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Posts Tagged ‘petrology’

Hard to believe, but this is my 100th post.   I thought about doing something serious, but honestly, let’s just go with what has been successful for me: garnets throughout my geology career

Garnets in plane light from the Northeast Kingdom of Vermont (BA research):

ERG-25b from the Cow Mountain region of Vermont; garnet inclusions are clearly at an angle to the dominant biotite-muscovite foliation in the rock

Garnet back-scattered electrons (BSE) image from the Connecticut portion of the Bronson HIll terrane (MS research):

BSE image of 99ERG07c

Girl Scouts participating in a Rocks Rock badge workshop (Western Kentucky University, 2002).   We ran the workshop in conjunction with both the Campus Girl Scout troop & the Geology Club the year I taught at Western.

Juniors (4th-5th graders) examining a garnet schist for the rock ID portion of their badge

Wavelength dispersive X-ray spectroscopy (WDS) images from a sample from Payer Land in northeast Greenland (or the PhD project I didn’t end up working on):

WDS mapping of 438965. Garnets are green in the Fe map, blue in the Mg map, and very light in the CP.

In-situ picture of the rocks I did work on for my PhD from Alp de Confin in Switzerland:

bimodal garnet sample that contains kyanite (hard to see without direct sunlight in these samples) from the Adula Nappe

Outcrop photo from a 2007 Vassar field trip to Gore Mt in New York:

amphibole-pyroxene-garnet-plagioclase wall in the upper quarry at the Gore Mt mine

When I taught at the University of Pittsburgh at Johnstown, I started blogging.   At that time I started posting about photomicrographs, including one post about the “jellybean” mylonite from Payer Land.   Hmm, mylonites & garnets together 🙂

Garnet "fish" from the jellybean mylonite.

I started this blog when I moved from UPJ to Gustavus in the summer of 2010.  Over the course of the year, I posted several times during the year about garnets, but the most memorable to me is my summary of the 2010 GSA session in Colorado entitled “Garnet and Its Use in Unraveling Metamorphic and Tectonic Processes”.   Since I didn’t have any pictures to post the first time around, let me just say that garnet research is alive, well, and going in a multitude of directions according to that session.

And last summer I moved back to the Boston area.   Because I’ve only been teaching physical geology & solar system since moving, garnets haven’t come up much in conversation here recently.   In fact, my only garnet-related post was my submission to AW #43 earlier this week.   I obviously need to work on that!

pen & ink drawing of garnet gneiss from Best (1983)

So that’s a 100 posts.   Let’s hope I pick up the pace a bit, add a few more garnets, and hit 200 in less than 18 months 🙂

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Live-blogging is not something I could have managed during GSA, since I’m usually taking notes for myself during the good talks (and trying to catch up with “brilliant thoughts” at other times), but I’m going to try summarizing the ridiculously good session that occurred on the final Wednesday of GSA at the beginning of the month in Denver.

I will note that I did attend other sessions at GSA, but most of the time there were only one or two “amazing” talks (e.g. Jane Selverstone’s talk on the microdiamonds found in the eastern Swiss Alps).   The session Wednesday had me writing & thinking the entire time–it was that good.

The session title was “Garnet and Its Use in Unraveling Metamorphic and Tectonic Processes” and was organized by Ethan Baxter, Mark Caddick, and Jay Ague.   Talks were held in the morning and the afternoon was dedicated to the poster session in a rapidly emptying hall (last day at GSA generally has attendance issues).   The wonderful thing about this session was that it look at a rather variety of ways that garnet could be used within metamorphic rocks and did a great job of bringing people together from different areas of our sub-disciple to talk & think about different ways to look our rocks.   And the organizers did a very good job of picking appropriate speakers and ordering the talks, so that there was a flow over the course of the morning.

As stated above, the last day of GSA is frequently a bit sparse attendance-wise.   (In 2001 in Boston I had a poster the final morning of GSA and various people appeared with suitcases in tow.)   And since the topic wasn’t exactly “sexy” (unless your a metamorphic petrologist!), I personally was thinking the room would be half to quarter empty by the time my own talk occurred at 11.30.   Yeah, that was erroneous logic.   Turns out the session was full for the entire morning.   There were people standing for every talk and new chairs had even appeared during one break.   Lots and lots of people were interested in garnet on that day 🙂

For me, the greatest benefit of the talks was all of the different moments when I thought to myself, “oh, I need to consider that” or “maybe we should do that with these rocks?”   Let me give you a short run-down of the high points (and yes, if you’re not a petrologist, this is probably the time to move on to something else):

