Stirring tales from the deep past.

Posted on 12 April, 2016 by Metageologist

My cup of tea is sitting nearby1, the rocket-fuel for the mind is sitting in a piece of man-made metamorphic rock and lying on the saucer is a humble object that bears mute witness to ancient, earth-changing events.

Tea in England is typically taken with milk and sometimes with sugar – lots if it’s “builders’ tea” – and a small spoon is required to add and blend the ingredients. These spoons can be made of silver or even gold. I’m not a Duke or a Prince, so mine are made of stainless steel.

Steel is an impure form of iron and has been made for thousands of years. The use of Archaeological periods (Iron Age succeeding Bronze Age) works because smelting iron tools is harder to do, but gives a better product than bronze. Strong and sharp iron tools are excellent for slicing through both fields and people. Successful Iron Age societies such as the Roman Empire were based on both swords and ploughshares, working together.

Iron-carbon phase diagram. From Wikipedia

Much as with ceramics, small differences in the processes used can make a huge difference in quality of the end product. Molten iron easily mixes with carbon, which was originally introduced from the burning charcoal used in the furnaces. Mixing different amounts of carbon within iron results in different phases (with different atomic structures) – as the phase diagram above shows. This sort of diagram will be familiar to anyone who’s studied igneous petrology. It can be used to predict which minerals are produced in which order when a molten substance is cooled. Producing a molten mix of carbon and iron of the correct composition and cooling it at a rapid rate results in a fine-grained mix of different materials that is strong yet not brittle.

Our ancestors worked all the best forms of steel by trial and error over generations and in many different places. Getting hold of iron ore was never a major issue as iron is one of the most common elements on earth and beyond. It’s atomic nucleus is one of the most stable and a common product of stellar furnaces. Indeed the earth and other rocky planets are so rich in iron that as well as forming part of many silicate minerals in the mantle and crust, left over iron trickled down into the centre of the earth where it forms a dense magnetic core.

Banded Iron Formation. Source: James St. John on Flickr

Much modern iron comes from deposits that have their roots in a remarkable transformation of the earth- the “great oxidation event”. The early earth was a very different place with no oxygen roaming ‘free’ in the air – this reactive element was everywhere bound up with other elements. Iron at the surface was mostly in the reduced ferrous form of iron that easily forms compounds soluble in water. The first forms of life that lived via photosynthesis were tremendously disruptive. The Oxygen they produced never reached the atmosphere but quickly reacted with the surrounding seawater. Often it reacted with iron, changing it from ferrous into ferric iron that forms compounds that are not soluble in water. These same compounds are familiar to us as red rust.

Slowly, photosynthesising organisms turned dissolved iron into layers of iron minerals on the sea-bed. Over several billions of years, grain by grain, these bacteria produced huge volumes of iron-rich sediment. These banded iron formations are found in ancient rocks around the world and form the great iron-ore mines of Western Australia that have helped build modern industrial China.
Eventually the critters won. The earth ran out of ferric iron and the oxygen stopped reacting in the sea and started bubbling into the atmosphere, building a world, our world, that would in time support oxygen breathing animals who sip tea.

The iron in my spoon, was once transformed by the caress of ancient slime, but now it is nearly pure metal, strong and shiny. Surrounded by Oxygen, it is under threat – the Oxygen could bind with it again, undoing the smelting process and turning it back into rust. To counter this – to make it stainless steel – just over 10% of Chromium has been blended in. Chromium is not resistant to Oxygen’s charms either, but the resulting oxide does not let Oxygen pass through. This process of passivation means my spoon has a thin protective film over it, keeping it shiny even in the hostile toxic environment of my tea cup.

Chromite in Serpentinite. Source: James St. John on Flickr

The Chromium in my spoon probably came from South Africa. It is a relatively uncommon element and places where it is concentrated enough to mine are not easy to find. The best place is in a large igneous intrusion formed from the melting of the earth’s mantle. The magma itself isn’t very rich in Chromium, but the biggest intrusions cool very slowly, which allows layers of different minerals to form. We’re still not exactly sure what the exact processes are, whether the minerals sink to the bottom of a pool of molten rock or form later at the slushy mushy semi-solid stage (probably it’s a bit of both). As the owner of a spoon we don’t need to know, just to be grateful that some of the layers are so rich in a mineral called Chromite they form a rock called chromitite.

