Andalucia: a history of stuff

Andalucia is a province in Spain, at the far south west of Europe. Its long and varied human history has seen it linked to the middle East, north Africa and the Americas. The creation of these links brought new foods, metals, diseases: new stuff into Andalucia. Sometimes the impact of arrival created ripples that reached out far across the world.

Geological history

Some 200 million years ago Andalucia was within a massive continent called Pangea, close to both Africa and North America. Slowly Pangea broke into pieces, a new ocean basin – the Atlantic – filling the gap as the Americas drifted away. Some 50 million years ago, Africa was pushed north in Europe creating a long mountain range. Andalucia was part of this. Mountains form when the crust is thickened, pushing rock into the sky. A similar process at the base of the lithosphere 1 also forms a thick balancing ‘root’. Under Andalucia, this thick root ‘fell off’2 and sank into the hot convecting mantle beneath.

Mountain belts are surprisingly fragile. The sudden removal of the heavy root caused the over-thickened crust to collapse, flowing sideways and bringing deeply buried rocks up to the surface3. Fragments of mantle called peridotite, not often seen at the surface, form brown mountains around the town of Ronda. The collapse of the mountain belt went so far that its centre is now the very western part of the Mediterranean, the Alboran Sea. The mountain became a great hole in the ground.

Most of Andalucia is made up of sedimentary or metamorphic rocks folded and twisted by these dramatic changes. An exception is the basin of the Guadalquivir river. Here the weight of the collapsing rocks pushed down the rocks to the north, making a depression that has filled with recent sediments – a feature called a foreland basin. The edge of this flat basin makes a clear line that is easily visible from satellite views of Southern Spain.

From Wikimedia http://upload.wikimedia.org/wikipedia/commons/5/53/Andalucia_satelite.jpg

Andalucia from Space. Image from Wikimedia 

People

People, at first hominins such as Neanderthals, have lived in Spain for over a million years. The little we know of prehistoric humans comes from their use of materials. First their gradually more sophisticated use of stone tools, then from about 5000 years ago the smelting of metals: first copper, then bronze (copper plus tin) and then iron. Mines in Andalucia have been been involved since the start, notably the Rio Tinto area. Here a Carboniferous massive sulphide deposit has yielded silver, gold and copper and spawned a global mining company.

Image of Rio Tinto mines, Andalucia. Image from David Domingo on Flickr under CC

Rio Tinto mines, Andalucia. Image from David Domingo on Flickr under CC

Around 3000 years ago (1100BC) the Phoenicians reached Andalucia, founding the town of Cadiz. A culture that reached across the Mediterranean they were also involved in trade with the British Isles. Tin from Cornwall in England was smelted with Spanish copper and the resulting bronze traded on. The olive tree reached Spain at this time, brought from the eastern Mediterranean.

A mere thousand years later, the Romans took control – their province of Hispania Baetica covers much of modern-day Andalucia. They introduced deep mining to the Rio Tinto, using the characteristically Roman combination of slaves and very big wheels to pump water up from the depths. Andalucia was a renowned source of many products for the wider Roman Empire, including silver, olives, emperors, philosophers, dancing girls, and garum, a sauce formed from fermented fish guts rich in umami flavours.

Yet Moor invaders

After the slow collapse of the Roman Empire, the next major influx of change in Andalucia was the Moorish invasions. The Islamic Moorish army4 conquered most of Spain between 711 and 718 AD, in time creating a kingdom of Al-Andalus with its capital in Cordoba. Once more Andalucia was part of a multi-national empire with good trade links. Valuable crops from further east, such as figs, citrus and pomegranate were introduced for the first time, as were sophisticated irrigation systems, some of which are still in use. By the 10th century, Cordoba was the most civilised city in Europe, it’s Grand Mosque was one of the wonders of the Muslim world.

Cordoba Grand Mosque

The site of the Grand Mosque was originally a Christian cathedral (before that, a Roman temple). Built over 200 years and funded in part by mining proceeds, the Mosque was built with local stone and brick, but also recycled Roman stone columns. Some of these were found locally, but others were brought in from much further afield, from the rest of Spain and perhaps wider.

