not late at all, measuring with a geologic time scale

As the rest of the geoblog sphere is busy offering up their entries to this month's Accretionary Wedge, I am finally able to offer my entry for last month's call.

Why a month behind? Because the call went out while I was in Scotland, and the counter top, ok, floor, I wanted to photograph was in Sweden. So I had to wait till I got home, and then I had to wait another couple of weeks before I found time to head to the city center to get the photos, and then I had to wait till I had time to actually upload them. However, I think you will agree that this particular building stone was worth the wait.

This is the floor at my eye doctor's office, and what a lovely floor it is. Look at the garnets! Look at the banding! I would love to know the location of the quarry that produced this rock.

"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.

orogenesis

I have been far too busy enjoying the fact that I moved to Scandinavia for love to post very often, but I feel compelled to play in this month's Accretionary Wedge. My favourite geology word would have to be orogenesis. I love mountains better than any other land form, and the process of their formation is endlessly fascinating. The microscopic changes which happen in the metamorphic rocks at depth during mountain building are rich with information about the processes that built the peaks.

Spooky deskcrops

This month's Accretionary Wedge asks us to share photos of our favourite deskcrop, with bonus points if it is spooky. As one who moves, often, I have managed not to accumulate a personal collection of rocks, but rather catalogue them appropriately and store them in the collections of which every university I was with when I collected them. However, I have many photos on my computers of my samples. My personal favourite "spooky" rock photo is this back-scatter electron image of a monazite grain. Every time I see it I think I'm looking at the skull of some sort of humanoid creature.

This was one of the many grains in Tasmanian metamorphic rocks analyzed for U-Th-Pb dating for my PhD thesis. Analysis of a point near the optic lobe of the skull of this grain gave an age of 508 +/- 5 Ma for this grain, which is in good agreement with the main Cambrian metamorphism within Tasmania.

Barrow’s kyanite zone

It has been a while since I made time to play in the Accretionary Wedge. This month the topic asks us to share (a) geologically significant photo(s). This announcement reminded me that I never did type up a summary of the field trip I did last year after the MAPT conference, so I’ll share a couple of photos from that trip, and explain why they are important.

The field trip went to look at the area Barrow made famous when he used it to formulate his theories on metamorphic facies. The Dalradian series is located just north of the Highland Boundary Fault in Scotland. Different parts of the series were metamorphosed at different pressures/temperatures, which resulted in different combinations of minerals in each area, though the composition of the rocks is similar.

In our field trip we started out near the fault, and stopped in each of the key areas along the way, visiting outcrops of the chlorite zone, the biotite zone, the carnet zone, and the kyanite zone. Most of the geologists on the trip are accustomed to looking at rocks in thin section, where, once you’ve learned how to identify them, it is a very easy matter to identify minerals and tell at a glance which zone the rocks come from. However, when Barrow did his field work in this area, he did it largely without thin sections—instead he looked at the rock outcrops themselves, and broke off pieces with his hammer and looked at them in the field.

The man was a talented enough petrologist that he was able to identify the minerals in the field, and to tell when he’d moved from one mineral assemblage to the next. Having spent a rainy day tramping around the lower part of the Scottish Highlands accompanied by geologists who knew in advance what minerals to expect in each outcrop, I still had difficulties spotting the index minerals in the outcrops. Or rather, I did, until we reached the kyanite zone. There is simply no mistaking the large blue crystals of kyanite in these rocks!

Here's one I prepared before

Is it cheating to participate in this month's Accretionary Wedge by simply linking one's first ever blog? How nice of them to request a topic upon which I've already written, since I've not had much time for writing recently.

I've been taking time off from geology related stuff--I've had just over two weeks of adventures visiting friends and family in the US after finishing up my PhD project in Tasmania and before heading to Europe for my first post-doc position (the degree itself will be complete sometime in the next few months, after the examiners have had a chance to look over my results and I've had a chance to do any corrections they wish to see). I fly to my new home in another four hours, with luck I'll be posting again regularly soon.

Connecting Microscopic and Continental Scales

This month’s Accretionary Wedge is on the topic of “connections”, which is a terribly appropriate topic for geologists, since everything is interconnected in this universe, and we (generally) study that portion of the universe which we call “Earth”. The planet itself is a ball made of up a variety of layers some of which, over time, do a bit of mixing up. They call this mixing “plate tectonics”, which theory has been around in a changing, but reasonable stable form since I was a child, and existed in a much earlier form (as early as 1915) called “continental drift”. The theory of plate tectonics is a very inter-connecting, unifying theory which helps explain the current distribution of continents around the planet, which parts of the continents (and the ocean floor as well) are wrinkled into mountains (and how the relative motion between two “plates” causes that wrinkling, either in the form of very high mountains when two plates “crash” into one another (e.g. the Himalayas, where India is colliding with Asia) or in gentler mountains where they are sliding past one another (e.g. the San Andres Fault).

Plate tectonics is also useful when looking at rocks, like those I’ve been studying for my PhD project (see photo in my user pic!), which, while at the surface of the earth today, have evidence that they were once buried quite deeply. Every mineral has its “favourite” temperature and pressure—the conditions wherein it will grow, given enough time to do so. Many minerals have a range of compositions, and they will vary their composition based on the temperature and pressure under which they are growing. Garnets will swap in magnesium (Mg) and out iron (Fe) at the same conditions that biotite swaps in iron and swaps out magnesium. This exchange has been studied extensively, going back to the 1970’s with experiments involving putting a tiny amounts of biotite in with large amounts of garnet (and vice versa) with known amounts of Fe and Mg into tiny containers which they then squished (that’s a technical term) with lots of (a known quantity of) pressure and high heat for an extended period of time. Then they checked the mineral which was present in smaller quantity to see how its quantities of Fe and Mg had changed. Doing this a number of times at different conditions permitted them to work out how much iron and magnesium each mineral prefers at a variety of pressure-temperature combinations. As a result of such experiments and comparing them with the proportions of Mg and Fe in biotite and garnet in real rocks, we now have a method of working out just how hot and how squished (how much pressure) rocks must have been to grow the minerals they currently have. This helps us work out the details of the past configurations of the continents—it is known how deeply something must be buried to reach a given pressure, so from the composition of the minerals we can figure out how deep the rock used to be, and therefore what its prior plate tectonic setting would have been. It is all interconnected, from the microscopic level to the scale of an entire continent!