In part two we learnt about the specific samples I am looking at, where they come from and how they came to be as they are today.
In this final part, we will finally learn how my samples look now, and what sort of things I might have to do in order to say sensible (and most importantly useful) things about the processes on display inside them.
For all my big talk in part one, it’s becoming clear that we aren’t going to learn everything about mechanical-chemical feedback from this one little system. But we’ll know more than before. This step from big grand ideals to the final seemingly tiny achievements in concrete understanding is one of the hardest things about research. You always have to keep in mind why you started the project even while you are trying to understand very individual things in tiny detail.
With that in mind, let’s look at the tiny details!
At first glance, there doesn’t seem to be much shape change/deformation in my garnets. Inspection at low magnification shows that they haven’t been squeezed out of all recognition, in fact many of them show the same nice regular shape that they have had ever since they crystallised from the molten rock.
Appearances, however, can be deceiving. Under the microscope, some interesting features can be seen. Currently they are being referred to as inclusion trails, although there are other things in geology already referred to by this term, so we really ought to change it to prevent confusion. I shall present some images to show you what they are like, if you wish enlarged versions and a few extra comments, follow this link to my photo gallery.
What can we see? The most important thing to look for in any geological image is the scale. Our first picture is very zoomed out, that yellow bar is a whole millimetre! Now we know the scale, we can try and figure out what we’re looking at. This image is a microscope image of a thin section, a slice of rock 1/20 of a millimetre thick, thin enough that it is transparent to light shone through it from underneath. The whole thing is stuck to a glass microscope slide as it is obviously rather fragile otherwise! Don’t worry about that black splodge, that’s just some graphite paste on the underside of the glass plate which needs to be cleaned off.
The part of the image we are interested in is the garnet crystal itself, which is the slightly reddish-brown cracked looking stuff filling most of the image in the centre and left (this image is already zoomed in enough that you can’t fit a whole garnet into the field of view). At the far right edge of the garnet there’s a weird fragmented looking part, but that is just where during preparation of the thin section some of the material has broken away, leaving a hole through to the glass slide beneath. Several other small holes created during preparation of the sample can be spotted elsewhere in the garnet, like at the left had edge.
When we look at what’s happening inside the garnet we can straight away see that there are patches of different colours or shades. These patterns and shade changes are caused by those supremely tiny microinclusions that I previously introduced. At this scale the ‘trails’ are barely visible as a pair of very fine dotted black lines cutting diagonally SW-NE through the centre of the picture.
When we zoom in to 10 times higher magnification, we can see lots more. Note the scale in image 2 above! That yellow line now represents 0.1 millimetres (100 micrometres). Now the individual microinclusions are visible as tiny dots, and you can finally get a sense of just how tiny they are. What’s REALLY exciting though is that trail of (in comparison) HUGE inclusions going straight through the middle of the image. And either side of them there seems to be a thin pale band where the microinclusions are completely gone! Why is that? And why has it only happened in that area, and not in the bits next to it?
Well of course we don’t properly know, or I wouldn’t have a job. What we can infer (among other things) by firing electrons at the garnet is the orientation of the lines of atoms in the crystal, enough to see that near these trails the crystal structure is slightly bent. Aha! So these zones of coarser inclusions seem to a) have been made out of/ somehow used up the original tiny inclusions (otherwise we wouldn’t see a bleaching zone) and b) this change seems to be triggered in areas of the crystal where deformation is particularly strong.
The movement and recrystallisation of the tiny inclusions to form bigger ones is the chemical part of my chemical-physical interaction, and the deforming garnet is the mechanical part. It is clear that the way these trails form is intimately connected with deformation of the host crystal structure. So just by figuring out the specific mechanism that led to these strange patterns, I will be learning new things about chemical mechanical interaction and how it occurs.
But how on earth can we go from simply describing these inclusion trails to working out how they formed? There’s no magic probe-a-tron that will tell me ‘BEEP- These inclusions recrystallised because distortion of the garnet allowed diffusion to travel faster in these areas -BOOP’. There isn’t even a magic probe-a-tron that will tell me whether or not diffusion was the only process involved. Looking at the picture it looks like lots of tiny inclusions just decided to collect into fewer, bigger inclusions. But did they also suck up some additional elements from outside the garnet crystals, or maybe chemically react with the surrounding garnet crystal? The difference between inclusions that hop around and make friends in an uncaring lump of garnet and inclusions that move around while also exchanging elements with the garnet around them is pretty big, but you can’t tell the difference if you are looking at the final result using only a normal microscope.
So the best way to start working out what things are important to our process is to start asking some very basic questions, because otherwise all we are working with are assumptions. You can probably think of some good questions yourself, but here are a few examples:
Are the types of mineral we find in inclusion trails the same type and/or chemical composition as the tiny inclusions found nearby?
Does the size of inclusions or width of bleaching zone differ depending on what types of minerals are found as inclusions in different trails?
Does the size of inclusions or width of bleaching zone differ depending on the chemical composition of the garnet the trails are in?
Is there a relationship between amount of deformation and appearance or size of trails?
You will notice that almost all of these could easily be rephrased as testable hypotheses, e.g. “Hypothesis 1: The size of inclusions in the trail is different depending on the chemical composition of the garnet the trail is in.” At that point all the theorising has been done for the moment and to get any further you need to start answering a new set of practical ‘how?’ questions, such as “How do I make sure to only measure the composition of the garnet and not accidentally measure some tiny inclusions as well?” or “What do I even mean by amount of deformation??”
That’s how even initially simple questions can lead to months of work trying to solve some technical problem before you can even start collecting the data you think you need. Of course one of the problems you are having might actually turn out to be very informative in the long run...
So what I am doing now and for the foreseeable future is using a wide variety of analytical instruments to attempt to answer hypotheses such as the ones quoted above. Only once I’ve collected lots of data will I be able to start doing any theorising, working out things that must or cannot be true considering the answers I’ve found. Naturally there will be many unexpected things along the way that will throw up more, as yet unimagined questions...
So there you have it, an overview of why I’m doing my PhD, what I’m looking at, and some of things I want to know to start answering the big questions (although these big questions are very small compared to the stated goal . I hope those that have reached this far have a better understanding of my project, my samples and also the scientific approach to observing anything you don’t understand (start by describing what you see, then set hypotheses and test them).
I also hope that by doing this you understand just why my project is really interesting and rather fun, and that I will be able to share new developments with you and you will (at the very least) understand why they are exciting!
Happy hypothesis making!
Der Tom.
No comments:
Post a Comment