Exciting extraterrestrial eclogites

Posted on 26 August, 2013 by Metageologist

Eclogites are beautiful rocks that on Earth are associated with the process of subduction – where pieces of crust sink into the deep mantle region. A recent paper by Makoto Kimura and 5 other Japanese authors, describes the first ever evidence of eclogitic rocks found beyond Earth, formed within an unusually large asteroid now found only as tiny pieces.

ZS Unit Aosta valley, glaucophane bearing eclogite (scanned thin section 50x30mm) showing that glaucophane may be stable under eclogite facies, together with omphcite + grt _ rutile

A terrestrial eclogite in thin-section

Our authors were studying samples of a meteorite1 called a chondrite. It contains numerous small fragments (“clasts”) of different types. The paper focuses on a single set of clasts that contain distinctive eclogitic minerals – omphacite and pyrope-rich garnet. The sample also contains more typical minerals such as olivine and orthopyroxene. All of the Sodium and Aluminium within the sample is found within the garnet and omphacite – indicative of formation under high pressure. Based on the black arts of geothermobarometry our authors estimate formation under conditions of 2.8–4.2 GPa and 940–1080 °C.

On earth, eclogitic minerals are associated with subduction because this is a process that makes rocks experience high pressures and provides mechanisms for getting them back to the surface where we can enjoy then. This meteorite sample formed in a different world 2 – so there is no need to infer subduction on another body, but it is still remarkable (comment added later). To quote the paper: “It is believed that meteorites formed in small asteroidal bodies under very low-pressure conditions, except for the high pressures produced during secondary impact events, as recorded in features such as shock veins.” But these high-pressure minerals do not appear to have been formed in a shock vein, but within the interior of an unusually large asteroidal body (that was later smashed into pieces).

How large a body would this need to be? On earth, pressures like this are found at 100km depth. Does this mean the asteroid could have been 100km in radius? No. The pressure is caused by the weight of the rocks above and so relates to the gravitational pull of the entire body. The smaller the body, the lower the force of gravity. Back of the envelope calculations suggest that in order to achieve these pressures, the asteroid would need to have a radius of 1000s of kilometres – getting into planet territory. By comparison, the pressure at the core of our (unusually large) moon has been estimated to be 4.5GPa 3, which is only slightly higher than the upper pressure estimate from these samples.

This study is based on a tiny fragment of rock – only three thin sections. But from this, we can infer there once existed a huge piece of rock, now smashed into countless fragments. All thanks to our understanding the way minerals behave under different conditions.

Update: I do like Twitter. Various geotweeps found this story as interesting as I did. @lockwooddewit has long suspected that some types of meteorite (such as kamacite Fe-Ni ones) “suggest major differentiated body existed“. Pieces of eclogitic mantle would be consistent with this. Ryan Brown (@glacialtill) pointed out that “we know planets were differentiating w/in the first few million years of the soar system- few survived though“.

One thing that struck me my untutored eye was how remarkable it would be that a large body could form and be destroyed and the only trace be a tiny fragment in one meteorite. Andrew Alden (@aboutgeology) points out in a post that there is an obvious candidate – Theia – “the “Mars-sized object” that is thought to have collided with Earth, way back in the Hadean Eon, to create the mess that formed the Moon.”

One striking thing about the paper is the lack of speculation about the source of this material – the guessing all comes from me and the folks mentioned above. The last paragraph suggests there is more to come from Kimura et al. – “The precursor materials of the clasts, and the genetic relationships between the clasts and the host CR chondrite, are not yet clear. We are now measuring the isotopic and trace element compositions of the clasts, which will shed light on this issue.” Studies like this have a great record of tracing events from the early solar system. I look forward to their next paper.

References

Many thanks to @TriclinicFlow (Konstantinos) for alerting me to this paper:

Kimura M., Sugiura N., Mikouchi T., Hirajima T., Hiyagon H. & Takehana Y. (2013). Eclogitic clasts with omphacite and pyrope-rich garnet in the NWA 801 CR2 chondrite, American Mineralogist, 98 (2-3) 387-393. DOI: 10.2138/am.2013.4192

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

Cornwall: tin, pasties and the world

Posted on 24 March, 2013 by Metageologist

The county of Cornwall is like England’s foot, stretching out languorously into the warm waters of the Gulf Stream1. Now a relatively poor area, best known for fishing and tourism, it has a proud industrial past based on mining, notably of tin. Once the most important thing about Britain, Cornish tin is now distributed across the world. The up and downs of mining also scattered Cornish miners globally. If you look in the right places in the right ways, traces of both can still be found.