There is a massive frozen magma chamber where this has happened in South Africa. Called the Bushveld intrusion it is rich in Chromite, and also most of the earth’s known reserves of Platinum group elements. It was formed from a million km³ of magma that was intruded into the crust 2 billion years ago. To put that in context, that would fill the Baltic Sea fives times over. Four Bushveld’s worth of magma would fill the Mediterranean Sea with some left to slop over the side, making a hell of a mess. The Bushveld is an amazing thing, if a little overshadowed by the gold and diamond mines and the massive eroded meteorite crater that sit nearby.

Science and history isn’t just something that sits out there somewhere, in museums, or distant galaxies. It can be found within the most ordinary of objects. These stories aren’t just stories. We can’t identify which ones, but some of the iron atoms in my teaspoon were affected by the great oxidation event. The same atoms really were there billions of years ago. My choice of tea as a drink is not a simple act of will; I’m guided to it by global events hundreds of years ago. Look deeply into anything around you and there are amazing stories to tell.

Tasting the earth: mantle geochemistry

Posted on 11 January, 2015 by Metageologist

If seismologists listen to the earth then geochemists taste it.

Like experts blind-tasting a glass of wine and recognising where it came from, geochemists studying the deep earth aim to find out where a particular liquid came from. Their liquid – basaltic magma formed from melting of the mantle rocks – is now solid, so ‘tasting it’ involves dissolving it in Hydrofluoric acid or vapourising it in the bowels of a machine with an unlovely name.

A wine buff can sniff out where a wine came from because they’ve already sampled lots of known vintages. Geochemists have a much harder job – basalt samples don’t have labels. They are formed from melting of the rocks below, but was the material that melted from the deep earth or shallow? Is it from oceanic crust that’s been subducted and remelted or material that’s sat around since the earth was formed?

Mantle geochemists still have more questions than answers, but that’s because what they do is really hard. They are like first-time wine-tasters who’ve been given anonymous bottles and only a fuzzy satellite image of France to work with.

Image stolen from the Basalt winery. I’m sure they won’t mind.

What to sample?

Most basalt is produced at mid-ocean ridges, where oceanic plates move apart and the underlying shallow mantle rises up, decompresses and melts. Known as MORB1 this is plonk. Widely produced, homogenised and of little interest to the true connoisseur.

Basalt from oceanic islands (OIB) is mantle geochemists’ favourite tipple. Found only in select areas far from plate boundaries it has many flavours but can be distinguished from MORB by a trained nose. Thought to be formed by material rising up in hot plumes from the deep mantle it carries whiffs of what is lurking down there.

Of particular interest at the moment are dark intense picritic lavas. Formed under higher temperatures in smaller batches they tell us more about what happens when a mantle plume first nears the surface.

Tasting

The process of producing basalt from the mantle is complex, depending on the composition and mineralogy of the melting material plus the depth and pressure. Also a lot may happen to the magma before it cools as the surface as lava. Iceland has rhyolite lava flows – very different in composition to basalt, but ultimately formed from mantle melt2.

So to study the mantle that was melted tasting the basic chemistry of the lava is not enough as changes due to later processing can obscure the smell of the source material. More sensitive mechanical noses are required, that can sniff trace elements or isotopes that may be unchanged by later processing and hold the tang of the source mantle.

Terroir

Mantle composition, as inferred from basaltic melt, is very variable, leading to the identification of a ‘zoo’ of acronyms, from DMM and HIMU (sources for MORB) to EM and FOZO (for OIB).

A key concept is ‘enrichment’. Particular elements are ‘incompatible’ which means that if they are in a rock that melts, they are strongly partitioned into the melt. As the ‘enriched’ melt moves away you are left with a ‘depleted’ residue. Continental crust is extremely enriched, oceanic crust less so.

For this reason the churned up mantle contains portions which are depleted by having had oceanic crust melted from it (DMM) and other enriched portions which contain recycled oceanic crust (HIMU). Small amounts of continental crust may enter the mantle – perhaps the mantle frozen to the base of continents may fall off. Also continental material (sediment, stones frozen into icebergs, the Titanic) may end up on ocean floor destined to be subducted. EM and FOZO are sources that may have been enriched in this way.

