Cratons – old and strong

Posted on 19 December, 2012 by Metageologist

Cratons are pieces of continents that have been stable for a over a billion years. As earth’s plates drift along, mountains periodically rise and fall, plate boundaries appear and disappear. But cratons are like great-grandmothers at family gatherings, while younger crust moves excitedly around them, they sit quietly, occasionally remarking on how different things were when they were young.

Every continent has cratonic areas, notably the core of North America, Scandinavia, Siberia, India and most of Australia. They may be covered by a thin shawl of sediment, but often they expose ‘basement’ rocks such as gneiss. Economically they are very important – most of the world’s diamonds come from cratonic areas as do many other valuable deposits.

Cratons are stable because they are strong. The geology of the Himalayas illustrates this – the modern day plate boundary between Indian and Asia is at the southern edge of the Himalayas. The cratonic Indian plate is barely deformed, in great contrast to the vast pile of deformed soft young crust in the Tibetan Plateau to the north.

Cratonic crust is strong, being unusually cold and dry, but that is only part of the picture. Continental crust is the upper portion of continental lithosphere. It’s lithosphere that puts the plates into plate tectonics, it’s a rigid layer on the earth’s surface, as opposed to the hot flowing mantle that lies beneath (the asthenosphere). Lithosphere consists of the crust and an underlying layer of mantle that has become ‘stuck on’. This layer of lithospheric mantle can be 2 to 3 times thicker than the crust above and so contributes a great deal to the strength and stability of continental lithosphere.

It turns out that the lithospheric mantle beneath cratons is unusual. In oceanic crust, as it ages it cools and eventually it sinks down into the mantle again. It is thought that continental lithospheric mantle can fall off as well, but in cratons this doesn’t happen. Finding out why is an interesting challenge but before we start that journey, a brief interlude….

Geochemical interlude

My feelings about geochemistry have changed over time. As a doctoral student I was exiled out into an annex 10 minutes from the main department because a new piece of geochemical apparatus required a lab to be expanded, swallowing our little rock-filled office. I could relate to metamorphic petrology and analysis of mineral chemistry, but the isotope stuff, seemingly requiring months of juggling Hydrofluoric acid to get a couple of data points, left me cold. Years later, with a broader, wiser perspective I find myself marvelling at the way isotope geochemistry leads to so much awesome science.

There are two flavours of isotope geochemistry, stable and radiogenic. Many chemical isotopes are unstable and over time spontaneously turn into different elements, the bits left over being thrown out as matter and energy – radiation. For geologists, this real-life alchemy, known as radioactive decay, is mostly used to tell how old something is. The rate of radioactive decay is constant, so measuring the current abundance of isotopes and working backwards tells you the time elapsed since an event happened. Getting from measurements to age involves assumptions about the sample which may be incorrect – my own research successfully predicted a published age was incorrect. Nevertheless modern geochemists have a wide range of extremely robust and accurate techniques at their disposal. Datable events include a crystal forming from a magma or growing in a metamorphic rock, a surface being exposed to the atmosphere, a critter growing, a mineral cooling below a particular temperature, when magma was extracted from melting rock, even the age of ground water.

Stable isotope geochemistry relies on the fact that two isotopes of the same element, for example Hydrogen and Deuterium, are not precisely the same. Most physical and chemical processes treat them identically, but some do not, the slight different in mass means slight differences between isotopes. For example during evaporation of water the lighter H₂¹⁶O will vaporise first, creating a difference in isotopic composition between the water vapour and the remaining liquid. This phenomena of isotopic fractionation affects water in other ways, such that there is a relationship between fossil oxygen isotopes (in ice or fossil sea shells) and the temperature of the sea they came from. This tremendously useful technique is just one example of the seemingly magical way tiny differences in the physics of atoms can shed light on diverse geological problems. There are many other applications, but the technique shines most when we have very little other evidence to go on. Research into the formation of the earth and moon relies heavily on studies of arcane isotopes. We have only limited information about lithospheric mantle from beneath cratons (remember them?) and isotope geochemistry tells us many interesting things…

Back to the cratons

A recent review by Cin-Ty A. Lee, Peter Lufﬁ, and Emily J. Chin of Rice University focuses on the creation and destruction of continental mantle (I’ll mostly discuss creation).

If lithospheric mantle beneath cratons was the same composition as the rest of the mantle then over time it would cool and become denser and unstable. A consensus has emerged that it does not because at some point in the past it was involved in melting and up to half of its volume was removed. The rock left behind after the melt flowed away is known as depleted peridotite and it is stronger and more buoyant than normal mantle, so it remains stable for billions of years.