If you took a chunk of rock and analysed it, atom by atom, you’d find most elements inside it somewhere. In order to have a rock that is an ore, of interest to miners, it must contain enough of what you want so that it can be extracted commercially. Therefore the formation of ore deposits is often associated with processes that take specific elements from large volumes of rock and put them into much smaller volumes. Granites, which are common in Cornwall, are good at doing this. As granite intrusions cool, water that was within the magma, or from the surrounding rocks, is heated and flows widely. Hot water is good at dissolving and transporting certain elements. At some point this hot water cools and the elements dissolved within it are precipitated out, often into fractures called veins. In places around the Cornish granites there are veins rich in cassiterite – tin oxide.

South Crofty Tin mine in Cornwall. Phone from exnottsminer under CC

South Crofty Tin mine in Cornwall. Photo from exnottsminer under CC

Cornwall’s tin travels far

The Bronze Age (in Europe 3000 BC to 500 BC) saw extensive trade networks develop across Europe. The technology of smelting copper and tin to form a durable alloy is first seen around the copper deposits of Cyprus. The warm Mediterranean world had few tin deposits, forcing them to trade with cold barbarian lands to the North. We know that from 2000 BC tin mining started in Cornwall, initially focusing on alluvial deposits (river gravels containing ore).

The Phoenicians, a now vanished pre-Roman civilisation in North Africa, traded directly with Cornwall. The name “Britain” comes from the Phoenician name “Baratanac”, meaning “Land of Tin”. The Greek historian Herodotus, who is the source for much of the little we know about the ancient world, describes how tin comes from the Cassiterides, ‘lands of tin’ that sat beyond Gaul (France). [See comment below for an informed correction of this paragraph] It’s thought that the Phoenicians, who managed the trade, might have been a little cagey about the exact whereabouts of this economically valuable land.

When great military powers invade far-off lands, there are always people who say that their true motivation was to get access to valuable natural resources. We don’t know of any ancient Romans waving banners saying ‘No Blood for Tin’ when Julius Caesar invaded Britain, but modern historians have suggested Cornwall’s tin deposits were a motive.

Given the lack of documentary sources for these ancient periods of history, its obvious that archaeology has a role to play. A recent paper used the black arts of isotope chemistry to study ancient tin. The isotopes within metals can be used to uniquely characterise where they came from2: the different geological settings leave a distinctive isotopic signature. Despite having an impressive 10 different isotopes it’s proved relatively difficult to do this with tin, but the authors have shown that a Bronze Age artifact in central Europe (the “Himmelsscheibe von Nebra” or sky disc of Nebra) contains tin from Cornwall.

Bronze Age metal disc from Germany, containing Cornish tin. Image from Wikipedia

Another recent scientific study looks at Cornish bogs. By studying metal concentrations in layers within the bog, they can trace when mining was active. Local smelting would spread tin through the environment to be captured in the bogs. From this evidence they suggest that there was only a little mining before the Roman period and that the arrival of the Romans, with their Southern European work ethic and trading mindset3 greatly increased the rate of mining.

To get this direct evidence of the ancient movements of Cornish tin takes big machines run by dedicated scholars. Evidence of the movement of Cornish miners in the Nineteenth Century is easier to find, and tastier too.

Pasties – a world tour

The Cornish pasty, now protected by the full might of European Law, is a folded and crimped pastry circle containing beef, potato, onion and swede. It was popular with the tin miners, as it was a convenient meal that could be eaten with dirty hands.

Cornish pasty. Image from Hammer 51012 under CC as I couldn’t be bothered to take my own picture of one.

The potato reminds us that this is not a Bronze Age dish. In many ways the pasties heyday was the Nineteenth Century, which was also a big time for Cornish mining. By this time all the surface ore had been found and miners were digging down deep, following the veins into the earth. Mining in Cornwall had its peak in the early 19th Century. The Royal Geological Society of Cornwall was founded in 1814 and is the second oldest in the world. Mining terms such as vug and gossan are Cornish in origin.

The mid-Nineteenth Century saw Cornish mining start on a slow but terminal decline as massive deposits were opened up in Bolivia and East Asia. As work dried up, tens of thousands of Cornish miners emigrated to new mining districts across the world, where their skills were in great demand. A Cornish saying of the time said that “a mine is a hole anywhere in the world with at least one Cornishman at the bottom of it!”4

Some went to America. The Californian gold rush of 1850s attracted many Cornishmen and Cornish Pasties are still found for sale in the Sierra Nevada to this day. Others went to the iron and copper districts of northern Michigan and also left pasties behind5.

They reached South Africa and Australia, but also non-English speaking countries such as Mexico and Brazil. They are credited with bringing football (“soccer”) to Mexico and also the ubiquitous pasty. Mexicans may add hot chili sauce to theirs, which would be frowned on in Penzance, but they did open the world’s first Cornish Pasty museum in 2011.

The business of mining and the shipping of raw materials is not glamorous, but it is important. Our civilisation depends on technology that needs particular materials, whether the tin in a Bronze Age sword or the rare metals in your mobile phone. Written histories, often created by politicians or the winners of wars, sometimes overlook the importance of such lowly matters. The study of objects, whether metallic or cultural, can help redress the balance a little.