Previously I explained the very big picture of my PhD. But if you want to study a process involving deformation and chemical reaction, you need to have samples in which both of those things have occurred! This post is all about what is in my samples and (as far as we know) how it came to be there. Once again remember that lots of this geological stuff is fascinating, but it is the background to my research, not the subject itself.
It’s time to reveal at my project title. Project III.1 is called ‘The physical and chemical behaviour of solid inclusions during recrystallisation of the host mineral’. Let’s look at some parts of it. ‘Physical and chemical behaviour’ should be obvious, all it’s saying is what I told you last time, that this PhD is about a certain process. We can ignore ‘during recrystallisation’ for now, that part is being more specific about what physical and chemical interaction is going on, and I’ll deal with it in part 3. The most important thing is that to study this ‘recrystallisation’ we are clearly going to need a sample with some ‘solid inclusions’ sitting inside a ‘host mineral’, that has then experienced chemical reaction and physical deformation at the same time.
My samples come from south-eastern Austria. They are garnet crystals. Garnets form large, symmetrical almost spherical crystals in many rocks. They have a reasonably simple chemical formula as minerals go (good for me!) and are also quite tough, not usually letting much in or out of them after they form. My garnets are unusual, because they crystallised straight from molten rock (or ‘magma’, hence why we call these ‘magmatic’ garnets). Most garnets grow gradually in metamorphic rocks, but metamorphism is slow and messy and it is rarely possible to understand everything that happened to a garnet or in what order, so these magmatic garnets are much better as a starting material for the experiment that nature is going to carry out for us. All natural samples have lots more unknown factors than experiments done in the lab, but we still try to choose samples that start out as simple as possible.
The magma that the garnets crystallised from wasn’t blasted out of a volcano, instead it sat underground in a nearly vertical sheet of rock called a ‘dyke’, cutting through the layering of older rocks around it. From radioactive dating, we know this all happened around 250 million years ago (that’s just under 4x as old as T-Rex, dino-fans). The magma in the dyke was of quite a rare composition. It is what is called a pegmatite, the last little bit of a granite to solidify. All the unusual elements that hate to sit in crystals are in this liquid, so some very unusual minerals can be found, made from elements like titanium, yttrium, phosphorous, boron, fluorine, chlorine and others.
These ‘unhappy’ elements that don’t fit so nicely into crystal structures are the key to making the next thing we require in our sample, the inclusions. When the garnets formed, they probably incorporated some of the elements mentioned above in various places in their crystal structure (crystal structure is the regular repeating 3D pattern of atoms that makes up a crystal, the simplest would be salt, with alternating Cl and Na in a cube structure). At high temperatures when atoms are wobbling around a lot, you can more easily wedge atoms of many different sizes into a crystal structure, but when the crystal cools it can be more stable to throw out the badly fitting atoms, which are more stable forming their own crystals in new minerals.
In my garnets, this process (called ‘exsolution’, which just means ‘unmixing’) means that after cooling instead of lovely clear garnet crystals, we end up with garnet crystals clouded with literally billions of extremely tiny ‘inclusions’ of other mineral types. So tiny are the inclusions that happened to form in these garnets that individual inclusions (less than 1/1000th of a millimetre across) are impossible to see even in a microscope unless you use a really big magnification. Some pictures to illustrate this are here!
So we have a host mineral, filled with inclusions. Now all we need is to deform this host mineral (causing it to ‘recrystallise’) and reactions can happen involving the garnet host and the tiny inclusions. Then we’ll have our natural, deformed sample where we can try and pin down some hard facts about this feedback stuff!
The force that will take and deform our inclusion filled garnets sitting minding their own business in their pegmatite dykes is one that most will recognise: the Alps. 90 million years ago proto Italy was busy crashing into mainland Europe. This collision first caused the rocks in which our garnets sat to be forced deep under the earth’s surface, but by a fluke of interacting forces (that we still can’t 100% perfectly model), these rocks were squeezed out again (geologically) soon afterwards, a bit like toothpaste*.
All you really need to take from this is that the rocks in which our garnets sit were rather badly tortured during formation of the Alps, and that’s the cause of the deformation (remember, all I mean by that is distortion/shape change) in our samples. Deformation is good, because without deformation, we can’t study the chemical reactions and feedbacks that might have occurred during it!
So now we’ve made and squashed our samples, all that remains is to tell you about the structures that all this squashing and reacting produced. It’s my job to understand in great detail what controls the formation of these structures, so we can come to some general conclusions about the processes of mechanical deformation and chemical reaction in this special system.
Join me in part 3!
* How do we know all this? New minerals can be formed during this process, which we can radioactively date. Some of the minerals only form if a rock is at extremely high pressure and temperature, they tell us how deep the rock went. Others formed later when the rock cooled down a bit, and the difference in times and the pressures that we think the minerals formed at can give us a rough estimate of how fast the rocks were moving upwards.
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