Fragments of mantle peridotite in lava.

Pieces of continental lithosphere do reach the surface, sometimes as huge slabs such as at Ronda in Spain. Some weird volcanic deposits start at mantle depths and bring up pieces of the surrounding rocks. Sometimes they contain diamonds, but also pieces of the lithospheric mantle. Armed with these samples, geochemists have applied a massive array of techniques.

The geochemistry of melting rock is well understood and it allows us to infer many things about the conditions under which it happens. For example comparing ancient with modern samples, we can tell that older samples melted at higher temperatures (1,500 to 1,700◦C 1,300 to 1,500◦C) and saw more melt extraction (50 to 30% versus <30%). This is consistent with the fact that the early earth was hotter.

Melting is affected by pressure as well as temperature – deeper mantle rocks contain different minerals (at these pressures, garnet at depth, spinel above) which melt in subtly different ways. Cratonic peridotites melted at shallow depths (90km) but were later moved deeper (180km).

Not all samples are of peridotite, next common are pyroxenites, sometimes referred to as eclogites. Chemically these are similar to basalts formed at modern mid-ocean ridges. Their oxygen isotopes show wide variation, such as is seen in oceanic crust following hydrothermal alteration. Other physico-chemical processes could in theory cause similar patterns, but it seems only low-temperature processes explain the patterns seen. The fact that they were once part of oceanic crust makes these pyroxenite rocks very different from the peridotites (which have only ever been part of the mantle).

Further evidence that pyroxenites were once near the surface comes from sulphur isotopes from eclogitic inclusions within diamonds. Fractionation of these isotopes occurs in ways only explained by chemical reactions high in the atmosphere. These diamonds with eclogitic inclusions themselves have carbon isotopes that are extremely ‘light’. Light carbon is a sign of life – organisms preferentially contain light carbon. In these ancient rocks, life probably means bacteria. So we can lengthen the list of “lovely things bacteria help to make” – cheese, soy sauce, wine, yogurt, healthy digestion and now certain types of diamonds.

NB for visitors from the German wikipedia page on Diamonds, I’m delighted you’re here. Here is a better reference for the science on this.

This is a wonderful thing. A bacterial mat sitting on the sea-floor billions of years ago ends up being stuffed down into a subduction zone. It enters a world of crushing pressure and extreme temperature where it is transformed utterly, its carbon forming a diamond. Later a weird eruption happens and the diamond is squirted back up to the surface for us to marvel at. Only isotope geochemistry (and some generous assumptions on my part) allows us to tell this story.

How does continental mantle form?

Our authors discuss how continental mantle forms (spoiler: we don’t really know). One model is that a large plume of hot mantle material rises up underneath a continent. This would indeed form lots of depleted peridotite (underneath a big pile of lava). However this model is inconsistent with the evidence described above – melting would be expected at greater depths (200km) than is seen.

A plume model also fails to explain the eclogitic parts of the mantle. One mechanism to explain how portions of oceanic lithosphere end up in the sub-continental mantle is that when subducted instead dropping steeply down they remain buoyant and stack up beneath the continents. In this model, the sub-continental lithosphere grows by progressive capturing and stacking of oceanic lithosphere. Its been argued that parts of the Farallon plate were captured beneath western north America in recent times. Hotter oceanic lithosphere (common in the Archean) subducts at a shallower angle and so is more likely to end up getting stuck to the base on the contintent.

This model explains the evidence for shallow melting and subsequent burial, plus the presence of former oceanic lithosphere. Indeed it predicts a larger proportion of eclogitic pyroxenite than is observed. Our authors propose a process of “viscous drainage” whereby the crustal portion of the stacked ex-oceanic lithosphere (now dense inclined sheets of garnet pyroxenite) slowly ‘drains’ down and out, leaving the peridotite ‘framework’ behind.

Models of the creation of continental crust emphasise the importance of island arcs. An oceanic island arc (where oceanic crust, subducts under oceanic) may collide with a small continent, turn into a continental island arc and ultimately become a new piece of continental crust. This mechanism will not give thick lithosphere on its own however – so some process of ‘orogenic thickening’ (squeezing it so it’s thicker) is required.

You will perhaps have sensed that the paper drifts into speculation a little. This is appropriate in a review paper and inevitable as we are on the ragged edge of what we can know about these rocks, so distant in space and time. The paper ends with a ‘future directions’ section full of as yet unanswered questions.

“How do continents form?” is a simple question to ask but we still can’t give a complete answer to it. We still don’t know the secret to great-grandma’s longevity.

