"Directly" experiencing Subduction-Zone processes

I haven’t made time to participate in an Accretionary Wedge for a long time, so when Ron Schott called for the "Most Memorable/Significant Geologic Event That You’ve Directly Experienced", I thought it was about time to come out and play this game again.

In his call he gives some examples of a few processes that take place in human observable time frames and at surface pressures and temperatures (e.g. earthquake, landslide, flood…) and he repeats the part of his challenge about our being able to directly observe and experience the process we write about.

However, the sorts of geologic processes which most fascinate me are not those which create sediment at the surface of the earth, nor yet processes which produce fine-grained igneous rocks. To my eyes the most significant geologic processes are those which are responsible for creating the pretty rocks that drew me into geology in the first place—the ones with beautiful large crystals. Metamorphic processes, and also igneous processes when they take place deeply enough to permit significant crystal growth, are my favourite of all of the many geologic processes. However, the pressures and temperatures which are responsible for making particularly pretty rocks are well in excess of what our frail bodies can tolerate, which means that the process isn’t something I can ever "directly experience".

Or is it? How can we ever know what is happening within a subduction zone?

This question was not only of interest to me, but also to the international research team I joined when I began my last post-doc position. While none of us could go down the subduction zone ourselves to find out what was happening there, we were able to bring a tiny bit of the subduction zone setting into our lab.

Using a piston cylinder apparatus I regularly performed experiments which match the sorts of pressures and temperatures one would find if one could enter a subduction zone. While real rocks can spend millions of years working their way down a subduction zone and then back up again, I only held my samples at high pressure and temperature for two to four weeks at a time. As a result the crystals I grew from my powdered starting material did not achieve the large, stunningly pretty, sizes one can find in metamorphic rocks, but they did grow up to 100 µm in length (remember that there are 1000 microns in every millimetre), and many were lovely to look upon in the BSE images.

Having had the opportunity to perform such experiments I confirmed for myself that yes, pressure and temperature do matter to the minerals in a rock. If one takes the exact same starting material and "cooks" it at different settings one will get a different assemblage of minerals for each combination tried.

The below images show some of the results for one of the compositions I tested, at three different pressures (2.65, 2.8, and 3.0 GPa), and three different temperatures (600, 625, and 650° C). As you can see, the sets of phases present are very different for each experiment. Even the phases which are present in all experiments are present in different abundances when the pressure, temperature, or both are changed.

The few photos I have shared with you today are just a glimpse some of the experiments I have done. All of them together have transported my imagination to the depths of a subduction zone, and brought the merest hint of a subduction into my lab. This is "directly experienced"enough for me.

List of abbreviations:

grt = garnet

mu = muscovite (or other white mica)

qtz = quartz or coesite*

ctd = chloritoid

anth = anthophyllite

tlc = talc

*In all cases the SiO₂ phase is labelled "qtz", even when it is at pressure high enough for that phase to probably be coesite—the microprobe does not differ between those two phases, and I did not check it with another technique (such as Raman) because the difference between quartz and coesite wasn’t relevant to my work, which was focused on questions related to the stability fields of talc, biotite, and garnet.

Well, that changes everything. Perhaps.

I have heard it said that sometimes the most exciting moments in science are when you look at something and say “that’s odd!”. I had one of those moments today. I have been working on the paper from my post-doc research this week, and the main focus of my work has been the figures. I now have very good, clearly labelled BSE images of every sample analysed and am starting to reference them in the text of the paper. The process consists of saying things like “Garnet ranges in texture from thick rims overgrowing seeds to a spray of very small grains” and then adding parenthetical notes behind each type pointing to figures that show each sort. In the process I also listed those samples which have no garnet at all, with reference to their photos.

This is where the “that’s odd” moment came in. For one of the samples my table of results shows that it has garnet, and that I have five good analyses of it. However, the photo of that sample shows that there is no garnet at all, unless you count the seeds of a vastly different garnet composition that were included in each experiment to give the new garnet an easy place to start growing. Clearly something is wrong here, so I checked the data file and determined that the five garnet analyses we have for this sample were done by my predecessor in this project years before I arrived. In my own records I also have three analyses of garnet seeds from this sample. Comparing his data with my own shows that the garnet analyses he did are clearly *not* the seeds—for this sample the seeds are grossular garnet (the calcium end member) but the garnet he analysed are iron-rich with only a little Mg and Ca. Sorting all of the garnet data for all of the samples shows that his analyses for this particular sample fall nicely in the middle of the pack in terms of composition.

So what is going on? I can think of a couple of logical possibilities off the top of my head. One (kind of obvious one) is that I somehow analysed the wrong sample myself and the photo I have is not the sample I think it is. Another is that I did analyse the correct sample but there is something wrong with his data. A part of me hopes that it is the latter—not just because of vanity and the hopes that I did everything perfectly, but more because it would clear up a problem that has been bothering me with my results since I first obtained the data.

I ran my experiments at three different temperatures: 600, 625, and 650 C. For the two higher temps garnet was present for every pressure I tried. For 600 C, on the other hand, I found no garnet at all in any of the metapelite runs, but it was present in 3 of the 4 metagreywacke runs. Now, had those three been adjacent to one another I wouldn’t have complained. But they weren’t. Instead I had garnet present at 2.2 GPa, garnet absent at 2.5 GPa, and present again at 2.65 and 2.8 GPa. This makes it difficult to draw pretty diagrams. How much nicer they look when a phase is absent for one half and present for the other half of the diagram.

