"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.

Wish I had seen this one years ago

Earlier this month I discovered that it is possible for me to download entire textbooks from the internet. Now, it may be that the reason I can do this is because I am on a university computer and they have an account with the publisher which makes it possible, but since a high percentage of readers who might be interested in the book I am about to talk to you about are likely to associated with a university in some capacity, there is a reasonable chance that you will have access to it as well, if not as a download, perhaps in paper at your uni library.

The book which I am enjoying reading just now is Phase Equilibria in Metamorphic Rocks, Thermodynamic Background and Petrological Applications, written by Thomas M. Will, published in 1998 by Springer, DOI: 10.1007/BFb0117723. This is volume #71 of a Lecture Notes in Earth Sciences series.

When I first started my PhD project in the field of metamorphic petrology I was coming into the field cold—it had been years since my last geology course of any sort, and I had had no background courses on topics like thermodynamics or the many calculations associated with phase equilibria. Consequently, when I started reading papers which included formulas that explained how the authors had arrived at their estimates for the pressure and temperatures at which their minerals formed I found myself skipping over the equations and looking for the sentences that explained what was done. Over time and after reading many papers which did this sort of thing I started to gain a partial understanding of the topic. Enough to apply the tools to my own samples and arrive at numbers that, hopefully, actually reflect the history of the samples.

However, while I could follow a recipe for calculating pressure and temperature of formation for metamorphic minerals, I choose which recipe to apply based on the fact that my samples were similar to those in the published papers, not because I had any real understanding of the models which underpinned the calculations. Terms like “ideal” vs “non-ideal” mixing painted pictures in my mind because I know what those words mean grammatically, not because I actually knew the difference between them as applied to the crystal structure of a mineral.

If you are in a similar boat, and would like to actually see good definitions of those terms, along with others like “entropy of mixing”, “activity-composition (or a - x)”, this may be the book for you. So far I have only managed to read the first 40 pages, but already I have a *much* better understanding of why it is that one gets such hugely different results in terms of pressure/temperature estimates depending on which model one uses for a given mineral, and a much better understanding why programs like Perple_X have so many models available to choose from. The author is kind enough to work through example calculations in a step-by-step basis, so that the reader can learn how it is done. Sure, when actually doing the geology you aren't going to do these calculations by hand—at the very least you will have a template full of formulas set up in a spread sheet, if not using a more complicated program to do the work for you, but it is always nice to understand *how* the program does the calculations—this makes it much easier to spot if a typo in the data-entry stage resulted in a geologically implausible answer being spit out by the program.

Science is the ultimate game for adults

I have just come from listening to a lecture by Dan Shechtman, this year’s Nobel Laureate in chemistry. The topic of his speech was his 1982 discovery of quasi crystals which has led, all these years later, to his achieving a Nobel Prize for his work.

He began his talk with an introduction to the science of Crystallography, which was founded in 1912 when Sir William Lawrence Bragg and his father Sir William Henry Bragg first starting using X-rays to study the diffraction pattern caused by crystals. (Prior to that breakthrough people studying crystals measured the angles between the faces of crystals, which had been done at least as far back as the 1600’s—Johannes Kepler published studies of snowflake crystals in 1611, and in 1669 Nicolaus Steno reported consistent sets of characteristic angles for quartz crystals, no matter where the crystals came from.)

During the next 70 years crystallographers studied 1000’s of crystals using X-ray diffraction, and they all conformed to a set of rules which became accepted as the definition of a crystal:

*They all had a periodicity; if you measure from the center of the diffraction pattern to the first spot, and then move out again that distance in the same direction you will encounter another spot, and another again each time you repeat the pattern.

*they all had a rotation of symmetry; this means that if you rotate a crystal around an axis you will get a repeat of the same pattern every certain number of degrees. The possible amounts of rotation were 2, 3, 4, and 6.

This means that a crystal with a 2-fold rotational symmetry can be rotated half way around (180 degrees) and will look exactly the same as before it was rotated. A crystal with a 3-fold rotational symmetry can be rotated to three different positions (each 120 degrees apart) which have the exact same pattern. 4-fold means that each rotation is 90 degrees apart, and 6 fold is 60 degrees apart.

As Professor Shechtman emphasized in the early portion of his talk, this list was it—crystallographers knew that no other rotation symmetry pattern was possible. There was no such thing as a 5-fold rotational symmetry, nor were there any crystals with rotational symmetry greater than 6. This was so well accepted that when he was a student he once had an exam question requiring that he prove that 5-fold rotational symmetry was impossible.

He shared with us his answer on the projector screen. It works better with images, but I will try it with words alone (see this page for more details). Start with a dot on the center of your page, then put five dots around it in a ring, each 66 degrees apart, all the same distance from the center dot and from one another. If 5-fold rotational symmetry is possible one could then take any pair of those encircling dots, rotate around them in five steps, plotting a new circle of dots around each, and the new dots around one would line up with the new dots around the other. However, as he showed us on the screen, this does not happen—if you colour one set in blue and the other in red they clearly fall near, but not upon, one another.