Peridotite picture from dun_deagh on Flickr under Creative Commons.

Craton map from Pearson and Wittig under Geological Society of London fair use policy.

PEARSON, D., & WITTIG, N. (2008). Formation of Archaean continental lithosphere and its diamonds: the root of the problem Journal of the Geological Society, 165 (5), 895-914 DOI: 10.1144/0016-76492008-003
Lee, C., Luffi, P., & Chin, E. (2011). Building and Destroying Continental Mantle Annual Review of Earth and Planetary Sciences, 39 (1), 59-90 DOI: 10.1146/annurev-earth-040610-133505

Eclogites: back to the surface

Posted on 24 October, 2012 by Metageologist

Eclogites are beautiful rocks that form deep within subduction zones. The vast majority of subducted oceanic crust becomes more dense than the surrounding mantle rocks and travels to the strange world of the deep earth. Lucky for us, small volumes make it back to the surface. How does this happen? As so often in the Earth sciences, a combination of detailed field work, laboratory analysis and modelling yields many insights.

ZS Unit: valtournanche Garnet omphacite + late Gln rims around grt. All images in this post courtesy of Samuel Angiboust.

To the Alps

The Alps are a major mountain belt stretching across Europe. They are the result of the closure of the Tethys ocean which used to separate Africa and Europe. The details are complex – oceanic crust remains in the Mediterranean and there are small fragments of continental crust in the mix too. In the western Alps, stretching through Switzerland, Italy and into Corsica, there are large areas of oceanic crust (known as ophiolites). These ophiolites were subducted 50 million years ago and turned into eclogites. They now sit on the surface sandwiched between other less exotic metamorphic rocks, firmly part of the continental crust.

Very recently these rocks have been studied by Samuel Angiboust who (with others) has produced a series of papers. I’ve written about one before, concerning traces of ancient earthquakes. He’s kindly provided me with some photos, so I can give you a view of the rocks before I describe his other research.

Ophiolites are interesting as they allow us to study oceanic crust without getting our feet wet. These Alpine ophiolites have another added layer of interest – they have also been metamorphosed in complex ways. Here’s an example of what I mean. Ophiolites typically contain parts of the top layer of oceanic crust, ancient lava flows made of basalt that formed pillow shapes as it cooled underwater. Here are some from the Alps.

Eclogite facies pillows, reddish radiolaritic mud between them Monviso Unit (70km depth!)

Eclogite facies pillow basalts.

The structure is unremarkable – pillowed lobes with thin layers of sediment (radiolarian chert) in between. What is special is that this rock has been buried 70km under the surface. Every mineral grain in it has been recrystallised – nearly every atom in these rocks moved as the rock was utterly transformed under the new high pressure conditions. This movement of atoms is only on the millimetre scale (or less); the edge between sediment and basalt is still sharp. The rock is superficially the same yet utterly transformed.

There is no obvious sign of deformation in this outcrop either, despite them being shoved down into the earth and pulled back out again. Much of the ophiolite is extremely deformed, but little patches of calm often remain even among the most tormented rocks.

Slabs or Knockers?

How do high-pressure rocks get to the surface? One possibility is that eclogite rocks are broken into fragments and somehow squeezed to the surface as tectonic blocks. Some models of subduction zones include a channel of soft serpentinite immediately above the subducting plate. In such a ‘mélange’ rigid pips of eclogite may be squeezed towards the surface. Such things do happen: accretionary wedges, the piles of sediment that form above subduction zones, often contain ‘knockers’* of high-grade rock (including eclogite).

Is this how our Alpine rocks got back up? How could we tell? Angiboust and colleagues look at the record of metamorphism. There are large joined up areas of eclogite, cross-cut by serpentinite shear-zones. If the eclogite came up as pips then each pip would have a different metamorphic history and have reached different depths in the subduction zone.

Metamorphism

In a 2011 paper, available for free Angiboust and others summarise a mass of data from the Monviso Ophiolite, found along the French-Italian border. They found a wide range of metamorphosed rock types, from limy muds (now calc-schists), mudstones (now metapelite) to meta-basalt and meta-gabbro. This allowed them to apply a wide range of techniques to estimate to conditions under which these rocks were metamorphosed.

Eclogite facies Mg-Al metagabbro, Monviso ophiolite. Green patches are Cr-rich pyroxene (omphacite).