So now I sit and eagerly await a reply from my erstwhile boss to the email I just sent him. Depending on how he replies I will have work to do changing the write up to match the new situation—do I get to delete the analyses of garnet for this sample that he did, and write the paper with a story that makes sense to me, or will I need to beg him to find the real sample and put it into the microprobe and take new BSE images and do an analysis or three to confirm that the composition my predecessor saw all those years ago are correct, or…?

November writing challenge update

Back at the end of October I put my hand up to participate in a science writing challenge for the month of November by finishing up two papers that really should have been written long since. On 1 November I started my new job, and didn’t write at all that first week as I focused on the 1001 tasks necessary to starting a new job, including moving into my office, rearranging the office furniture to suit my working needs, meeting my new colleagues, obtaining employee cards, library cards, etc.

Early in week two my office computer arrived, and another two potential writing days were lost installing programs and generally making the computer ready to use. However, on Thursday and Friday of that week I actually sat down and finished up the current draft of the paper based on my PhD research. The previous version I had done (while back in Australia in July) was good in terms of presenting the basic facts of what was done and what the results were, but I had been kind of sketchy in the discussion and conclusions section. (I think that is a common failing on my part that I should work on—I am totally comfortable reporting facts, and I am totally comfortable with editing a previously existing discussion/conclusion section, but actually confessing what *I* think is relevant or important about those facts? That is getting kind of personal.)

Be that as it may, since it was necessary to finish the paper I somehow found the necessary motivation/inspiration to just write it—I went back to my PhD thesis, looked at the points covered in that section there, and chose which ones to address here. Not only did I write it all down, in many cases the version in this paper is much clearer and more eloquent than what I had typed when finishing up the degree. Spending a couple of years thinking about other areas of geology actually helped give me some new insights on that project.

In this, my third week on my job I have continued to split my time between needful tasks for this project (doing background meeting, arranging a trip to the mining company with whom I will be working, obtaining a card to let me use the uni gym (free to employees during business hours), etc.) and finishing up previous papers. Since I had sent a copy of the PhD paper to my erstwhile advisor in Tasmania on Friday that meant I could focus on working on the paper from the experimental post doc position I finished last December.

This week I have managed to do some editing of the text and make major progress on a set of figures that should have been done long since: BSE images of every experiment, annotated to label the mineral phases present.

Why hadn’t I created such images previously? Because I documented each microprobe session in CorelDraw. My standard operating procedure was to look at the sample, determine a region to work on, take a picture, transfer it to my personal computer, open it in CorelDraw, and create a new layer for the day’s session in which I would make colour-coded circles superimposed over the picture at the locations for each analysis point. I would give each circle a name that matched the name recorded in the microprobe (such as RC1-NMg1 for the first garnet analysis on sample #RC1-NM). Repeat for each analysis, taking additional photos as needed.

This works very well for recording things, and one can easily go back and compare the results with the appearance of the phase analysed. However, the layers can get confusing for those samples with multiple microprobe sessions (due to the difficulties in getting good results for some phases).

The new, improved, pdf images I am creating for each sample make things much easier—they are all labelled with the pressure and temperature of the experiment, which bulk composition was used, and which phases are present. The phases are colour coded to indicate the quality of the data—if they are in bold print I had five or more good analyses of that phase for that sample which agree. If they are in normal print there were three to five good analyses, if they are in red there were two (or fewer) good analyses, and if the red text has a question mark next to it I know that the phase is present, but it was too small to get a clean analysis so I do not actually know the composition of the phase. Having this information right there with the photo of the sample is very helpful. It is also pretty easy to see why the red text is in red in most cases—those phases really are smaller or too amorphous to get good readings—one can see that in the photo, too.

There is still a week and a half left in November—I think I may be able to finish compiling these images today, so there is a chance that I will be able to complete all of the other tasks necessary for this project before the month ends. However, even if I do not complete this goal, I still thank Anne for having inspired me to set it—I am certain that I would have found plenty of tasks to keep me busy instead of writing these older papers if I had not stated publically that I would do it.

Extrapolation of the likely composition of a mineral from mixed analyses

I have mentioned before the difficulties of using a microprobe to analyze very small phases. The electron beam with which we do the analysis can, with care, be focused to about 1 micron diameter (remember that there are 1,000 microns in every millimeter). However, should the mineral phase of interest be smaller than one micron, in any dimension, the analysis will yield the composition of not just that mineral, but of whatever happens to be next to it as well.

The below photo shows one of my experiments for which this was a problem. As with all back-scatter electron images the amount of brightness or darkness of any given part of the image is based on the composition of the sample in that location. Brighter areas contain more heavy elements, darker areas more light elements. The brightest grains in this image are the large pale grey crystals, which often have dark centers; these are garnets. The dark centers are the pyrope (Mg-garnet) seeds that were included in the experimental powder to give the new garnet, which is much higher in Fe (iron), a place to start growing from. The narrow stick-shaped crystals which occur in a group on the left hand side of the image are chloritoid. Unfortunately, as you can see by the scale bar on the bottom of the image, they are too narrow to obtain a good analysis. Through careful searching of the sample we found a few places where the chloritoid grains were slightly larger than the others—these were the ones we analyzed, in hopes that we would be lucky. Alas, 15 times we tried, and 15 times we failed to obtain an analysis which was only chloritoid, but instead they were "mixed" results of both chloritoid plus another phase.

