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

The Grampian / Taconic orogeny in Ireland – when arcs attack

Posted on 20 March, 2013 by Metageologist

Ever since the plate tectonic paradigm-shift of the 1960s, geologists have strived to understand ancient rocks in terms of the movements of plates. The geology of north-western Ireland can be explained by what happened when a subduction zone ran out of oceanic crust back in the Ordovician. Let me take you back to before that happened.

Imagine you are floating in the sea. In 480 million years time the crust below will be in Ireland. The sea is warm – CO₂ levels are high and you are fewer than 30° south of the equator. Apart from cheeky trilobites nibbling your toes, it is an idyllic place to be. You are near a large land-mass. It’s barren looking – plants are just about to learn how to survive outside the sea 1 so there is little to soften the landscape. The tumbler of gin and tonic2 you clumsily dropped has sunk into sediment that will one day be part of the Dalradian Supergroup.

Your mood has soured. Not only has your drink gone, but you’re getting creeped out by the cloud on the horizon out to sea. It’s not a fluffy, friendly one but a dark spreading plume of volcanic ash. Far out to sea, there is a line of volcanoes and they spell D-O-O-M for the peaceful spot you’ve found. Every year the volcanic arc gets a little bit nearer. Eventually it will smash into the continent behind you, grinding over the top. The peaceful sand and mud washing around below your feet will be sediment no more. The collision between the volcanic arc and the continent will transform the Dalradian sediments into the contorted metamorphic rocks that today make up much of NW Ireland.

An accident waiting to happen

Here’s a diagram showing a section through the land below. Black is oceanic crust, yellow stippled is sediment3.

Let’s start from the left. Notice that the edge of the continent is thin and tapering – it was stretched out when the ocean basin formed. The oceanic crust attached to the continent is being stuffed down back into the deep earth. As it sinks, it is squashed and heated, starting a complex process that ends with molten rock reaching the surface. Over time this magma has formed a volcanic arc, a small piece of thickened crust.

This situation can’t last for ever. Once all of the oceanic crust of the lower plate has run out, normal subduction comes to an end. The upper plate slides over the thinned continental margin and ends up lying on top of the continent – a process called obduction. The island arc and forearc basin are squashed against the continent. Like a bulldozer hitting a pile of sand, the arc collision compresses and thickens the crust. The Dalradian sediments become thoroughly deformed and heated and are now a wildly complex set of schists and marbles. This process creates a mountain belt, an event known as the Grampian orogeny.

As shown in the diagram below 4, the orogeny does not stop plate convergence. The old subduction zone has been destroyed, but another one is created in the opposite direction. This process, called ‘subduction flip’ changed the tectonic stress regime; it’s believed to have led to a process of ‘orogenic collapse’ whereby the thickened crust extends and thins, bringing metamorphism to an end. The overall tectonic event was remarkably quick, around 15 million years.

Where are they now?

This type of orogeny can be recognised as it leaves distinctive rock types behind. Ophiolites, pieces of oceanic crust within continents are found in several places in Ireland; the largest example is the Tyrone ophiolite, but traces of oceanic crust can also be found in Mayo, near Westport, in the Clew Bay and Deerpark Complexes. The story goes that there are no snakes in Ireland because Saint Patrick, while fasting on top of Croagh Patrick, threw them down into Clew Bay. That the sea-facing side of the mountain contains a lot of green serpentinite suggests the origins of the story have a geological angle5.

Map of NW Ireland. Red – continental basement, Yellow-Dalradian, Blue – lower-mid Ordovician volcanics, Brown – Ordovician sediments, purple serpentinite & melange. Data from Geological Survey of Ireland, displayed using Google Earth

Parts of the volcanic arc can now be found around Lough Nafooey, just to the south of the South Mayo trough, which corresponds to the fore-arc basin. The Dalradian sediments and underlying older crust make up most of the land to the north west of these rocks.
Let’s revisit the cross-sections, labelling the modern day equivalents. Before the collision:

and immediately afterwards:

A wider context

Ireland was only a small section of a continental margin that stretched from rocks now in Greenland down into the eastern US and Canada. The Grampian orogeny also affected Scotland and the same arc-collision event is found in eastern Northern America where it is known as the Taconic orogeny.

Figure 1 from Hollis et al. 2012

Recognising the connection across the Atlantic helped geologists understand the causes of these complicated patterns in the rocks – different sections show different parts of the orogeny and synthesising research across a wider area leads to a richer understanding. The patterns are complex – more so than I’ve shown above. There were probably multiple ‘accretion events’ where different arcs collided at different times. The pattern of plates in modern day SE Asia is seen as an analogy here – they may have been multiple subduction zones and arcs within the wider ocean.