Metamorphic petrology relies on the fact that changing conditions of temperature and pressure transform the minerals present in rocks in predictable ways. Our authors used THERMOCALC, which combines our knowledge of the thermodynamic properties of rock-forming minerals with analysis of the composition of a rock sample and its minerals to estimate to conditions under which those minerals formed. (For more details on what that sentence means, see my posts on metamorphism).
A technique new to me was Raman Spectroscopy of Carbonaceous Matter (RSCM) that analyses the degree to which carbonaceous material (boiled up fossils) has been converted to graphite. The conversion to graphite depends only on the maximum temperature reached – unlike most other ways of estimating peak metamorphic conditions, it can’t be reset and doesn’t depend on pressure. Very neat.

Eclogite facies Mg-Al metatroctolite, Monviso Dark chloritoid, green omphacite, pink garnet, white talc (where olivine was. See Messiga et al JMG 1999).

The point of all this painstaking work is to estimate the most extreme conditions reached by these rocks, to discover how deep they reached. By studying over 60 rock samples they were able to distinguish two domains, within which the peak metamorphic conditions were extremely consistent. The deeper Lago Superiore Unit reached conditions of 530–550°C and 26–27.5 kbar, corresponding to burial to 80 km depth. All of this unit reached the same depth, suggesting it hasn’t been broken up since – this is no pip, but a large slab of rock (20–30 km-long, 2–3 km thick) that “corresponds to a more or less preserved portion of thinned oceanic crust detached from 80 km depth in the Alpine subduction zone“.

If this is the case, then how did it get back up to the surface? One reason might be that it was quite wet.

Hydrothermally altered eclogite (Aosta Valley, Italy, Zermatt Saas Unit) With garnet, chloritoid (black), talc (whitish). Enrichment in Fe-Mg (+ pyrite) Due to seafloor hydrothermal alteration led to this very unusual Paragenesis (sample Angiboust, photo Marco Moroni, UPMC). This rock sample belongs to a fossilized hydrothermal system. Fossilized eclogitized “black smokers” can be occasionally seen on the field in Aosta valley (see Martin et al., 2008)

The role of water

In another recent paper, Angiboust and others discuss the influence of water in eclogites. As newly created oceanic crust cools it can take up a lot of water from the overlying ocean. This is bound up within minerals and so is taken down into the subduction zone. Looking at the Zermatt–Saas ophiolite (Switzerland/Italy) they identify portions of oceanic crust that were extensively affected by hydrothermal processes, perhaps because they formed at a slow-spreading ocean ridge. Now eclogitised, these have between 2.5 and 6% H2O (by weight) which is unusually high. Eclogites more usually have between 0 and 1 %.

All this water affects the minerals that are stable under eclogite facies conditions. Glaucophane is the mineral that puts the blue in blueschist facies – it’s normally stable only under lower pressures. In these ‘wet’ eclogite rocks it remains stable.

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.

Lawsonite and phengite are other water-bearing minerals stable in these rocks. All of these water-bearing minerals are significantly less dense than more typical eclogite minerals such as garnet and omphacite. Based on detailed analysis, our authors estimate that these hydrothermally altered rocks are 5-10% less dense than normal eclogite. These are crustal rocks – the mantle rocks beneath were also pumped full of water turning them into serpentinite. Serpentinite is olivine plus water and about 15% lighter than ‘dry’ mantle rocks.

So overall, hydrated oceanic lithosphere is lower density than both ‘normal’ lithosphere and the surrounding mantle rocks. This keeps it buoyant. This is important as these rocks nearly reached the depth at which normal eclogite becomes denser than the surrounding mantle, at which point subduction becomes irreversible. In these ‘wetter’ rocks low-density hydrated minerals remain stable at higher pressures, so the entire slab is harder to subduct.

Back to the surface

A clue to how these Alpine Ophiolites got back out of the subduction zone comes from the wider geological context. They are now stacked up alongside continental rocks that also reached high pressures. Soon after our ophiolites were subducted the ocean basin closed and continental crust starting being stuffed down into the subduction zone, reaching depths of 80km. This continental subduction became an unstable situation, as continental crust is much less dense than oceanic material. Subduction stopped and the deeply buried rocks were rapidly exhumed, making their way back to the surface.

Diagram showing sequence of actions leading to ophiolite slices returning being attached to buoyant continental crust.

Our authors propose a two-stage process. Firstly our ophiolite slices became detached from the sinking oceanic plate. The plate was not smooth and had variable thickness, making this mechanically plausible . They see the detachment being associated with large earthquakes, that would have created tsunami washing across the shores of the Tethyan ocean basin. The next stage is where the thin leading edge of the continental crust is thrust underneath our ophiolite slices. This mixed buoyant material then rapidly returns to the surface, bringing the ophiolite up with it.