The Columbian exchange

Eventually Moorish Spain came to an end as Christian rulers conquered the Muslim lands. The final Moorish kingdom of Granada fell to Ferdinand and Isabella in 1492, the same year that they sponsored Christopher Columbus to mount an exhibition across the Atlantic. As  the Spanish kingdoms turned into a global Spanish empire, lots of incredible things started flowing back into Andalucia. First Seville and later Cadiz were the main ports for the Atlantic trade. Gold and silver beyond imagining passed through Andalucia – enough to create a century of inflation across Europe. Some of this stuff ended up in the the Grand Mosque in Cordoba, which is now a cathedral again. Sitting within the vast pillared area of the mosque is a Christian church full of beautiful things made of American gold and silver. The choir stalls are made of American mahogany – lots of plant material crossed the Atlantic too.

Ingot of South American silver as brought over by Spanish treasure ships. Cadiz Museum.

Ingot of South American silver as brought over by Spanish treasure ships. Cadiz Museum.

A popular Spanish dish is called ‘patatas bravas’ and consists of potato, tomato and chili – all foodstuffs that spread across the world following the ‘discovery’ of the Americas. Andalucia was the first stop for many of these vegetable treasures. Botanical gardens turned seeds into plants, to be studied and propagated. A fine building in Seville is the old tobacco factory, dedicated to processing another new crop. The setting of Bizet’s opera Carmen, its walls are mottled with yellow and brown, like a smoker’s fingers. More immediately bad for the health, syphilis was first recognised in Europe in 1494, most likely brought back by Columbus’ sailors.

The British dimension

As an Englishman who likes history, I often visit other countries in an apologetic mood. A dimly remembered story about Francis Drake daringly ‘singeing the king of Spain’s beard’  is rather less jolly when you are sitting in Cadiz, the town that was attacked. Drake was engaged in warfare on behalf of his Queen, but also behaving like a pirate, raiding Spanish treasure ships. Still, no one seems to mind any more; it was in 1587 after all.

The British drink everything and anything. Not content with home-grown beer, gin and whisky we also crave grape-based booze. When Francis Drake returned from attacking Cadiz, he brought 2,900 barrels of sherry, a type of wine made only near Jerez in Andalucia. This went down rather well – we’ve been drinking it ever since, even getting involved in its manufacture.

Tonic water is sweet fizzy water flavoured with quinine, best taken with gin. Quinine, an extract from a South American tree was for centuries the only effective way of countering the effects of malaria. First popularised by Spaniards returning from Peru it was introduced to gin by the British in India. In the early 18th century the Royal Navy had a number of bases on Spanish soil, including in Andalucia, and passed the habit on. It remains popular in Spain to this day.

Image from Amanda Slater on Flickr under cc

Image from Amanda Slater on Flickr under cc

A final anglo-Spanish connection is marmalade. Seville is full of orange trees, of a particular kind, bitter and rich in pectin. They are pretty inedible raw, but for some reason 5A Scottish tradition and industry grew up of preserving them as jam, with pieces of the skin floating in jars of pungent and yielding orange delight. Paddington Bear, James Bond and Alice in Wonderland all eat marmalade, along with many real people, such as me. The bitterness gives it a very grown-up feel, with some of the grimy delight of cigarettes and whisky, but in a healthy breakfast-friendly form.

The future

What of the future? Recent research suggests that soon6The oceanic crust beneath the Atlantic will start to plunge down under Spain into the earth’s mantle. The collapse of the mountain belt I mentioned at the beginning left a tear in the crust and this may grow and extend into a full-blown subduction zone. This will bringing volcanoes to fertilise the soil with ash and earthquakes to shake the buildings (if any remain). As slowly as finger-nails grown the Atlantic will vanish and Spain and the Americas will be reunited once again.

Story of an atom: diamond

This is the third part of a story told to me by a Carbon atom in my brain. It started with her tale of how she ended up on earth, followed by an inside view of the Carbon cycle.