How do I know for certain that they are mixed? Look at the below graphs and you can see for yourself. The upper graph shows the composition of all of those mixed analyses with respect to how much aluminum and how much silica they contain (blue-green hollow triangles). It also shows the region (grey circle) within which all of the matrix mica in this sample plots, and the location of where kyanite (Al₂SiO₅), also present in this sample, plots. As you can see, there is a clear trend going from the solid green triangle towards the mica, and another trend going from the solid green triangle towards the location of kyanite. The lower graph shows the trends for iron vs aluminum. By plotting this data for a variety of different combinations of elements I have come up with my best guess as to the composition of chloritoid is in this sample (solid bluish-green triangles). Is it as accurate as if I'd been able to get a good measurement? No. Does it give me information I can use when doing other parts of my data analysis? Yes, yes it does. Playing with graphs is one of the fun parts of my job—the information they convey communicates so very clearly.

Final experiment of the project

What is probably my final experiment (for this project) is running. We started it yesterday morning. My current contract runs through December. At 2 to 3 weeks per experiment, this one is probably it. This fact left me torn as to what conditions to choose for the run.

I've got a couple of previous experiments which turned out to be very difficult to analyze due to improper sealing of the capsules. Therefore I'd love to re-run them, with properly sealed capsules, in hopes of better grain sizes, so that I can get good analyses of the phases present.

We've got some data from experiments run by a predecessor of mine which use a slightly different bulk composition. In theory the difference between the two bulk compositions shouldn't really matter—most of the difference is in the amount of SiO2, which is still in excess in the bulk composition I use (as we can tell from the presence of quartz). However, when I calculated what phases should be present for each bulk composition using Perple_X, it said that at the P/T of those old runs the new bulk should have different phases. Therefore I'd like to re-run one of those to see if there is, in fact, any difference in the results with the slightly different composition.

And, finally, both my boss and I wanted to do another run in a P/T space that is expected to have both garnet and talc, since this project is about defining the stability field for talc. That makes three different experiments I want to run, and sufficient time available to do one of them. Decisions, decisions!

We eventually decided to go with a totally new run—while the data from the "bad" runs isn't complete, we do know that there isn't talc present at those conditions, which is the single most important question. Therefore there is more value in obtaining new data than clarifying old data in this case.

All in all, this project has convinced me that 1.5 years is simply not enough time for such a project. However, I am pretty certain that if it had been a 3 year contract that was now approaching its end I'd still be sitting here thinking about the other experiments I would have liked to have run, had there been time/budget to do so. Ah, the life of a research scientist—so many questions to be asked, so many things to do to try to answer them, but no matter which ones we choose, it never, really, feels like we've got time enough to obtain all of the data we would like to have…

potential calamity averted by a careful change in the erosional surface orientation

My most recent experiment is one I’ve long been looking forward to analyzing—this is the highest-pressure run I’ve done yet. We gave it nearly 500 hours in the piston cylinder, while I was busy with lots of travel and a minor bout of food poisoning. Once we finally downloaded it and I rescued the capsules from their nest I discovered that they were stuck together. We always run two capsules (2 mm diameter gold tubes, welded shut around the experimental charge) in every experiment, with different compositions in each.

Usually it is enough to set them in the palm of my hand and rub them back and forth a bit to cause them to break apart before I carefully break off the last of the MgO, salt and graphite that clings to the capsules. This time they stayed stuck together no matter what I tried, so I consulted with my boss, who suggested that I try carefully grasping each with jeweler’s pliers and seeing if I can break them apart that way. Alas, they did not separate, instead one of the two capsules started to tear open.

Therefore I stopped trying and instead we put both capsules into a single epoxy mount. Knowing that it would require a fair bit of luck for the capsules to be oriented within the mount such that it would be possible to expose the insides of both capsules when I went to polish them I put off the task and spent the weekend visiting with friends. This morning I finally decided to look at the mount and try my hand a polishing them. When I looked I determined that the important ends of each capsule were not on the same plane with respect to the surface of the mount, but it looked like if I were careful to put more pressure on the one side than the other it might be possible to change the dip of the top of the mount such that both capsules were intersected. Much to my delight I managed to accomplish exactly this task. I now have one mount with two capsules, both exposed, all polished and turned in to be carbon-coated in preparation for tomorrow’s microprobe session.

This makes me very happy. I did not like the alternative, which would have been polish enough to expose one of the two, analyze and photograph everything in it and then polish it away to expose the other one for analysis. This way we will be able to go back and re-analyze anything we want at any time, rather than having one of them cease to exist in order to reach the other.

play with the data long enough and it becomes possible to find a way to see patterns in it

One of the things I’ve been struggling with in my current research is how best to communicate the results from the various experiments I’ve run. My experiments have thus far yielded a total of ten different phases, with as many as seven of them appearing in a single experiment. I use two different capsules at each pressure and temperature at which I run experiments; each capsule has a different bulk composition. Therefore I’ve been displaying the result graphically, by using 8-pointed stars divided into an inner ring for one of the composition types, and an outer ring for the other. The resultant triangles representing the phases present are either left blank if it isn’t present or filled in with colour-coding if it is. One phase, quartz, is always present (save for when we reduce the starting SiO2 to eliminate it), so it doesn’t need a triangle of its own, and another occurs only in one high-P run, so it appears as a different colour triangle replacing one attached to a low-P phase; this is why I’ve been able to get away with using only eight points for the star.