This mechanism is a little elaborate, with many particular circumstances being required. However the authors emphasize that the preservation and exhumation of such a big fragment of oceanic lithosphere would have been possible without the mechanical intervention of underlying, buoyant continental crust. That’s why Alpine ophiolitic rocks (and involved volumes) are so unique to understand the steady-state processes potentially occurring at depth in active subduction zones. In his current research, Angiboust is finding evidence for similar large tectonic slices within eclogitic ophiolites in Iran.

The overwhelming majority of subducted oceanic crust is lost forever into the mantle. The story of how eclogites avoid this fate is fascinating and surely not yet fully understood. I’m sure the study of these beautiful rocks will yield more surprises in the future.

*Clearly these weren’t named by an English person, who whom the term ‘knockers’ would have other connotations….

Many thanks to Samuel Angiboust, for the pictures and for providing useful feedback.

References

Angiboust, S., & Agard, P. (2010). Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel? Lithos, 120 (3-4), 453-474 DOI: 10.1016/j.lithos.2010.09.007

Angiboust, S.,, Agard, P.,, Raimbourg, H.,, Yamato, P.,, & Huet, B. (2011). Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps) Lithos DOI: dx..org/10.1016/j.lithos.2011.09.004

Silvana Martin, Gisella Rebay, Jean-Robert Kienast, & Catherine Mével (2008). AN ECLOGITISED OCEANIC PALAEO-HYDROTHERMAL FIELD FROM THE ST. MARCEL VALLEY (ITALIAN WESTERN ALPS) Ofioliti : doi:10.4454/ofioliti.v33i1.359

MESSIGA, KIENAST, REBAY, RICCARDI, & TRIBUZIO, . (2001). Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy) Journal of Metamorphic Geology, 17 (3), 287-299 DOI: 10.1046/j.1525-1314.1999.00198.x

Oceanic crust – down to the core

Posted on 25 September, 2012 by Metageologist

Almost all of what I write about in this blog concerns only 1% of the earth’s volume. All crust, all sedimentary rocks, the glories of mountain building, all occupy an insignificant portion of the earth. It’s the only bit we can get to – in geology, we are the 1%. This post is all about the 99%, the earth’s interior, the mantle and core, which is remarkably different to here. The inner core reaches 5430°C (as hot as the surface of the sun) and pressure is multiple millions times atmospheric pressure. Forget your Jules Verne, we are never going to reach the centre of the earth*.

We aren’t isolated from the interior though, it affects us in many ways – from the earth’s magnetic field to plate tectonics and volcanic islands. The influence goes the other way too. Oceanic crust is constantly being pushed down into the mantle (maybe 20% by volume over time) and it goes a surprisingly long way. Rocks that form part of the familiar 1% start on the surface but travel down into the extreme conditions of the deep interior.

Cross section of earth’s interior. Image from Wikipedia

Deeper and deeper

In a previous post, we traced the journey oceanic crust takes, from its birth in a mid-ocean ridge down into the mantle in a subduction zone. We left the story at 250km depth.

Here our crust has been transformed. Its minerals have changed into new ones, stable at these depths. Metamorphism and partial melting have taken away most of its water and some other elements that have large ions. It consists largely of garnet and pyroxene. The material surrounding it, called peridotite may in addition contain olivine. As it descends further, at around 2cm a year, our plate continues to change.

A huge amount of what we know about the deep earth comes from listening to earthquakes. Seismologists can identify the location of earthquakes within the sinking slab. This pattern of earthquakes (called the Benioff zone) shows the shape of the subduction zone. Seismic waves can be used in multiple ways to identify the properties of the materials they pass through. The liquid outer and solid inner core were identified this way in the 1930s.

We can also now spot sudden changes in the speed at which the waves pass (seismic velocity). This implies a sudden change in the properties of the rock. In the mantle, there are multiple places where we see such a change. They are known as discontinuities and they occur at specific depths.

The shallowest mantle discontinuity is at 300km depth. It’s only seen intermittently and is most common beneath continents and island arcs. It’s been interpreted as a phase change in minerals made of SiO2 (silica). At the surface, the stable form of silica is quartz, but at depth it is transformed, first into coesite (discovered 1953) and then, at 300km depth into a mineral called stishovite (1961). Stishovite is dramatically denser than coesite so the transformation from one into the other changes the rock properties, making it visible to seismologists. Nearly all rocks contain SiO2, but in the mantle it is all locked up into other minerals. Only subducted oceanic crust has enough SiO2 to contain ‘free silica’. For this reason, the existence of the 300 km discontinuity in a particular place is evidence for the presence of subducted oceanic crust [1].