Therefore when he noticed a crystal which showed a 10-fold axis of rotation on a transmission electron microscope on the afternoon of 8 April 1982 is was more than a bit surprised, as one can see with the triple question marks he wrote in his notebook. His first thought was that it must be the result of crystal twinning, which had been known to cause the appearance of five-fold rotational symmetry but was really the result of having more than one crystal contributing to the diffraction pattern. So he zoomed in to the limits of the machine and took the diffraction pattern again, several more times, and each time he received the same result—a 10-fold rotational pattern, and from areas so tiny that the possibility of there being more than one crystal contributing to the pattern had been eliminated.

How was this possible? Well, the substance he was studying does have a rotational symmetry that had previously thought to be impossible, but it achieves it by lacking in periodicity—the pattern does not repeat if you jump a set distance in a given direction. As a result of his discovery the definition of a crystal has changed to say “any solid having an essentially discrete diffraction diagram”. These days we call substances such as the one he discovered “quasicrystals” and it turns out that they are reasonably common. But in the early days after the publication of his discovery he met with much resistance to his ideas—one scientist was even heard to declare that there are no quasi-crystals, only quasi-scientists”. However, as happens with science, other laboratories repeated is results, both with the compound he first noticed the phenomena in, and then with other compounds, and gradually the acceptance spread throughout the scientific community.

After he finished his speech there was time for a few questions from the audience. One of the University faculty members ask him a question—she pointed out that this university, like many others, has difficulties attracting students to the study of science, and she wondered if he had any advice in how we might better convince young people to study science. His reply included one of the best quotes I have heard in ages “Science is the ultimate game for adults”.

I believe that he is, in fact, correct with that description—studying science is fun, it requires us to use both our logical and creative parts of our brains, and to push both to their limits. It provides enough challenge to prevent us from ever being bored, and it comes with possibility of making discoveries which change the way we view our world. What could be more fun than that?

Can’t make it to AGU this year? Neither can I, but I will still visit the poster session

It is once again the time of the year for the big AGU conference in the US. I made it to the one two years ago, and found it kind of overwhelming in its hugeness, with talks and posters on any number of topics which fall into categories of which I know nothing, because geology is nothing if not a huge broad catch-all of a science. Fortunately for us that meeting, like many others these days, has a fully searchable program, permitting us to find those talks or posters we should hear/see because they relate to our own research, and those talks which are totally outside our current research but still fall into topics which are of personal interest. This year, in addition to having the computerized program, they have also made much of the poster session available on line. Therefore those of us who are staying home can still have the fun of wandering the crowed isles and looking at interesting research results. So ahead, go on over, look at some posters, and learn something new. Perhaps you will even find something interesting enough to send the author a note inviting conversation on the topic.

reading for fun and education

Sixty days ago I received the job offer to start my new job, and was given a small pile of literature for background reading before I started. As a result I started my “1000 words a day” challenge once again, after having had nearly a year off since last I had done that. The challenge is simple: do a little reading in the geologic literature every day, and keep track of how many days in a row you manage before you miss one. If you miss one re-start and begin the count from zero once again. How much is 1000 words? Well, today’s post is 691 words long. It doesn’t take much time to read that many words, but I have found that if I read that many I often keep reading the article until I either reach a natural breaking point or finish the article. Some days I read quite a bit more, others I just barely make the goal (do I actually count the words? No, not any more. And when I did at the beginning of the first time I undertook the challenge I didn’t count all the words, just how many words were in the first line of a paragraph, and how many lines long it was to calculate a rough word count for the paragraph, and then figured out how many paragraphs of that size it would take to reach at least 1000 words. It isn’t the precision that matters, but the consistency of actually reading (and thinking about what you read!) every day.

Since I started this time I have remembered to read every day. This has been easy during the week days since my job begun—the pile of literature I need to read and understand in order to get my knowledge base to where it needs to be for my research project is quite substantial, and growing all of the time as I find references to more and more articles I wish to read. Managing it on the weekends is a bit more of a challenge, but I have managed, so far.

So, what have I read over the past 60 days? I have completed reading six articles that relate directly to the ore deposits in the area near where my project is based (ok, one of those wasn’t a single article, but rather a PhD thesis which was comprised of five different papers, so really I have done 10 articles total on this subject), four articles on the concept of 3D modelling, and one article on geochemistry.

That last one (MacLean 1990)* explains how one can use ratios of immobile elements to calculate what the unaltered composition of a suit of altered rocks must have been. I am gathering from my reading (many of those local papers cite this technique paper) that this is a very useful way to determine what types of rocks were present before the hydrothermal alteration associated with the formation of ore deposits. It works especially well when the precursor rocks are volcanic and changed in composition due to fractionation of the magma. When this is the case one can plot the current compositions on the same diagram as the curve which shows the expected changes in composition due to fractionation, and extrapolate from the trends in the current compositions back to the likely original compositions when the rocks cooled from their magma. The paper mentions that these sorts of calculations are easy to set up in a template in a spreadsheet, and that they will give away such templates upon request. I wonder if that offer is still open two decades after the paper was published, or if people use a different technique to accomplish the same sort of task today. I will have to do a search for papers which cite this one to work my way forward to the modern techniques, if they have changed. What did people do for research before it was possible to easily look up who had cited a particular paper?

*MacLean, W. H. (1990). "Mass change calculations in altered rock series." Mineralium Deposita 25(1): 44-49.