However, there are times when it is necessary to communicate with text or a table, rather than with an illustration, and this is where I’d been stymied. I simply wasn’t seeing much in the way of a pattern with my data in terms of mineral assemblages. A mineral assemblage is the group of minerals which are all stable at the same pressure/temperature; they would have been the products of the reaction(s) which produced the assemblage. (When doing experiments we talk of phases rather than minerals—a phase is a particular composition of a mineral (many minerals can have more than one possible compositions, so may be considered a family of mineral phases)—in general only one phase within a family will be stable at a given set of conditions) .

Today I finally discovered a way to organize my data so as to see patterns in the assemblages. This required using colour coding and playing with the data, combining them into groups until I was able to determine that the “important” phases of the list of 10 are talc, garnet, and biotite. The others are either ubiquitous (quartz, chloritoid and muscovite) or only show up in a few of the runs and can be considered "minor" (zoisite, lawsonite, kyanite, carbonate). Once I’d worked that out, I was able to split the data into four groups each of which are +/- the minor phases and + the ubiquitous phases.

However, it was also necessary to consider each of the two bulk compositions separately to see the relationships between the groups, and I had to draw circles around the stars on my original P-T diagram to see how the groups relate to pressure and temperature. (I love having a drawing program which lets one draw circles on layers that can be made visible or invisible, so that one can see only the circles relevant to a single composition at one time.) Once I had all of the groups for each bulk composition circled it was easy to see that:

The experiments using the metagreywacke composition only have groups A, B, and C thus far. These groups plot on diagonal trends for this composition such that with respect to temperature B is less than both A and C, but with respect to pressure C is less than both B and A.

The experiments with metapelitic composition have groups A to D which plot in a grid such that with respect to temperature B is less than A while D is less than C and with respect to pressure C is less than A while D is less than B.

Now that I can see these patterns I shall really look forward to obtaining the results from future experiments to see how they relate to this overall pattern.

“It Happens”

The lessons we learn in this life come with a cost—be it the time we spend reading books, or the consequences from the mistakes we make. Yesterday’s lesson has to do with polishing the capsules for my experiments. The cost: I need to re-do two weeks of work, and will have less data than I’d hoped to put into the conference abstract that is coming due soon.

What happened? Well, with my experiments the first step is to seal powder with a composition similar to that of natural rocks into tiny gold capsules (2 mm outside diameter, ~5-6 mm long). These capsules get placed into a nest of MgO (which looks just like normal chalk-board chalk, but contains Mg instead of Ca), which goes into a graphite tube; the tube goes into a cylinder made of salt, and that cylinder and its contents goes into a large steel container, which is hooked up to a piston-cylinder apparatus with thermocouple in such a way that it is possible to inflict quite high pressures and temperatures upon those little gold capsules. We run these experiments for two to four weeks, and once that time has elapsed the second step is to remove the capsules from their nest, clean off any of the MgO-graphite-salt that has become stuck to the outside of the capsule, mount them into epoxy and then carefully polish the resultant disk until the capsule is at the surface, with the inside of the capsule exposed.

During the experiments the powder within the capsule undergoes chemical reactions and minerals grow (just as happens in real rocks when they are metamorphosed by enduring such pressures and temperatures in the real world). Because we start with powder the minerals within the capsule are often not as well interlocked with one another as is the case with real rocks. Therefore our normal procedure is to polish just until the uppermost surface of the gold has been removed, and then add additional epoxy, which soaks into the powder and holds it all together, letting that dry before doing the final polish in preparation for analysis with the electron microprobe. Yesterday that isn’t quite how it worked.

Before I left for AGU we “uploaded” my fourth experiment into the piston cylinder machine. That “run” completed its two weeks of mineral growth at high pressure and temperature (650° C, 22 kbars) during my absence, and my boss “downloaded” it, and left the package of salt-graphite-MgO-gold capsules-contents in my experimental drawer for me to deal with upon my return. Last week, after my return from my post-AGU holidays, I removed most of the salt-graphite-MgO layers from the now somewhat deformed gold capsules (it would be odd if they didn’t deform under that much pressure), but a small amount of it remained suck to the outside of the capsules—small crystallized bits with one edge wedged between wrinkles in the capsules surface, or merely adhered to the outside. This is normal, and our standard policy is to just ignore such small contaminants on the outside of the capsule, since our analytical methods are able to focus upon individual mineral grains within the capsule, so long as they are 1 μm or larger (remember that there are 1000 μm in every millimeter).