There is not much free silica even in our sinking slab of crust, so as it passes through the 300km discontinuity the transformation is relatively minor. Bigger changes await.

Into the lower mantle

At 410km we are leaving the upper mantle and entering the transition zone, marked by several seismic discontinuities. Here were are reaching pressures of 24GPa (250,000 atmospheres) and temperatures of 1600°C. Now olivine is no longer stable. It transforms itself into other minerals with the same composition (polymorphs), first wadsleyite (1966) and then ringwoodite (1969).

These minerals with the funny names aren’t found in crustal rocks, aren’t stable under conditions humans can reach. The first line of evidence came from meteorites. Most meteorites are pieces of a rocky planet that formed a core and mantle but was then smashed into pieces by the hurly-burly of the early solar system. Both wadsleyite and ringwoodite were both first recognised in meteorites and are named after Twentieth Century scientists. Samples from meteorites give us an insight into high pressure conditions and a rare opportunity to find actual samples of them. The only other place is tiny pieces inside diamonds.

Blue Ringwoodite. Why are all high-pressure minerals beautiful colours? Image from Wikipedia

At a depth of 650km there is a significant discontinuity that marks the beginning of the lower mantle. This is the point where most of the minerals mentioned above are no longer stable. Under the intense pressure minerals with extremely dense, tightly-packed mineral structure are formed. Here Ferropericlase (aka Magnesiowüstite) and Silicate perovskites become the dominant minerals. Note the silicate perovskites have the same mineral structure as perovskite (a calcium titanium oxide) but a different composition.

It used to be thought that sinking crust couldn’t get through the 650km discontinuity. Maybe some slabs don’t, but ours does, continuing its journey into the lower mantle.

Much of what we know about the lower mantle comes from experimental work. In order to simulate the crushing pressures and high temperatures within the deep earth, experimenters have to take tiny samples and squeeze them between two diamonds and blast them with frickin’ lasers. These ‘diamond anvils’ allow scientists to peer through to materials under deep-earth conditions, studying the minerals that form there. Only recently have scientists managed to create diamond anvils able to achieve conditions within the deepest parts of the earth. This brings home quite how extreme and alien the deep earth is.

Diagram showing conditions in deep earth, plus conditions reproducible in a Diamond Anvil Cell. Taken from http://www.earth.ox.ac.uk

Research on the deepest earth is cutting edge and not yet settled so the story of our plate’s progress becomes a little vague. A recent addition to the menagerie of extreme minerals is post-perovskite (2004) a mineral stable in the lowest 200 km of the mantle, a material so exotic it doesn’t yet have a proper name (being just ‘stuff perovskite turns into’). It’s been linked to the deepest seismic discontinuity, the D” (D double prime) which is a layer just above the core-mantle boundary. This mysterious boundary zone may be linked to the fate of our oceanic slab. As our crust ends its long journey it encounters a boundary it cannot cross. The core is made of metal, which doesn’t mix with our silicate crust, so the slab ends up lying flat on the base of the mantle, forming the D” layer.

Everything is connected

In one of earth science’s many dizzyingly lovely links between different domains, the very deepest layers of the mantle have dramatic influences on the earth’s surface. Firstly, large igneous provinces – big areas of vulcanism not obviously linked to plate tectonics, seem to originate in mantle plumes that start at the core-mantle boundary (these in turn may cause mass extinctions). One theory is that the presence of old oceanic crust leads to the instabilities that drive plume formation [2]. Here geochemistry, specifically isotope geochemistry, is our tool for understanding places we can’t reach.

Another link between the surface and the deep relates to the magnetic field that helps homing pigeons and old-fashioned geologists find their way. Some researchers speak of hot dense “thermochemical piles” created by deep subduction which influence heat loss from the outer liquid core. This core creates earth’s magnetic field and a recent paper links patterns of magnetic polarity reversals with these piles and so with patterns of deep subduction and plume formation.

The reality of deep subduction seems clear, even if the details are still coming into focus. A recent comparison of numerical modelling of subduction with cutting edge seismic imaging (seismic tomography) shows a good match [3] – oceanic plates really are going all the way down.

Although the deep earth seems an alien place, turning familiar minerals into substances we are only starting to understand, still it is intimately related to our surface world. A lump of basalt that forms at a mid-ocean ridge, that gets subducted and transformed into new minerals again and again, may ultimately rise up as part of a mantle plume and get melted, ending up as a new form of basalt. Crust and mantle are intimately linked – the 1% and the 99% together make up a single planet, after all.