Primitivo

Geochemists don’t just worry about the mantle, but the whole earth. Chondritic meteorites have long been thought to be a model of the bulk chemistry of the earth. Strip out iron and other elements into the core, account for the enriched crust and you can calculate the bulk composition of the mantle.3.

Compare known mantle compositions with the theoretical bulk composition and you get a gap, leading to the idea of a hidden reservoir of ‘primitive’ composition (e.g. closer to chondritic). Conceptually this is similar to the idea of ‘dark matter’ in physics – a thing invented to explain inconsistent pieces of evidence, but for which there is no direct evidence. Only time will tell if hidden reservoirs in the mantle will be found or go the way of the luminiferous aether.

Basalt in a vineyard

Paradoxes and problems

The idea of hidden reservoirs was extremely popular over 20 years ago, when it seemed that subducting plates stopped at 660km depth, where a ‘phase change’ in minerals alters the stiffness of the flowing mantle. This suggested that the lower mantle could be of very different composition. But modern seismic imaging suggests whole-mantle convection is possible, suggesting that over billions of years the mantle will have been thoroughly stirred – with the exception of a mysterious layer at the base of the mantle.

Mantle geochemists often talk of ‘paradoxes’ – patterns of ratios between elements and isotopes that aren’t consistent. There is a lead paradox, and an Argon one, plus a ‘heat-Helium imbalance’. Explaining these in terms of a primitive reservoir is one way, but others are possible. Let’s look at Helium.

Helium comes in two flavours. The first ³He is just two protons and a neutron and from the earth’s point of view it’s primoridal, it’s always been there and never changes. In contrast, when the great hulking nuclei of Thorium and Uranium fall apart they leave small fragments – making ⁴He in the alchemical process of radioactive decay.

The ratio of the two Helium isotopes is fairly consistent for MORB sources, but wildly variable for OIB. Material with a high ratio has been interpreted in terms of a primitive reservoir, rich in primoridal ³He. An alternative explanation is that the source is extremely low in ⁴He due to it being depleted in Uranium/Thorium. Or maybe the ³He bubbled up from the core.

Tasting the earth does not give you all the answers, but it is vital part of the picture. As I continue my tour of the deep earth, geochemistry will often have an important role to play. The difference between OIB and MORB is a powerful argument in the armoury of those who favour mantle plumes and as seismologists start to see odd things at the base of the mantle, getting a whiff of the chemistry here becomes very important.

Tasting ‘black cherries’, ‘tar’ or ‘cat-pee’ in wine is a clever trick. Tasting blobs of 4.5 billion year-old rock or recycled oceanic crust in basalt is even cleverer. Cheers!

The Himalaya: mountains made from mountains

Posted on 6 April, 2014 by Metageologist

Good building stones get reused. Sometimes the only traces of very old buildings are their stones, built into more modern ones. It’s the same with rocks and mountain belts. Stone that now forms parts of the Himalaya was once part of a now-vanished mountain range.

The Himalaya were formed by the collision between the Indian and Asian plates. For 50 million years, the Indian plate has been pushed down into the Himalayas where it is squashed, mangled and changed by heat and pressure. Working out the details of this process of mountain building has taken decades of careful study. Modern isotopic techniques are now so powerful that researchers studying Himalayan rocks can peer through beyond the effects of the recent mountain building to see traces of older events.

A recent open access paper by Catherine Mottram, Tom Argles and others looks at rocks in the Sikkim Himalaya, around the Main Central Thrust (MCT). As you can guess from the name (and the Use Of Capitals) this is an important structure; it can be traced over 1000km across the Himalaya and separates two distinct packages of rock known as the Lesser and Greater Himalayan Series.

Figure 2c. Cross section of MCT in the Sikkim Himalaya

As the rocks of the Indian plate were stuffed into the moutain belt, much of the movement of rock was along near-flat faults, known as thrusts. These stack up layers of rock, shortening and thickening the crust. Thrusts near the surface may be a single fault plane, but at greater depths rocks flow rather than snap and a thick thrust zone of deformed rocks is formed. This makes drawing a line on a map and calling it the Main Central Thrust rather difficult. Should the line be placed where the rock types change, or where they are most deformed, or where there is a break in metamorphism? Each approach has its advocates.