Therefore I dropped them off to be mounted into epoxy on Friday, and picked them up on Monday morning. I did a bit of polishing on Monday, just enough to remove the surface epoxy, revealing the tiniest bit of gold at the surface of the disks (one per capsule). Then I decided that I was really too sleepy to be engaging in such work (not only had I not yet completely adjusted back to this time zone after my flight from the US the week before, I’d also, foolishly, stayed up quite late both nights of the weekend) and I set the project aside for the day, and returned to it Tuesday evening.

In retrospect, I now believe that my brain was still not functioning at full capacity on Tuesday (no doubt do to once again, foolishly staying up late on Monday night). Why do I think that my brain wasn’t working properly? Because when I looked at the disks to be polished I could clearly see a bit of the “stuff” adhering to the outside of the capsules, so when I started polishing I told myself that I needed to polish past that “stuff” to get to the capsule itself and its contents. Therefore, each time I paused in my work to look at the capsules under the microscope I was convinced that the non-gold “stuff” I was looking at was naught more than the external contamination from the salt-graphite-MgO casing that had been around the capsules during the experiment. It never looked like the dark, powdery stuff that I’d found within the capsules of my earlier experiments, and, so, thinking I’d not yet polished deep enough to expose that powder, I kept polishing. And polishing. And polishing. Eventually, I suddenly had a gap in the gold between the top and the bottom ends of the capsule for one of them, and looking at it from the side view, saw that there wasn’t much left to the capsules at all. Confused, but undaunted (a *huge* clue that my brain wasn’t working properly), I then picked up the other capsule (I’d been switching between them as I worked, to keep them at more or less the same point in the polishing process) and proceeded to polish it, too, down past the middle of the capsule, with only the very ends remaining in the epoxy disk. Then I began to panic as I realized that if I hadn’t yet found the top of the contents, then, perhaps, what I had been thinking of as “outside contamination” was really the contents I’d been seeking. Oops. Major oops.

How was it possible to thus mistake it? Well, part of it is probably the difference in quality between this run and my earlier runs. The first couple of times I attempted to weld shut the capsules my seal was not, quite, perfect for all of them, which resulted in the water (which we put inside of the capsules along with the powder) to boil out of the capsules. Without that bit of fluid present the chemical reactions are inhibited and the minerals don’t grow as large as they do when the fluid is present. Therefore the resultant product is darker and more powdery than one obtains when the capsules are properly sealed and the fluid remains inside to participate in the chemical reaction. The hard “white stuff” I had been so carefully removing because it wasn’t the black powder I was expecting was, in fact, exactly what I wanted to see. Some combination of lack of sleep and three weeks of holiday (including the week at AGU) since last I’d done anything with these experiments conspired to make me fail to realize this on time to save any of it. Sigh.

My boss tells me that “it happens”. He did it once; our PhD student did it once. He also says that it is a mistake that doesn’t tend to get made a second time. I bet! I know that I shall be really, really paranoid about the polishing in the future to be certain that I never do this again. So now I need to go back to step 1 again and seal some fresh powder (plus water plus graphite) into new capsules, and give them a couple of weeks in the piston cylinder to grow new minerals. Then I can polish them, correctly, without throwing away the important part, and analyze the results in the microprobe. Then I can add that data to our growing database, and, perhaps, someday, one of you might compare the compositions of the minerals present in your real rocks with the data from my experiments and use that to determine at what temperature and pressure your rocks probably formed. When you do, and the list pressure-temperature combinations we tried isn’t quite as extensive as you were hoping it would be, remember just how much effort goes into obtaining each data set, and how easily things go awry making it necessary to start the process over from the beginning.

routine + templates makes things easier

With each of the experiments I am running I inflict elevated pressure and temperature on two different tiny gold capsules full of powder. One of them is always full of powder “NM” while the other contains powder “NP”. Even though it is generally possible to tell the two capsules apart before they go into the piston cylinder (by making a sketch of the actual shape of the welded ends, since no two are ever quite the same), the pressure they are subjected to generally means that it is harder to tell them apart when they come out. Fortunately, it is possible to use the microprobe to do a scan over a largish patch of the sample and obtain numbers which are, more or less, representative of the bulk composition of that region.

The first time I did this it felt difficult to compare those numbers with the actual, known, bulk composition of the samples, since this rough-area scan is never going to give precisely the same numbers as the bulk composition. The growth of minerals within the powder has caused some elements to be concentrated in some minerals, and other elements in other minerals. However, in general, the relative differences between the two bulk compositions can still be distinguished via the rough scan. Therefore I set up a spreadsheet with graphs, plotting the original, known, bulk composition in one colour (hollow symbols for NM and solid symbols for NP), and the rough area scans of the first experiment in another. Sure enough just as the original bulk NM is higher in Al₂O₃ and lower in K₂O FeO and CaO than is NP, so the first experiment has one capsule with higher Al₂O₃ and lower FeO, K₂O, and CaO than the other. The second experiment repeated the pattern, but now that I’ve added the third the graph is even easier to read, for now the symbols for the NP bulk plot in one distinct clump on each graph, whilst the ones for the NM bulk composition samples plot in another. This means that from here on out, I need only enter in the new data into the spreadsheet, and in a second’s glance at the chart I’ll know which is which.

Somehow, I really enjoy these tricks which make life easier. Besides, it is fun to set up the charts and graphs.