* I say never, but it may be possible, provided you got hold of a nuclear weapon and a hundred thousand of tonnes of molten iron.

References

Web-links

[1] Evidence for 300km discontinuity (abstract only)

[2] Link between oceanic-island basalts and deep mantle (pdf of paper)

[3] Deep subduction – comparison of geodynamic and tomographic models (open source paper)

Old-skool links

Q. Williams, & J. Revenaugh (2004). Ancient subduction, mantle eclogite, and the 300 km seismic discontinuity Geology DOI: 10.1130/G20968.1

W. M. White (2010). Oceanic Island Basalts and Mantle Plumes: The Geochemical Perspective Annual Review of Earth and Planetary Sciences DOI: 10.1146/annurev-earth-040809-152450

B. Steinberger, T. H. Torsvik, & T. W. Becker (2012). Subduction to the lower mantle – a
comparison between geodynamic and
tomographic models Solid Earth Discuss DOI: 10.5194/sed-4-851-2012

Oceanic crust – that sinking feeling

Posted on 9 September, 2012 by Metageologist

Some rocks lead a quiet life. Stable parts of continental crust just sit there for billions of years, doing nothing. In the oceans things are much more dynamic. Live fast, die young, stay pretty is the motto of oceanic crust. It goes on one of the most amazing journeys rock can take. Along the way it affects well nigh everything in and on the planet. Let’s tag along.

Making the crust

Mantle material, usually made of a dark heavy rock called peridotite, is odd stuff. Whenever I’ve seen it on the surface it always looks out of place somehow, too homogeneous, too brown, too massive. Yet it forms over 80% of the earth’s volume – excepting the metallic core it is what earth is made of. Found everywhere immediately below the crust, from between 5 to 75 kilometers depth, it is very very hot (> 1000 °C) – hot enough so that it flows (slowly to us, but fast on geological timescales) but still remains solid. The top, “lithospheric”, layer of mantle has ‘frozen on’ to the crust, becoming part of a rigid plate. The rest makes up the “aesthenosphere”, where the mantle is constantly flowing and mixing, like hot soup in a saucepan.

Our little piece of oceanic crust is about to be formed at a mid-ocean ridge, where two plates move apart. This makes space that is filled by part of the hot mantle rising up – whereupon the reduction in pressure causes it to melt. Rock is made up lots of different minerals and when it melts it usually doesn’t melt completely and produces a magma that has a different composition than the original rock. Shallow melting of mantle material gives basaltic magma. This rises up and pools into magma chambers. Here it may cool to form coarse gabbro, or flow upwards through cracks to form basalt lava flows, or cool in the cracks to form a sheeted dyke complex. This creates a characteristic layered pattern in the crust.

pillow basalts from http://www.flickr.com/photos/19311544@N00/2882765413

Pillow basalts. Image from Earthwatcher on Flickr. http://www.flickr.com/photos/19311544@N00/2882765413

All of this is taking place under water. A 2.5 km thick pile of water in fact. When magma reaches the surface, it flows out as lava but it cools extremely quickly and forms piles of distinctive pillow shapes. Water also flows down into the crust where it is heated up. When it comes back to the surface, at places called hydrothermal vents, it may form dramatic chimneys called black smokers, built up as minerals precipitate out of the suddenly cooled brine. Water is making important changes under the surface, changing the original igneous minerals into new ones, often by putting H2O into the mineral structure. Incredibly, it seems there may be bacteria living in this hot wet rock deep below the surface.

That sinking feeling

As our new crust slowly drifts at finger-nail speed (5 cm/yr) away from the ridge, it cools and grows denser, causing the sea-bed to sink. Sediment builds up on top. We are a long way from land and the sediment tends to consist of dead things with great names falling from above – diatomaceous ooze, globigerina ooze, the Titanic. It depends.

After 50Ma (million years) our crust is denser than the underlying mantle. It would sink down into it, but it is part of a rigid plate so it can’t – until it reaches a subduction zone, that is. Subduction zones, usually associated with deep ocean trenches, are found around the world. They are doorways – a place where our crust leaves the surface and enters the interior of our planet.

As it moves down into the subduction zone, our crust is pushed down under another plate. It bends down and starts to sink into the mantle – the older the crust, the steeper the angle. Some of the sedimentary cover is scraped off, to form an accretionary wedge above the plate. All this scraping and bending is associated with earthquakes, some of the strongest ones known. The bending of the plate allows water to get into the mantle part of the plate, further changing its composition.