Geological is 4-dimensional, and the Grampian/Taconic orogeny reminds us of this. The timing of the various accretion events varies because the edge of the continent was not straight. Promontories (sticky-out bits) were hit by the arc sooner than parts where there was more oceanic crust to be consumed. Another complication is that continents don’t break cleanly: fragments of continental crust can end up far from the main continent (for example Rockall in the Atlantic). The arc whose collision caused the Grampian orogeny in Scotland (now found buried in the Midland Valley) is thought to sit on continental crust.

These 15 million years were a very important time for the crust of the northern half of Ireland, but are only a small part of the wider geological history of Ireland. I’ll leave the story unfinished here and tell you what happened when the new subduction zone ran out of oceanic crust in future posts.

References

RYAN, P., & DEWEY, J. (1991). A geological and tectonic cross-section of the Caledonides of western Ireland Journal of the Geological Society, 148 (1), 173-180 DOI: 10.1144/gsjgs.148.1.0173
Hollis, S., Roberts, S., Cooper, M., Earls, G., Herrington, R., Condon, D., Cooper, M., Archibald, S., & Piercey, S. (2012). Episodic arc-ophiolite emplacement and the growth of continental margins: Late accretion in the Northern Irish sector of the Grampian-Taconic orogeny Geological Society of America Bulletin, 124 (11-12), 1702-1723 DOI: 10.1130/B30619.1
Bird, A., Thirlwall, M., Strachan, R., & Manning, C. (2013). Lu-Hf and Sm-Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland Journal of the Geological Society, 170 (2), 301-317 DOI: 10.1144/jgs2012-083

How to make a rock from scratch

Posted on 20 January, 2013 by Metageologist

“If you wish to make an apple pie from scratch, you must first invent the universe.” Carl Sagan.

I have a handsome piece of rock in my hand. How did it come to be, how was it made? A perfectly acceptable geological answer is that it formed as molten rock cooled slowly underground. But that’s not the whole story, it doesn’t say what melted, and where that come from and…

So, taking my cue from Carl Sagan, here’s the full story.

Inventing the Universe

The Universe was created in a ‘Big Bang’ (if you want to know what happened before that, you are reading the wrong blog). At first, only three elements existed, Hydrogen, Helium and Lithium, basically just simple arrangements of sub-atomic particles. As the universe calmed down a bit, clumps of gas grew and grew, increasing the density in their centre. Eventually the pressure squashed atomic nuclei so much that they fused together, producing energy. The first stars were born.

These nuclear-powered furnaces produced light and heat, but also performed alchemy, turning simple nuclei into larger ones, thereby creating new elements. Getting from a nucleus with a few protons and neutrons to ones with over 100 (as seen in heavier elements) is not easy. Several generations of stars were required, gradually building larger and larger nuclei. The heaviest elements only form in the extraordinary conditions that occur in the least few seconds of a supernova, where large gobbets of protons and neutrons are forced together.

The modern universe has seen several generations of stars come and go, during its 13.75 billion years of life. As wells as stars, galaxies and the like, it contains chemically complex clouds of gas and dust, the mixed remnants of exploded stars. Hydrogen is still dominant but plenty of other elements exist. Over 150 organic molecules have been recognised, including vast quantities of ethyl alcohol, otherwise know as booze. Our own solar system formed from such a gin-soaked* cloud over 4 and a half billion years ago. The atoms that the rock in my hand, and you, and everything else you can see are made of was in there. Arranged rather differently, but there nonetheless.

The cloud contained elements in different forms, such as tiny grains of diamond, formed in supernova shockwaves. They are found today in meteorites, precious little gems older than our solar system. Other grains were present (notably ‘CAIs’ or Calcium-Aluminium inclusions) but many elements were in the form of gas or ice. These ‘volatile elements’ are distinguished from refractory elements found in grains. Unsurprisingly compounds we know of as gas or liquid were in the volatile component, but it also included elements we think of as solid, such as potassium and lead.

Inventing the solar system

Some eddy, some chance event, created a part of the cloud, denser than the rest, that ended up as our sun. As its nuclear furnace ignited, strong solar winds started pushing through the rest of the cloud, blowing out the gas and ice, leaving only dust and larger solid fragments. The gas, ice and volatile elements were pushed out beyond a ‘snow line’ about 4 earth orbits from the sun, where some ended up as part of Jupiter and Saturn, the ‘gas giants’.

The earth’s composition today is measurably different from chondrites, a class of meteorites that records the composition of the original cloud. Indeed the whole of the inner solar system is depleted in the volatile elements that were blown away in the gas and ice.

Over 100s of millions of years the inner solar system formed into the four planets we see today. This was a violent process. Chunks of rock called planetesimals formed and then smashed together. Mercury and the earth-moon system clearly show the marks of major collisions early in their history. This violence is convenient for us, as it provides evidence for what is going on deep within the earth.

Inventing the earth

Many meteorites are fragments of planetesimals that have been smashed into pieces. These fall into two main camps, stony meteorites and iron meteorites. To a first approximation, stony meteorites match rocks we find on the earth’s surface today. Iron meteorites are clearly exotic to us surface dwellers, but they would feel right at home in the centre of our earth.