  • Frank Spear (invited talk) opened the session by asking whether we have to only consider grain boundaries as our effective bulk composition (i.e. what’s actually reacting to form new minerals / grow ones already present), which would lead to some very complicated math… It would also mean that grain size has a direct impact on what kind of garnet zonation patterns are found within rocks.   Thought-provoking, but also a bit scary from the re-invention side.
  • Thomas Lapen (disclaimer: Lukas Baumgartner is also one of my co-authors as well as Lapen’s) discussed differences in what stage of growth of garnet Sm-Nd vs. Lu-Hf actually records
  • Ethan Baxter runs a TIMS lab and his talk gave a good update as to what they could or couldn’t manage to date at this point (garnet can’t be dated in situ like zircon or monazite, you have to actually extract it, which puts limitations on what the resolution of the ages are); for instance, they dated 12 different regions within a 6 cm garnet to get the growth history (episodic not continual)
  • Ashley Russell (student of John Valley’s) presented material on using oxygen isotopes within garnet to address the issue of fluid presence / absence during growth within a high pressure / high temperature (HP/HT) region in the Czech Republic
  • I was happy to see Greg Dumond’s talk simply because he had complicated pressure-temperature diagrams (which is where mine are headed towards)
  • Bill Carlson (also invited talk) returned the session to the theoretical (this is what Spear & Carlson do better than almost anyone else) and discussed why not only large cations, but also small cations, diffuse slower than moderate-sized ions.   He ended with a teaser that this may be true for divalent cations (e.g. Mg, Mn2+, Fe2+, Ca), but not for trivalent cations, which he’ll discuss at AGU
  • Sumit Chakraborty looked at diffusion rates of elements within garnet comparing the various calibrations that have been made–especially of calcium within garnet–and argued that you need to go with end-members (grossular vs. andratite vs. uvarite) instead of simply Ca vs. Mg vs. Fe.   This talk & several others resulted in several serious discussions with members of the audience, but the long & short of the matter is that we need to understand how fast things diffuse & what controls them in order to model the process in real rocks.   This theme would also come to the front in Baumgartner’s poster (volumetric differences between garnet end-members) and Caddick’s modeling of diffusion vs. growth and the resultant patterns in garnet (also a poster)
  • I’m not reviewing my own talk…

Of the posters, I spent most of my time talking with Mark Caddick and the implications of his model (he has a paper out in November’s issue of the Journal of Petrology if you can 1) get behind the paywall and 2) are interested).   Baumgartner’s poster had a rather large crowd in front of it the entire length of the poster session and it was a veritable “who’s who” of metamorphic petrology.

(I want to apologize if I misconstrued anyone’s research–I’m reading my hasty notes in trying to summarize this and may have missed some blatant point or re-interpreted something to suit my own research needs.   And if I didn’t cover your talk, it doesn’t mean it didn’t influence me–just means that I had to pick & choose for length.)

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M&M lab

(Though several people have suggested I try doing this lab with something other than M&Ms (e.g. plastic counters, colored beads), I think its the fact you can actually eat the magma chamber at the end that causes this lab to be a favorite.)

So, what is the process that we’re trying to demonstrate to the students?   Once a melt forms, several things start to happen to the liquid:

  • it rises due to a lower density than the surrounding rock (buoyancy)
  • it interacts with the surrounding rock & starts to cool down
  • in some cases, the surrounding rock melts & adds into the existing liquid (assimilation)
  • sometimes the first melt meets a second melt and they mix together to form some form of intermediate melt (magma mixing)
  • in all cases, the melt cools to a point and starts forming crystals which can either continue reacting with the melt (equilibrium crystallization) or separate themselves chemically from what’s going on in the melt (fractional crystallization), which changes the overall composition of the melt over time
  • in more traditional models of fractional crystallization, the crystals would form & then sink to the bottom of the pool of melt (magma chamber) to collect and each successive layer of crystals would have formed from a later & later point in the melts history
  • how the melt during fractional crystallization changes depends on what the 1st then 2nd then 3rd crystals are made of: if you first take out X from a collection of WXYZ, then the rock will proportionally become more enriched in WYZ & depleted in X; the 2nd mineral removed has a different proportion to pull from, so maybe it takes XY, making WZ be more enriched & XY depleted

For this lab, we’re using the the more traditional sinking model and the M&Ms represent various elements removed to make up a variety of minerals.   The students are given a list of minerals that crystallize out at each stage, so they can figure out how many of each color M&M to move out of the magma chamber.   At the end, you have a “fully crystallized” layered magma chamber:

Mike, Jonathan & Carson's magma chamber

Sam, Andi, & Todd's magma chamber

My students divided into two groups and went about this in very different ways.   The seniors (upper picture) kept all of the “minerals” that crystallized out at every step together, so their colors look mixed together.   The juniors (lower picture) chose instead to group by element, so you can’t see each “mineral” that crystallized, but you have a better idea of how much “Si”, “Al”, or “Fe” was in a given layer.   If you look at the pictures, you should see some trends about how many “crystals” formed at each stage; how the chemical composition changed from layer to layer; and, if you’re really good, how the names of the rocks varied from layer to layer.

There are a few M&M magma chamber activities out there, but I tend to use the more complex version of Wirth’s (intro level also exists).   There are some limitations to what we can model & how the system has to be modified in a delicious candy form, so I tend to follow this lab with the not-as-beloved MELTS lab I adapted from Jim Brophy at Indiana.

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