Subduction zones and mid-ocean ridges are linked. In them, creation and destruction is broadly balanced. Oceanic crust is created from the mantle and returns there to be destroyed. The balance is not perfect: 0ceanic crust starts of different in composition from the mantle and all that brine it interacted with caused many chemical changes. The crust returns changed and can’t just turn back into peridotite – it stays as something more interesting.

As our crust starts to sink deeper into the subduction zone, the pressure increases. A lot. Imagine the pressure of a kilometre of rock sitting on top of you (I bet you can’t). For every kilometre the crust descends, the more rock is pressing down on it. The temperature increases too, but to a lesser extent – the cold crust takes a long time to heat up.

As conditions change, two things start to happen, metamorphism and metasomatism. Firstly the minerals forming the basaltic rock (such as plagioclase, pyroxene, olivine) become unstable and new minerals are formed –metamorphism. This process is fairly continuous as the conditions change, but the most dramatic (and attractive) transformation occurs about 2 million years after subduction starts, at around 50 kilometers depth. Here the basaltic rock turns into eclogite.

eclogite with rutile from http://www.flickr.com/photos/30659367@N00/60820842

Eclogite with rutile. Photo from Graeme Churchard. http://www.flickr.com/photos/30659367@N00/60820842

Many of the metamorphic reactions affecting the crust release water which flows from the subducting crust up into the wedge of mantle sitting above it. Water is a fantastic solvent, so it takes other elements up dissolved with it – this flow of material is called metasomatism. These elements tend to be ones that make large ions, like Potassium and Boron- they are harder to fit into the increasingly tightly-packed mineral structures that form at depth. It’s as if they are being squeezed out of the rock.

How on earth does a slab of rock force its way deep into the earth? The driving force is density. Old cold subducting crust starts off denser than the surrounding rock and the process of turning it into eclogite makes it 10% denser still. Over geological timescales the mantle behaves like a stiff fluid and a cold and rigid dense plate is able to force its way into it. The force generated by the sinking plate is called slab pull and is one of the major drivers of plate tectonics. Eclogites make the world’s plates go round.

After burial, rebirth

A lot of what we know about subducting crust comes from pieces of it that have somehow got back to the surface. We don’t know of any eclogite that has been deeper that 150km, so as our plate sinks further down we have to infer what is going on using indirect methods.

One such method is to study the pattern of earthquakes associated with modern subduction zones. Over time, plotting their distribution picks out the subducting plate and shows that typically it carries moving down to at least 650km depth.

One, rather important, consequence of subduction is the creation of volcanic arcs. These are chains of volcanoes, parallel with the subduction zone, typically about 100km along the surface from the trench. Many major modern day volcanoes, such as Mount St. Helens and Krakatoa are found in volcanic arcs.

subduction diagram from http://www.flickr.com/photos/44615724@N05/6128547564

Subduction zone – diagram from infringer1 on Flickr (http://www.flickr.com/photos/44615724@N05/6128547564)

Working out what is going on below volcanic arcs relies on the indirect tools provided by geochemistry – studying the composition of earth materials. We know the composition of what we start with (subducting crust and mantle materials) and of what we end up with (volcanic rocks) and comparing the two gives insights into the process. It’s complex.

At depths of 100-250 km the oceanic crust begins to melt. The resulting magma, along with the water mentioned earlier, rises up into the wedge of mantle above. This rising material then lowers the melting point of the hot mantle wedge, so in turn parts of that melt. This new mantle-wedge melt is what rises to the surface and forms the volcanic arc. On the way it may be further modified by melting and then mixing with the crust it is intruded into. The end result is that mantle melting ends up producing rock with a very different composition, called andesite.

This process is worth studying in detail as it is one of the main engines of continental crust formation, producing the stuff that most of you are currently sitting on. Over time, volcanic arcs have been the major mechanism for turning mantle rocks into continental crust. [If you are not sitting on continent, how’s the pineapple/cod cheeks tasting tonight? I’ll get to the creation of your oceanic island in the next post]

Subduction is involved in not one but two interlocking cycles of creation and destruction. Oceanic crust is created, but it is destined soon to return to the mantle at subduction zones, to make space for newer crust. Squeezing out of the water the crust gained from the oceans helps create new continental crust. Eclogite plays an important role in both cycles – it helps pull the mid-ocean ridges apart by slab pull and it sweats out the fluids that kick-off the creation of volcanic arcs.

I talk of destruction, but our crust still lives on, transformed. Maybe 40% of it has melted and flowed upwards, but it it is still a distinct slab, different from the surrounding mantle. By now it has travelled 250km down, after about 10 million years of subduction. It’s journey is far from over with 10 times as far to sink still. We’ll continue the incredible journey in another post.