Sunday, November 4, 2007

Teeny-tiny partial melt

I just got back from a two-day microprobe marathon - two days on what's essentially a scanning electron microscope that can do chemical analyses of very small crystals.

Here's one of our images:

It captures the coldest melt that occurs in metamorphic rocks: the partial melting of muscovite. Muscovite is the glittery, flaky mineral used for things like making eye shadow sparkle. It's very common in metamorphosed shales, but it gradually gets used up in metamorphic reactions as temperature goes up, until finally it reacts with quartz to grow sillimanite (the tiny fibers in the image) and potassium feldspar. And it can also melt, a little.

And this one did.

There's a tiny bleb that looks like a pair of sunglasses in the middle of the grain. Maybe if I outline it you'll see it:

Most of the bleb is made up of quartz (qtz). But the tiny bright spot in the middle is potassium feldspar (kfs) - exactly what you should see if the muscovite (mu) melted a little bit, and then the melt crystallized without ever escaping.

1.4 billion years ago, this rock melted. A little. (There were also a few little dikes in the field, but because we were in a contact aureole, we weren't sure if they were in situ partial melts, or if they were part of the granite. Now I suspect they were in situ melts.)


saxifraga said...

I found your blog recently and just began reading, but I wanted to stop by and say hi. I have been following the geo blogs from a distance because I wasn't being very specific about my own work/ field on my blog, but missed inspiration from other bloggers in my own field. I like the way you are writing about geology and teaching, so I guess I will come by here often. Nice explanation about Muscovite. I'm a sedimentologist and haven't thought about partial melts in quite a while.

Sabine said...

That's one of the things I miss most now that I am in the Petroleum industry, looking at thin sections (and lab science in general). Even our cores are outsourced (although sometimes I go out to the rig and play with cuttings). Very cool slide, I love mica!

Kim said...

Thanks, saxifraga. I kind of alternate between "oh, wow, cool" posts and thinking out loud about teaching - hopefully the balance works.

Sabine - interesting that thin sections aren't used much in the petroleum industry. We talk about whether to keep teaching optical mineralogy (we still do, though many schools have discontinued it), and I've often wondered how many of the skills that we teach are actually used in industry. I've never worked in industry; I graduated from college when geoscientists were being laid off, and there pretty much weren't any jobs to get. But it hurts, a bit, when deciding what's important for students who are going straight into the professional world.

saxifraga said...

Interesting comment and response to what is actually being used in industry. I haven't worked in industry either, but most from my grad school cohort work in either the petroleum industry or enginnering/ consulting companies since academic jobs in my field have been virtually non-existent in my part of the world for about twenty years.

I'm curious why optical mineralogy is on the way out of the curriculum? I gues I have never really used all the microscope excersise we got in real life, but somehow assumed that probably someone else did.

Kim said...

Saxifraga -

In many departments, optical mineralogy has been added into courses in mineralogy and petrology, rather than being a stand-alone course. (In fact, that's the way I learned it.) I think part of the reason was a desire to streamline geology majors, so that students could discover geology during their first (or even second) year in college and still complete a major in it. (In the US, at least, geology isn't taught very much in high schools. And then in the late 80's, when the oil industry was laying people off, it was hard to convince students to study a subject in which there were few jobs. So departments did what they could to keep their numbers of students up, so they wouldn't be eliminated entirely.)

I use optical mineralogy a lot - I work on ductilely deformed metamorphic rocks, and thin sections are really a key tool for understanding everything from metamorphic reactions to deformation mechanisms. But I'm not in industry.

saxifraga said...

Thanks for the explanation. I also had optical mineralogy as a part of the compulsory mineralogy course in undergrad (what we learned in that course was also used in the subsequent petrology course). It's the same here with geology not being taught much (if at all) in high school and universities have been seriously struggling to keep student numbers up after a long time with no or limited job opportunities (hence no need for new faculty). Things seem to be looking up a bit now, though with a better job market outside of academia, some of the dominant generation of geology professors retiring and some more explicit regulations for how geology should be integrated in the geography curriculum in high schools.