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

Michael Garcia mogarcia at hawaii.edu
Fri Dec 29 14:31:58 EST 2017


Dear Sumit,

thanks for your petrology teaching story which I read after your sample
request.
John Sinton and I use to teach a grad level course very similar to the one
you describe but it was focused on the oceanic lithosphere (we live in
Hawaii).
It worked well with combining phase diagrams, petrography and geochemistry
in addition to reading current papers on these topics.
We had labs where the students examined samples from key oceanic localities.
John has retired and so has the course.

Best regards, Mike

On Fri, Dec 29, 2017 at 4:44 AM, Sumit Chakraborty <Sumit.Chakraborty at rub.de
> wrote:

>
>
> 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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