As planets form they separate out into two chemically distinct portions- a silicate part and an iron rich part. Iron is the sixth most abundant element in the universe – stars make lots of it – and it is refactory. It’s the most common element in the earth. There is so much that some of it doesn’t bond with other elements but sinks down into the core. It takes some friends with it – siderophile (iron-loving) elements such as nickel, gold, platinum and iridium.

The remainder of the planet, the equivalent of the stony meteorites, is known to geochemists as the ‘bulk silicate earth’ and now makes up the earth’s mantle and crust. It is rich in lithophile or ‘rock-loving’ elements which like to bond with oxygen and hang out together. We can’t see the earth’s core directly, just infer its properties remotely, so iron meteorites give us a glimpse of a place we can never visit.

Geochemists are still settling the details, but the broad pattern is clear. Take the volatile elements away from the original cloud and you get the bulk composition of the earth. Extract out excess iron and friends and you are left with the bulk silicate earth. Here’s a rough graph of the composition of bulk versus silicate earth.

Taking out large amounts of iron into the core, leaves the other elements in increased proportion. Note how only six elements (oxygen, magnesium, silicon, iron, aluminium and calcium) make up nearly 99 percent of the bulk silicate earth. A silicate is a compound that contains SiO_4,, so looking at the numbers, its no surprise that these are common. What’s that? You’re wondering if iron and magnesium oxides are common? Oh yes indeed. The earth’s mantle (the vast majority of the silicate earth) is made of peridotite** which is made of olivine ((Mg,Fe)₂SiO₄) and orthopyroxene ((Fe,Mg)SiO₃) plus other minor minerals that contain calcium and aluminium.

Melting the mantle

Picture your favourite rock. Unless you are odd, its not peridotite. So where do the pretty rocks come from? They are found on the earth’s continental crust, which is volumetrically unimportant, but much more varied than the mantle. Here a whole range of chemical processes are active: weathering, biological activity, metamorphism. I’ll stick with just one as it made the rock in my hand*** and it is how crust forms from the mantle: melting.

The mantle melts for a variety of reasons (great overview here) and it is yet another process of chemical change. The proportions of the major elements are different between peridotite and the molten rock, plus the melt is richer in the minor elements, which aren’t particularly at home within peridotite. Melting and re-melting has allowed the continental crust to be enriched in the interesting 1% of elements and produce rocks very different from peridotite.

Note how oxides such as sodium and potassium which are negligible in the bulk earth are much more common in the continental crust. The same applies to most other rock-forming elements.

My rock, the red bars above, is a gabbro from Ireland which was melted directly from the mantle. It contains pyroxene, which handles the iron and magnesium, plus calcic plagioclase, which mixes the aluminium and calcium with silicate and a sniff of sodium. The gabbro now forms part of the continental crust and as it is eroded away it will end up enriching sediments and going through yet another cycle of chemical change.

Water, water everywhere

I’ll try your patience with a final wet coda. The magma from which my rock crystallised was pretty dry, but it intruded into wet rocks (metamorphosing sediments). After the magma crystallised, it cooled and water from the surrounding rocks crept in and created new wet minerals. Where did this water come from?

Water**** was driven off from the inner solar system by the early solar wind. It’s extremely volatile, so why is it on the modern earth? Over the whole earth there isn’t much (only about 500 parts per million) but its concentrated near the surface. One popular theory is that it came via meteorites – wet ones from beyond the snow line.

This is an extraordinary idea, especially when you consider how important water is. Despite being such as small proportion of the overall earth, water drives processes that influence the entire planet. Subduction, the process where oceanic crust (sometimes) sinks down to the base of mantle, is facilitated by water. Water is involved in the formation of eclogite which makes oceanic plates more dense and allows them to sink. It also drives mantle melting that forms continental crust that allows us to keep our feet dry. Venus lacks plate tectonics and is drier than the earth – is this explained by how many wet meteorites fell on one planet and not the other?

If a defining feature of earth is only here by chance, it certainly puts the search for ‘earth-like’ planets into context. When we find planets of earth size within the ‘goldilocks’ zone (of orbits that allow liquid water) slight differences in their history may mean they are still far from ‘earth-like’ in their ability to support life.

Whatever the events necessary to create life on earth, one of the things it does is make apple pies. My work here is done.

——————————————————————————————————————-

*being pure alcohol it’s nearer vodka than gin (juniper didn’t exist), which is a shame. No tonic in space either.

**olive and pyroxene are stable only near the top of the mantle. At greater depth and pressure other more exotic minerals (with similar chemistry) are stable. But that is another story

*** it’s made typing quite difficult, I should probably put it down now

**** I say water when I should perhaps use water/hydroxyl/hydrogen, but you’ll forgive the simplification, I’m sure

Picture of peridotite from the incomparable hypocentre on Flickr under creative commons.

This post draws a lot on the book Destiny or Chance revisited by Stuart Ross Taylor.