Two weeks left to finish analyzing the data from my first three experiments and prepare my poster for AGU. Somehow, I suspect that this will keep me as quiet on the blog front as the past couple of weeks when I had both unpacking to do and thesis corrections to make (since my household goods and the examiner’s report arrived on the same day).

Experimental petrology defined

A new friend of mine asked what I do, so I replied that I do research at the university here. He replied "Very Cool ----What is the research in -?" To which I couldn’t help but reply simply “Experimental Petrology”.

However, knowing that the vast majority of the people out there haven’t a clue what that means, I wrote him the following translation:

I am a Geologist who inflicts extreme heat and pressure on tiny amounts of powder (of known composition, which is similar to a specific rock type) for one to three weeks at a time. I then analyze the minerals which grew from the powder to determine which ones are present, and the precise composition of each. Once these experiments have been repeated for a variety of temperatures and pressures (each of which is comparable to specific depths under ground) it is possible to determine which chemical reactions are happening at which temperatures/pressures (for that specific composition). This information is then used (both by myself and by other geologists) to help calculate the temperatures and pressures at which real minerals in real rocks probably grew.

first experiment photos

Yesterday I wrote about the effect of water saturation on the size of the crystals grown in my experiments. Today I've got photographs:



These are back-scatter electron images, which means that the brighter the pixel, the heavier elements present at that point (and the darker the pixel, the lighter the element). The bright ring-like objects are rims of iron-rich garnet growing on the seeds of Mg-garnet that was present in the powder before running the experiment. The bright dots of the same tone of brightness as the rings are new Fe-rich garnets growing in the matrix. As you can see, there is a pronounced difference in quantity and size between the two samples. Note the difference in the scale bars between the two photos.

water is important for growth, even for minerals

One of the joys about the learning to run experiments process is that one gets to learn the results of both intended and unintended phenomena. In my experiments the intention is to seal powder of known composition into gold capsules along with a sufficient H₂O and graphite to ensure that the chemical reactions which take place when we elevate the pressure and temperature (to simulate what happens to rocks buried at great depth) take place in “water-saturated” conditions (which is to say there is enough water available for the growth of minerals which require water as part of their chemical formula, such as the micas). However, learning to weld the capsules is a difficult process (I’ve got a draft post on that topic just waiting for me to get photos that actually display the features I want them to show).

As a result of my welding trials and tribulations I’ve had mixed success in the “sealing” part of the above paragraph. Despite the issues with my first attempts at sealing, we ran my first experiment nonetheless, giving two samples a week and a half at elevated pressure/temperature (in this case 650 C and 25 kbars). Once they were “cooked” we had the gold capsules mounted into small disks of epoxy, then carefully polished the disks until the insides of the capsules were exposed. During the polishing stage we received our first confirmation that they had not achieved the same level of “sealed”. Apparently when properly sealed the presence of water inside the capsules ensures that the pore space in between the grains of powder are occupied, and as a result even the high pressures to which we subject them aren’t enough for the new minerals to properly interlock when they grow. As a result, while there are new crystals present, the texture isn’t very rock-like, and when polishing it is easy to accidentally remove clumps of the sample itself. This is the texture we were anticipating, and, for one of the samples run in the first experiment, this is exactly what happened. In these cases we polish only enough to just expose the inside of the capsule, then add more epoxy, letting it soak down into those pore spaces and let it dry before completing the polishing process without so much risk in losing what we are trying to polish.

However, in the other of the two samples run in the first experiment I must not have done the final welding properly, because the contents of the capsule were much harder, and held together better, meaning that the pore space was not held open with fluid when the minerals were growing. This was obvious during the polishing process, so I was able to go quite a bit deeper into the capsule (remember these are only 2 mm in diameter and about 5 mm long so “deeper” is only a relative term) before needing to add the additional epoxy.

Today we got to look at these samples in the microprobe, and as expected from the difference in their textures noted while polishing them, they are rather different from one another. The one wherein I had issues with the welding did contain some water; we know this because there are very small grains of mica present. However, neither was it water-saturated, so it lost some due to the poor seal of the capsule. It contains many, many very tiny grains of garnet (~1 micron diameter; remember that there are 1000 microns in every millimeter) which nucleated on their own, and very thin rims of garnet on the “seeds” which had been included in the powder to encourage garnet growth. The rest of the sample is even finer grained “matrix” minerals, which are going to be difficult to analyze. The other, water saturated, sample contains fewer, larger, grains of garnet, and the rims of new garnet growth on the “seeds” are much thicker than in the first sample. While it, too, is generally fine-grained, it will be easier to find single crystals large enough to get a good analysis of their compositions (which we need if we are going to accomplish our goals).

Having had this first look at the samples we’ve set the probe to create “element maps”, pretty full-colour pictures showing which areas are high (warm colours) and which areas are low (cool colours) in specific elements. Once we have these maps, we will use them to select the grains for the detailed compositional analysis. But even before we do that, I now have a better understanding of the difference between water-saturated and water-under saturated environments in terms of the ease at which minerals grow.

Background learning has meant a reduction in posting

As the weeks slip by in my new job I find that I’ve not been making time to post. I do have a couple of draft posts which are waiting for photos to illustrate them, but they’ve been in a holding state for ages. Somehow the daily press of learning new skills, applying them, attending language lessons 6 hours a week, and doing the homework, combined with a half-hearted attempt to develop a social life in my new country has conspired me to feel as though I can’t spare the time for posting here. However, my log of manner in which I am spending my hours indicates that I have been making plenty of time for personal e-mail and reading the blogs I follow. In theory some of that time could have been spent writing instead of reading.

