Telling stories about Irish Geology

Posted on 5 June, 2013 by Metageologist

I clearly remember the most important moment of my geological career. I was resting my back on a glacially-polished wall of gabbro, my feet in an Irish bog, talking to myself in the sunshine. As a young man with bushy hair and beard, tattered field gear, wellington boots and a battered rucksack held together by darning and staples, I was recognisably a geologist, a ‘hammer-man’ in local parlance.

I was looking around as I talked to myself. This is a natural enough thing to do in beautiful Connemara. I’ve spent a lot of time admiring the interplay of sun and cloud and rain and glacial peaks there. I can especially recommend gazing out to sea, enjoying the sense of being at the end of the world, with the whole Atlantic before you. On a practical note, the west is where the showers come from; a glance that way gives you your own personal very-short-range weather forecast.

This time I was looking right at the low bobbles of the Dawros peninsula and left to the rounded hump of Currywongaun and Doughruagh’s majestic black stack. My gaze wasn’t aesthetic but geological, conceptual. In my mind these were no longer hills, but bodies of molten rock, injected into the beating heart of a mountain range millions of years ago.

I was talking because I was telling myself a story. A scientific story, where every detail is backed with evidence and armoured against the necessary pedantry of the scientific process. My story was a good one – good enough to turn into a scientific paper – I had not wasted the last 2 years of my life on pointless data collection after all. Best of all, my story felt like The Truth. I could dare to let myself believe that I understood events that happened miles underground millions of years ago. It was a good feeling.

The Dalradian in 1995

The modern consensus about the rocks of Connemara is that they are Dalradian sediments, deformed and metamorphosed over a short period of time by the Acadian/Grampian orogeny. Back in 1995 when I was a scruffy PhD student1 this was far from being a consensus. Some scholars (including, not incidentally, one of my supervisors John Dewey) were promoting the short orogeny model, but others disagreed. They had evidence on their side, too.

Understanding the timing events in deformed metamorphic rocks is helped by correlating phases of deformation and metamorphism. For Dalradian rocks there was a consensus that the ‘D2’ phase involved burial, heating and intense folding. D2 is associated with the original Barrovian style of metamorphism. Later, D3 involves less intense folding and in some areas Buchan style metamorphism linked to heating following the intrusion of gabbro intrusions into already hot rocks.

In the 80s and 90s, radiometric dating began to be applied to these problems. Direct dating of metamorphism was not then possible and the most reliable source of ages were zircons crystallised in igneous intrusions. Intrusions like the Ben Vuirich granite in Scotland – in 1989, Rogers et al. dated this as 590 million years old. Previous studies had interpreted the granite as being older than D2, but younger than D3. Scottish gabbros linked to D3 had been dated to around 490 million years, meaning that D2 and D3, far from being part of a single quick orogeny must represent, not a single quick orogeny, but entirely different mountain building events separated by 100 million years. In 1994, remapping of the Ben Vuirich granite showed that it was in fact pre-D2 (Tanner and Leslie, 1994), allowing advocates of the quick-Taconic model to argue it was pre-orogenic and therefore not relevant to these debates.

This was going on slightly before and during my PhD research. The challenge of linking igneous intrusions to deformation sequences was core to my work.

D3 folds of a D2 fabric. Sillimanite blebs aligned axial to the D3 folds. Connemara. Wellington boots for scale courtesy of the Geological Survey of Ireland

The Geology of Connemara in 1995

The rocks of Connemara correlate with Scottish events. There are extensive gabbro and calc-alkaline intrusions associated with an intense D3 phase of metamorphism. Most of these intrusions are in the south and the pattern of metamorphism reflects this, with sillimanite grade metamorphism close to them and lower temperatures further north.

These gabbros were originally correlated with D2 deformation, but in 1990 Geoff Tanner, partly in response to the Ben Vuirich date, argued that they were post-D2 and pre-D3.

For my PhD, I focused on a set of gabbro intrusions in the north of Connemara known as the Dawros-Currywonguan-Doughruagh2-Complex (DCDC). I produced detailed maps of the structures in these deformed rocks, focussing particularly on fabrics and shear-sense. I also did a lot of metamorphic petrology, describing the large metamorphic aureole around the intrusions. After a few years of this, I had enough data to start putting my story together.

The Currywongaun contains xenoliths of partially melted granulite facies sedimentary rock. Within these xenoliths are fragments of folded rocks, suggesting that had been deformed before the gabbro was intruded. The gabbro was intruded into rocks that were already at amphibolite grade (550 °C). There is abundant evidence that the magma was affected by deformation during its intrusion – it is syntectonic. Small intrusions within the DCDC are deformed by a fabric that is the same as the D2 fabric in the sedimentary rocks surrounding it. Overall, this evidence points to a syn-D2 page for the intrusion of the gabbros.