Our authors took an isotopic approach, measuring Neodymium isotopes for the whole rock and Uranium-Lead in useful crystals called Zircon. Their analysis shows that the two packages of rock separated by the MCT can be distinguished using isotopes. The actual boundary is not sharp: they prove interlayering of the two rock packages within the thrust zone, rather than a single boundary. This is not surprising given that thrusting is a gradual process and thrust surfaces are not flat. Deformation seems to have started at the boundary between the Lesser and Greater Himalaya and gradually moved down over time.

—

The patterns of isotope measurements that can be used to distinguish between the Greater and Lesser Himalayan Series also tell us about what happened before India met Asia.

The zircons whose isotopes were measured are of two types, detrital and igneous. The first are grains that were eroded from old rocks and settled into a sedimentary basin. The second crystallised from molten rock: their ages record significant events. Together these sets of dates give a view of a long and complicated pre-Himalayan history.

Our authors attempt to reconstruct the leading edge of the Indian plate, as it might have looked before it crashed into Asia.

Figure 10. “Schematic illustration showing the pre-Himalayan architecture of the Sikkim rocks, during the mid-Palaeozoic. The Lesser Himalayan Sequence lithologies were once separated from the Greater Himalayan Sequence rocks by a Neoproterozoic rift. The Bhimpedian orogeny was responsible for closing the rift and thickened the Greater Himalayan Sequence, causing metamorphism and intrusion of granites. The failed closed rift may represent a weak structure later exploited by the Main Central Thrust. Lithologies are the same as in the legend in Figures 1 and 2.”

The Greater Himalayan Sequence had already been heated and deformed in the roots of a mountain belt long before the Himalayas existed. This a relatively common situation. Polyorogenic rocks such as these1 need to be treated with care, otherwise we might mix up events separated by millions of years. A single garnet crystal may contain different areas that formed in totally separate mountain building events

One of the detrital zircon grains dated in this study was 3,600,000,000 years old. We can only guess how many cycles of erosion and burial, how many splittings and couplings of continents this mineral has ‘seen’. As it was buried and heated once again maybe, like the bowl of petunias in The Hitchhiker’s Guide to the Galaxy it thought to itself: “Oh no, not again”.

References

Mottram C.M., Argles T.W., Harris N.B.W., Parrish R.R., Horstwood M.S.A., Warren C.J. & Gupta S. (2014). Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya, Journal of the Geological Society, 171 (2) 255-268. DOI: 10.1144/jgs2013-064

Argles T.W., Prince C.I., Foster G.L. & Vance D. (1999). New garnets for old? Cautionary tales from young mountain belts, Earth and Planetary Science Letters, 172 (3-4) 301-309. DOI: 10.1016/S0012-821X(99)00209-5

Radioactivity and the earth (and moon?)

Posted on 23 December, 2013 by Metageologist

“Castle Romeo” atmospheric nuclear test – March 1954. From CTBTO

We tend to think of radioactivity as an artificial thing; some argue that the first nuclear explosions in 1945 should mark the start of a new human-dominated geological epoch called the Anthropocene. These man-made explosions have left distinctive radioactive traces that may well outlive us all. It turns out that natural radioactivity, even fission reactions, played an interesting role in Earth’s history long before we came along.

A little background

Our sun is a nuclear fusion reactor, taking simple atoms such as Helium and Hydrogen and squeezing them together to create new elements, plus energy. This normal activity, along with dramatic events in a star’s history such as supernovae, have created virtually all the atoms you see around you. Radioactive decay is where large unstable atoms break-up, creating new smaller atoms plus various left-over bits, such as alpha, beta or gamma particles. Sometimes these particles hit other unstable atoms and cause them, in turn to break up. Put enough radioactive atoms of the right sort together and a nuclear fission reaction starts. When nuclear fission is used to generate electricity, the reaction is controlled. If used to kill people, a chain reaction is created to generate as much energy as possible.

Too much radioactivity is dangerous, damaging cells and DNA whether the source is natural radon gas or a nuclear weapon. But it’s not all bad. Some people regard plate tectonics as a pre-requisite for life on earth. It certainly makes things more interesting. Plates move because the mantle convects because it needs to release heat to the surface. This heat comes partly from radioactive decay within the earth – without it this planet would be a cold and dull lump by now.

