Friday, 30 December 2011

"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.

1 comment:

Rockdawg said...

Thanks for sharing. Your post reminds me why I love geology blogs so much. I get to see things I haven't seen in years. Its been a long time since I've seen metamorphic slides. I've always found them fascinating to look at even though I leaned towards sedimentary rocks instead.