I posted in the fall about the M&M fractionating magma chamber lab I did with my petrology students. That lab is based off of assignments that Karl Wirth (Macalester College) has posted on the SERC website here. There are two versions of the magma chamber that you can download: one for intro-level students that is a bit more basic and does not require as much math or graphing of information and a second, upper-level lab that I used this past fall.

Wednesday we ran the intro-level lab (modified) in my Iceland class. By this point the students had had: basic igneous rock names, how magma forms, and how magma moves through the mantle & crust.

Why go with Wirth’s lab? There are a few other M&M labs out there, but only the Wirth one seems to be on the SERC website (yes, I searched several different ways…). I’ve spoken to a few professors at other institutions about this idea and they’ve talked about what they’ve developed, but for ease of simply taking someone else’s hard work and modifying it to suit your needs, SERC is really nice.

What do I like about the lab?

- M&Ms are edible. The students really seem to appreciate this.
- The ratios chosen by Wirth for Si vs. Al vs. Mg etc. are visually easy to group into “large amounts” vs. “moderate amounts” vs. “small amounts” that make it easy to qualitatively ask “what is happening to the proportion of ___ as the different layers crystallize?”
- Though the intro lab has quite a bit less graphing, both labs do require the students to actually make X-Y plots. At the upper level, its a great way to demonstrate why Harker diagrams are used in igneous petrology and how crystallization sequence will vary the slope during solidification of the magma.
- Ratios are well constructed, so both the residue & resultant magma from each step moves progressively from more mafic to more felsic compositions.
- Students seem to understand crystal fractionation well after this lab better than when I simply did a thought experiment using colored filled-in circles on the blackboard previously.

Ok, so why don’t I just run the lab as written?

- With intro students, I was trying to do this lab within a 2-hr block of time, so I edited.
- I wanted to insert the igneous rock naming chart from Marshak (Fig. 6-18a) so the students could easily figure out the name specific layers.
- I tried to do something with assimilation & magma mixing using the same type of construction.

So, what did I change?

- I had a departmental TA count out all of the M&Ms.
- Instead of having all of the students construct all of the X-Y plots, I gave each student one X-Y plot to do on a large piece of graph paper that we had in the department on top of a bookcase and then talked about them as a group.
- I had the students construct two magma chambers (I really should have done three, but I only have 6 in this class).
- After playing a bit with the numbers, I added two more sections to the lab: one for magma mixing and a second for assimilation. The students were to first go through all 10 crystallization steps and questions from Wirth. Then, we pretended that instead of continuing on after step 6 crystallized with the regular crystal formation, something happened to the remaining magma to change its composition. In the first case, I gave them a second batch of M&Ms (simply took Wirth’s original numbers and divided by 2) and told them that suddenly a new batch of basaltic magma had risen into the magma chamber and mixed with the remaining magma after step 6. In the second case, I gave a batch of M&Ms that represented a shale (simply took the bulk composition out of a paper and went 1 for 1 from the wt %) and told them that these rocks had dropped into our magma remaining after step 6 and had melted. The third case was similar, but the rock that dropped in was a calc-silicate. In all three cases, I asked them to calculate what the composition of the new magma would be as well as what that would imply about the next minerals to crystallize. And then we brainstormed about what kind of evidence would be present if magma mixing or assimilation or crystal fractionation had occurred.

Did the changes seem to help?

- The lab only took 2 hrs, so that worked.
- When discussing assimilation vs. mixing vs. fractional crystallization a few days later, the students could at least intelligently answer questions–previous versions of this lab had only resulted in a strong understanding of fractional, with more nebulous knowledge of mixing & assimilation. I really believe they understand better if they do it, rather than listen to me lecture about it.
- Even with only one X-Y plot to draw, the students had to think about it, so maybe I should let them construct several just to re-inforce basic skill sets.

What might I change in the future?

- Though Wirth has a good set of minerals chosen and they do demonstrate solid solution, I found with the upper level class I had to emphasize that though there were 4 fayalite & 1 forsterite in a given layer, that really meant that what was present was a 20%Fo – 80%Fa composition of a single olivine crystal.
- All of Wirth’s minerals are anhydrous. Especially when trying to do this to talk about volcanic processes and the build-up of volatiles in a magma chamber, a hydrous chamber might be a better model.
- Wirth’s order of crystallization follows Bowen’s reaction series, which is probably fine for an intro class, but upper-level students need to understand that its only one possible path the magma could take. My current solution to this is to follow the M&M lab with one using MELTS, which the students aren’t exactly thrilled about.

Do you use an M&M lab? What parts of it do you find important? Frustrating? Work really well?