the atmosphere. It turns out that as well as persisting in diamonds, the MIF signal can survive a sedimentary cycle – sediments formed in an oxygen atmosphere may still contain a MIF signal derived from older eroded rocks. This is important as sediments containing a MIF signal are the best way to date the onset of Oxygen in the atmosphere. Its now clear that such signals need to be used with care.

There’s more great research on diamonds, tracking their movements and relating them to plate tectonics, but I’ll save some for another day.

REFERENCES
Schulze, D., Harte, B., , ., Page, F., Valley, J., Channer, D., & Jaques, A. (2013). Anticorrelation between low 13C of eclogitic diamonds and high 18O of their coesite and garnet inclusions requires a subduction origin Geology, 41 (4), 455-458 DOI: 10.1130/G33839.1