So like I said, I’ve fallen into this pattern of cycling around different places, occasionally going underground for a bit. One time I got buried was really special. Just… just a really *deep* experience, you know?

It started off as normal, I was in a bunch of organic matter that’d settled onto the bottom of the sea. Things got slowly hotter and more squashed, but this time it just didn’t stop. Things started falling apart – everyone got antsy and wanted to move around, some atoms badly wanted to escape. The organic gunk I was in broke down. Lots of water started coming out and disappearing upwards, eventually there was just us carbon atoms left. Lots of other atoms around us too, of course, lots of silicate minerals – big structures dominated by Silicon and Oxygen, all in rigid ranks. It got so hot and pressured that even they couldn’t cope and had to start rearranging themselves to get more comfortable. Some minerals just disintegrated and the atoms had to find other minerals to join1. I’d never experienced this before – I’d never been  buried so deep.

You know when you’ve drunk too much coffee and you’re stressed? You feel like you’re vibrating really fast but you’re quite uncomfortable and you feel stuck. Same with us atoms in the deep earth. One way to cope is to try and help each other. Take us Carbon atoms in that organic lump. At first we were just jumbled up in a heap, but eventually we sorted ourselves out a more comfortable arrangement – we got together in lots of groups of six, all joined up but flat. If we stacked up like this then things weren’t too bad 2. But it just kept getting more extreme. Eventually every single mineral changed, even quartz – I’ve never seen that before or since 3.

Things kept getting more extreme. All around us atoms kept shuffling around into new configurations that were more comfortable. Hydrogen was very uncomfortable, it became harder and harder for it to find a place in these new arrangements of Silicon and Oxygen and more and more it joined with Oxygen and disappeared upwards. Eventually, some of us Carbon atoms broke up and starting moving upwards in a liquid4.

We travelled a long way like this (but were still very deep) when suddenly: it happened. There was this crystal of carbon – the most amazing thing. You call it a diamond. I’d always been a bit snooty about atoms all locked together in a mineral – fancied myself as a free spirit. But I so wanted to join in this crystal of just carbon. The other Carbons beckoned me in and at first I couldn’t see how it would work. When I’ve been with carbon before, I was joining with 3 other carbons in a flat plane. In this crystal all 4 of my bonds were joined, each with another Carbon. Not just flat too. I only managed to squeeze myself in because we were so tightly squashed together and buzzing around so much 5.

Once I was properly in there, I didn’t mind the conditions. We felt so strong, all together, bound so tightly. After a while I started to lose myself – no more ‘me’, only ‘us’. I was merging myself into a greater thing. One great collective of Carbon, perfectly happy in one eternal unchanging moment….

Sorry, drifting off there. Amazing times, so special. It couldn’t last though: diamonds aren’t forever, not really. The first sign of trouble was when the rock around us started to melt. We ignored it, but suddenly the whole area around us started shooting up through a big crack! The pressure dropped incredibly quickly – we were in a panic because it felt like we might start breaking up – could we stick together in these new conditions? Luckily we quickly cooled as well, making it easier for us to stay together6.

We soon got used to the new conditions – we were still underground after all – and we remained strong, ready for anything. It was a rude surprise when the rock around us got crushed up and we saw daylight for the first time. We were having a lovely time bouncing those photons about through us when this human hand grabbed us and put us in the dark again.

We were still together through all of this, which made what happened such a sudden shock. We were whipped out of the bag and put in a funny metal box. All the air around us disappeared and then ZAP! A huge beam of light hit us. Hit me! There was so much energy that a bunch of us got blown apart, all our bonds broken. There I am, alone and floating in space, just I was in my first memory. I soon hit some weird thing and ended up back here on the surface, going through the same old cycles7. For a while it all seemed so shallow, so temporary, so lonely. I’ve talked to other Carbon atoms about it, but none of them know what I’m talking about.

Still it’s been nice talking to you about it. It looks like you’re about to break up this molecule I’m in, so I’ll be off soon, back out into the atmosphere. Who knows where I’ll end up next!

No atoms were harmed in the making of this story.

Exciting extraterrestrial eclogites

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:

A deeper look at the geology of diamonds

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