Fossil fission

Radioactive decay is massively useful to geologists as a dating tool. Rates of decay, usually expressed in terms of half-lives, are constant. If you can work out that a grain of zircon started out with twice as much Uranium-235 as it now has, then you know it formed 703.8 million years ago.

Let’s turn that round: 703.8 million years ago there was twice as much Uranium-235 around as there is now and therefore four times as much 147 million years ago. This means that the earth used to be hotter (more radioactive decay), which is why Archean geology is so weird (odd komatiite lavas, crust that dripped back into the mantle). It also means that fission reactions were easier in the past.

Much of the hard work of a nuclear weapons program involves enriching Uranium. From the Manhattan Project through to the Iranians today the most laborious job is taking natural Uranium (a mixture of Uranium-235 and Uranium-238) and increasing the proportion of Uranium-235. This is important because U-238 is more stable, with a longer half-life and less interest in breaking up. Humans increase the proportion of U-235 using centrifuges, or lasers, but a time-machine would do the same job.

Around 2 billion years ago, a Uranium-rich deposit in modern day Gabon was the site of seventeen natural nuclear fission reactors. Self-sustaining nuclear reactions, moderated by groundwater, lasted for about a million years. There are two excellent blog posts that cover the site in more detail.

Such natural reactions are extremely unlikely now, since much more U-235 has decayed into lead over the intervening 2 billion years. But what about the 2 billion years of earth history before the Gabon reactors started up? Were fission reactions active in that time frame? Some argue that they were, with explosive consequences.

Huge explosions and the moon

The deep Earth is a mysterious place. We know that the crust is relatively rich in radioactive elements but we don’t know much about their distribution in the mantle. One day Neutrino detectors may help map out the modern day distribution. How they were distributed earlier in the earth’s history is anyone’s guess.

Some people’s guesses (informed by computer modelling) suggest that heavy radioactive elements such as Uranium, Thorium and Plutonium, sank to the bottom on the mantle, near the core-mantle boundary. Plutonium is now regarded as a man-made element, but it would have existed in the early earth, as it would have had less time to decay since being created in a supernova. Geochemical models suggest that while substantially enriched, the average concentrations would still be too low to cause fission reactions.

Dutch scientists (R.J. de Meijer and W. van Westrenen) have suggested an amazing thing. Their theory is that concentrations of radioactive elements were higher in some areas than others (not unreasonable). They suggest that, just as human nuclear bombs are triggered by using conventional explosives to pressurise the radioactive material, a major impact on the earth would send shock waves into the inner earth and compress the material enough to initiate a nuclear reaction.

This reaction would take place in a large volume of rock and so would be create a huge explosion. Big enough, their modelling suggests, to fragment the earth and send lots of material into space. In time, some of this material formed a large moon orbiting the earth – the one we see today.

The moon? Really?

I suspect you are feeling a little sceptical right now, which I think is the right reaction. But bear in mind that we don’t really know how the moon formed. The best available theory is based on the idea of a massive collision with another large body. This has big problems because of the many isotopic similarities between the earth and moon. Any other body coming in would be expected to have had a different composition, traces of which would be present in the moon today.

The giant impact model is still the best. A recent conference on the moon’s origins discussed many ways in which the similarities between earth and moon could be reconciled with the model. The impact could have thoroughly mixed the material, or maybe the impactor had the same composition. Perhaps the moon originally came from Venus. We don’t know anything about the composition of Venus – it may be very similar to earth.

As far as I can tell, nobody discussed the nuclear explosion model at this conference. This may be because there is no actual evidence for it, just inference from modelling. In their latest paper R.J. de Meijer and W. van Westrenen predict distinctive patterns in Xenon and Helium isotopes in lunar material. Measurements of these elements on our current Apollo samples are contaminated by the solar wind, so samples of deeply buried lunar material would be needed to test it fully.

We’ll have to wait then. Perhaps some future lunar rover will dig up the required samples. If it does, it is likely like the Chang’e rover currently on the moon to be powered by Plutonium. Useful stuff, radioactivity.