[MSA-talk] Plate tectonic Ig/Met Pet syllabus

Sumit Chakraborty Sumit.Chakraborty at rub.de
Fri Dec 29 09:44:42 EST 2017



Hello everybody,

As noted by Liz and Mike, repetition seems like a good idea. I have 
taught an Ig Pet course for several years now (and a similar met pet 
course many years ago) with the general approach that the first part of 
the course introduces tools, the second part uses these on specific 
tectonic settings, rather than present all of it in one go. This is at 
the Masters level though.

In addition to reminding students of topics, there is a pedagogical 
significance of such repetition, I have learned: one learns phase 
diagrams, or trace elements, or isotopes as tools but as I tell my 
class, when you go to an outcrop, there is not a signpost saying "use 
phase diagram x-x on me"....one has to decide which tools to use, and 
how to combine the tools to get the story out from the rock. That is an 
"inverse process" from what learns in class. So, what we do is go 
through the first part of the class learning the tools (mentioning 
possible applications e.g. textures in thin sections related to phase 
diagrams - see the Philpotts book for some examples). This is the 
forward process, if one will. There are associated labs to the lectures 
- from reading phase diagrams and lever rules to doing mixing and 
fractionation exercises, using different trace element partitioning 
models etc.

In the second part, I focus on three "rock types / tectonic settings" 
where one uses all the tools to get the "story" - Mid ocean ridges, 
continental arc volcanics, and granitoids. This is the "inverse" part, 
if one will. (Winter's book is laid out a little like that, although of 
necessity, it focusses on petrology).

Here we begin with observations - tectonic setting, outcrop scale, thin 
section scale, various chemical data, geophysical data (e.g. thermal 
structure)...and then do get into using all the information we can (i.e. 
tectonics, geophysics etc. as well), along with the tools developed in 
the first part, to understand the origin of the rocks and to look 
critically at alternate models. For example, with mide ocean ridges, we 
get into details like fast vs. slow spreading ridges, the difference in 
morphology of the ridges and their bathymetry, related to differences in 
compositions of plagioclases etc. We go through how N-MORB's and 
E-MORB's came to be distinguished, how Iceland, Hess deep and Pito deep 
are all different beasts, the debate about "depth of magma chamber" that 
the older ones among us may remember (connect back to phase diagrams), 
and that ended up with seismics finding no large magma chambers (only an 
axial magma lens)..... and on the whole discussing how a "boring basalt" 
is anything but. In the process, we end up discussing and seeing that 
mid ocean ridges are not just "basalts", in fact, most of it is 
"gabbros". Real places, real rocks, and stories as they unfolded and are 
still evolving.

I have some similar material for continental arcs (where one can discuss 
fractionation and mixing processes in a lot of detail, and the interplay 
between tectonics, thermal structure, and kind of magmatism). One good 
entry point to get students thinking is: In their simplified intro class 
pictures, the isotherms fold down in subduction zones, giving some of 
the lowest geothermal gradients known - how come that is associated with 
such a concentration of magmas and volcanoes? The usual "water sinking 
melting temperature" does not work too well for melting peridotites in 
the wedge.....and that makes them sit up....through the mid ocean 
ridges, they have been primed to not expect "mantle comes up - melts - 
basalts erupt" kind of scenarios. For granitoids, it is more of a 
hodge-podge, no particular setting, but more focussed on I-types vs 
S-types etc. All along, I try to remind them of the eternal "source vs. 
process" question.

I have some thin sections and hand specimens to go with these...the lab 
part on the second half of the course is on documenting whatever 
processes one can "read from the rocks", emphasizing that (a) one can 
read a lot, but also (b) some parts of the story have to necessarily 
come from other kinds of observations, and one needs to combine these to 
get the full picture. And of course, in each case, the story is still 
evolving - the textbook is not the last word...one can always get 
something from the last meeting one has been to, to provide an example 
of how ideas are changing.

Disadvantage of the approach - I do not get to deal with many important 
classes of rocks (no layered complexes - shock! compared to my own 
Skaergaard based ig. pet classes; no continental basalts, no 
leucogranites, syenites, kimberlites, anorthosites....). The idea being, 
you know the tools, you know how they may be combined and used, through 
reading and self-learning you should be able to work out those other rocks.

Hope this helps, best wishes for the New Year,

Sumit

-- 
**********************************************
Sumit Chakraborty
Professor, Institut für Geologie, Mineralogie und Geophysik
and
Director, RUBION

Ruhr Universität Bochum,
Universitaetstrasse 150, D-44801 Bochum
Germany
Phone: +49 – 234 322 4395
                      8521 / 8155 (Sec.)
Fax:   +49 – 234 – 321 4433
Email: Sumit.Chakraborty at rub.de
Web: 	http://www.gmg.rub.de/petrologie/
	http://www.rubion.rub.de/
**************************************************
                            



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