Ah, writing; it is such an easy way to communicate with people—simply enter the words into a keyboard at a time of your own choosing, and your audience will have the option to read it later, at a time of their choosing. No need to arrange a face-to-face meeting with them to share your information. No need to repeat yourself over and over—one typing session can communicate to hundreds of people, should you wish it to (and, at times, even if you don’t so wish, so it is always best to be careful what you commit to the written form, lest your words be shared in a venue unexpected).

So, what science have I been doing while I was busy not writing (or, in a couple of cases, not taking the photos to accompany the writing)? So far it has been simply learning the mechanics of setting up my experiments. #1 has been run, #2 is in progress, and #3 is approaching ready to go. Next week, I am told, we will look at the results of #1 in the microprobe and see what there is to see. It will be interesting to compare the two samples. My experiments are being run with about 5% H₂O and a small piece of graphite sealed into the gold capsules with the powder. When sealing the capsules it is important to do the welding with the capsule bottom surrounded by water to keep it cool so that the welding process doesn’t boil off the water before it is sealed. I failed to do this with the first capsule I filled. Likewise, I am not certain that I actually managed to get the capsule completely welded shut. If there is a small opening in the capsule it is possible for the water to boil out of the capsule early on in the experiment, and so not be available for chemical reactions.

Given the huge difference in texture between the two samples which comprised my first experiments, this may have happened for one of them. Apparently when water is present in the capsules the result is an amount of porosity in between the grains of powder, despite the high pressure of the experiment, so the end product is soft and easily torn out of the capsule after the run, if one isn’t very careful in the polishing process. However, when water isn’t present the grains of powder are pushed more closely together, and the new minerals have a chance to interlock as they grow, resulting in a much more coherent sample. One of my two samples polished up without a tendency for powder to be plucked out of it, so it is likely that this one operated under “dry” or nearly dry conditions. The other required much more care as it was soft. It was necessary to coat it with additional epoxy before the final polish to keep it from being lost. It will be interesting to compare the results of the two samples. Are there any hydrous minerals at all in the one we suspect was “dry”? I will have to wait till early next week to learn the answer to this question.

Learning to create capsules for experiments

Having recently started a position as a post-doc with an experimental petrology team it is now time for me to make the transition from reading a huge stack of background material to actually doing experiments. This requires learning a whole new skill set, like welding. Here follows the notes I’ve made on the first few steps of creating sealed capsules full of material to be taken to elevated temperatures and pressures during my forthcoming experiments. This is mostly for my own information, but someone out there might find it interesting or useful (or have helpful comments they can add), so I’ll share it here.

*Step one: Prepare the holder in which the capsule will be placed while filling it

During the filling process we use small metal disks into which holes of varying diameter have been drilled as a holder for the capsules (different sized holes are needed because different experiments use different sized capsules). First find a disk which has a hole with the correct diameter (or make a new hole in a disk if necessary). It needs to be just big enough to insert the tube into, without being loose. Then use fine sandpaper to carefully polish the metal around that hole so that when you get to step 6 you will have an easier time of filling the capsule.

*Step two: cut the tube for the capsule

For my experiments I will be using capsules made of gold tubing that is 2 mm outside diameter (1.4 mm inside diameter).

Obtain the correct diameter and metal tube (our lab uses both gold and platinum/gold alloys in a variety of sizes, I’m to use gold for mine) and then cut off a ~7 mm length from one end. To cut the tubing place it on a metal plate, then place an x-acto blade upon the tube and use the blade to exert a gentle pressure to roll the tube back and forth until the blade cuts through without squishing the tube. The back-and-forth motion is essential. This is not “sawing”, which uses a serrated blade to tear chunks out of a material which is stationary, but rather the tube itself rolls during the process as the blade slowly cuts into it.

*Step tree: Pinch closed one end of the tube

To seal the first end of the tube use pliers which have slightly rounded side edges. It is important to use the correct tool, as the sharp sided pliers can pierce a hole in the gold between the unpinched and pinched portions of the tube. (However, one of my colleagues uses the straight-sided ones, but he is very, very careful.) We use a three-part closure, which looks much like a symmetrical peace symbol. In order to create this do it in stages, don't try to squeeze it to final tightness on the first go, that won't work. Instead do a little at a time, slightly pinching on one side, turn the tube 1/3 of a rotation and pinch again, repeating around the tube, tightening the pinches a bit more each pass. Ideally, one wants it to be closed all the way to the center without leaving a hole at the triple junction. Unfortunately, this is difficult to achieve when working on such a small tube with pliers which are so much larger than the tube. Once the end is pinched shut trim the three edges using a cutting tool with beveled edges. The reason for wanting beveled edges to the cutter is so that the place where the two edges of gold meets is as narrow as possible, which will make welding easier. Hold the cutter at a slight angle so that once trimmed the triple point at the junction is slightly higher than the far ends of each of the three seams. (This photo, above left, taken through the microscope, shows a crimped tube held in place in the clamp and ready to weld.)