Folded fabric in a block of quartzite within a gabbro intrusion. The whole area is a xenolith, fractured by partial melting

There’s more. The larger gabbro bodies are extensively hydrated. Gabbro intrusions start off relatively dry, but if they are intruded into already hot metamorphic rocks they drive even more metamorphic reactions that produce water. As the gabbro cools a little (down from an initial 1200 °C) the water surrounding it makes its way into the intrusion. This water metamorphoses the gabbro into wet amphibolite, which is weaker than the gabbro and so is preferentially deformed. The west edge of Currywonguan is the site of a fantastic shear-zone that was active at high temperatures. In the country rocks, these fabrics cross-cut granulite facies D2 fabrics and can be correlated with D3 folding.

So the gabbros were intruded during D2, but affected by D3 folding while still hotter than the surrounding rocks. A spot of primitive (but effective) thermal modelling allowed me to show that the pulse of heat associated with these relatively small intrusions would have vanished within half a million years. So D2 and D3 were close in time – consistent with the ‘quick-Taconic’ model.

A sketch map of the Dawros-Currywongaun-Doughruagh intrusion from my paper.

Academic hurly-burly

Every good story needs a bit of conflict. For my story, it came from the radiometric dates. A 1988 paper dated the Connemara gabbros at 490Ma. Whereas 1993 and 1996 papers gave dates of around 470Ma for the D3 metamorphism. This gap of 20 million years doesn’t fit my story.

At this point, you may well be thinking that I must be wrong. Hard science, numerical analysis of isotopes must surely trump my hand-wavy field-based studies? No. Remember the paper was from 1988. Back then people still thought digital watches were a pretty neat idea. They dated zircons, not with high-precision laser beams but by chucking a bunch of them into acid. They didn’t even abrade them first. The date was based on the assumption that there was no inherited lead in them. I had no doubt that the date was wrong and I said so in public, at conferences and eventually in an academic paper.

Not everyone agreed. At a conference, after a talk I was told by a angry geochronologist that I couldn’t expect to be taken seriously if I went around saying radiometric dates were wrong. He hadn’t produced the date I disagreed with, but he clearly took it personally. This incident, the venom with which he disagreed with me, marked the start of the end of my geological career.

Closure

Some scientific disagreements remain unresolved for years. Not this one.

Anke Friedrich, a German PhD student at MIT started working on Connemara a few years after me. Soon after I’d been told off for doubting a published radiometric age, she proved it was wrong by redating the same rocks. The whole suite of dates she produced was powerful evidence for the ‘quick-Taconic’ model. All of the magmatism in Connemara and therefore the associated metamorphism and deformation lasted only 12 million years.

I was pleased to be proved right, of course, but I don’t actually remember when I first heard about it. I was so certain that I had to be correct that my reaction would have been quite mild. In this, I wasn’t unusual – I am far from the only scientist to have been hugely certain that they are correct. What strikes me now is how far away this is from the way science is ‘supposed’ to work. How can this be?

Leaning against my syn-tectonic gabbro, feeling I knew The Truth felt great. It helped motivate me: I’m only human. When I was ‘doing science’, writing papers, this feeling was irrelevant to the process of presenting evidence and suggesting hypotheses. Further, Anke Friedrich’s paper is rightly much more highly cited than mine. Her range of radiometric dates is the best scientific evidence for the ‘quick-Taconic’ model within Ireland. ‘Science’ is bigger than what goes on in scientists’ heads – its a process, not just a bunch of people’s opinions.

References

ROGERS, G., DEMPSTER, T., BLUCK, B., & TANNER, P. (1989). A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup Journal of the Geological Society, 146 (5), 789-798 DOI: 10.1144/gsjgs.146.5.0789
TANNER, P., & LESLIE, A. (1994). A pre-D2 age for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland Journal of the Geological Society, 151 (2), 209-212 DOI: 10.1144/gsjgs.151.2.0209
WELLINGS, S. (1998). Timing of deformation associated with the syn-tectonic Dawros Currywongaun Doughruagh Complex, NW Connemara, western Ireland Journal of the Geological Society, 155 (1), 25-37 DOI: 10.1144/gsjgs.155.1.0025
Friedrich, A., Bowring, S., Martin, M., & Hodges, K. (1999). Short-lived continental magmatic arc at Connemara, western Irish Caledonides: Implications for the age of the Grampian orogeny Geology, 27 (1) DOI: 10.1130/0091-7613(1999)0272.3.CO;2

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