*Step four: Weld the pinched end shut

The voltage necessary for welding will change based on a variety of factors, including the diameter and length of the capsule, the thinness of the seam, the sharpness and length of the graphite in the welding tool, and what, if anything, you use to cool the capsule as you work. Unfortunately, our

welder isn't very precise and it can be difficult to adjust it to the perfect voltage for any given job. For this size we tried a variety of settings between 25 and 30 V, the 25 V was clearly too low--the welder left it looking "dirty" and coated with black, which is graphite from the welder being left on the gold. At 30 V it was too high; there is too much melting. In between that range the exact value was hard to find, and as variables change, so does the perfect voltage for the task. One variable which can make a huge difference is the sharpness of the graphite point. We have two different sharpeners, one of which makes a sharper point than the other. Using a “point” created by the duller of the two sharpeners at a voltage which isn’t high enough for that point and then switching to a point created by the sharper of the two sharpeners without turning down the voltage will result in the entire end of the tube melting.

The bit of advice I obtained the next day seems to have made a difference—don’t try to touch the gold with the graphite point, but rather hold it just barely close enough to cause an arc between them, and then try to draw that arc along the length of the seam. This isn’t easy, but I did wind up with useable results. Alas, the photo to the upper left doesn't show the welding very clearly--gold is just too darn shiny to photograph well through a microscope with a cheap camera when resting in a brass holder (this is after adjusting the brightness/contrast/intensity to make it visible at all).

I was also told that when welding I should try to start at the outer edge and draw the graphite point towards the middle, which brings excess gold from the edge towards the center to fill the small hole at the triple-junction. The goal is also to wind up with a flat bottom after welding.

*Step five: Prepare the welded tube for filling

After welding the tube it is necessary to re-shape the tube so that the capsule will have properly rounded/curving edges. We have a form (photo to the left) into which the tube is placed carefully so that the widest parts will be inside the form and not pinched between the two halves of the form (one chooses the correct diameter chamber within the form for the tube in question, of course). Once it is positions correctly the form is closed, and the tube is pressed back into a cylindrical shape. Once it has been re-shaped in the form it is put it into the holder (prepared in step one, and resting on a metal plate) and insert into the gold tube a small rod which has a diameter which just fits into the tube (in this case the rod needs to fit into a space 1.4 mm wide). Gently tap the rod with a mallet so that the bottom of the capsule flattens against the underlying metal plate and spreads out to match the curve of the sides of the hole in which the tube rests (take care as to not strike it so hard as to tear a hole in the gold tube!).

After much effort I now have three small capsules with one end of each sealed and flattened, and the other end still open and ready to fill. Stay tuned for steps 6 and 7 once I get them working. Having finally managed to get the tubes ready to fill, I chose to rest on my laurels and call it good for the day.

The pet scientist writes another letter to her school

The following paragraphs were written in reply to the teacher at the primary school in Tasmania, Australia with whom I am long-distance partnered as a “Scientist in the School”. He suggested that I “email the students a paragraph on what you are doing in Europe”, because such a letter “can be used to generate questions from the class”. I made my reply as brief as I could manage, but it still turned out to be a full letter, rather than the requested “paragraph”.

My career as a scientist is in the process of undergoing a major transformation. I just spent the past four years working out the pressure and temperature at which Tasmanian Metamorphic rocks formed based upon the compositions of the minerals in the rocks. I did this by comparing them with the compositions of minerals formed during experiments other scientists have done. Now, instead of comparing real rocks with someone else's experiments, I shall perform my own experiments, so that others may compare their rocks with my results. To do this I have moved to Europe, where I have joined a team of scientists who do experiments.

The experiments we do require very special equipment and lots of time. First we take a powder of known composition, and then we seal it in tiny (about 3 mm wide) gold or platinum capsules. We then encase the capsules in a series of slightly larger containers, each layer chosen for specific reasons. Once we've got them all bundled up inside all of the special layers we put them into a special press, which then puts a large amount of pressure and heat on the package. We choose a temperature and pressure which is within the range at which real rocks are metamorphosed underground, and we leave it at that pressure and temperature for a few weeks. (My experiments will run at ~600-700 degrees C and 20,000 to 30,000 bars of pressure.)

During this time tiny minerals start to grow inside the capsules, just as they do in real rocks which are subjected to such extreme conditions of temperature and pressure. After they've been "cooked" long enough, we open up the package, remove and cut open the capsule, and polish the rock surface inside (it is now a rock instead of a powder because the minerals have grown from the powder, and they interlock together just as a real metamorphic rock does). Once it is polished we use a special machine called an Electron Microprobe, which allows us to analyze the composition of the minerals. It takes several hours of hard work to determine which minerals are now present inside the capsule, and what their compositions are.

Over the course of the next year I will repeat the experiment several times, each time using the same starting composition, but different pressures and temperatures. After each "run" I will make notes of which minerals, and what compositions of minerals are present in each capsule, and at what temperature and pressure they grew.

Once I have enough data from a variety of different "runs", I will draw graphs that show the patterns: the relationship between which minerals are present at different temperatures and pressures, and how the compositions of those minerals change as well. Then, other scientists, at other universities, can use those graphs to compare with their real rocks, and try to work out at which temperature and pressure their rocks must have grown.