[Petwksp] M&Ms, poker chips, and reality

[Allen Glazner]

At the risk of re-offending Bowenists, I'd like to respond to Ben's
suggestion about zoned plutons. It's not good pedagogy to have students
simulate a nice theoretical model that doesn't seem to apply in nature. I
can show you lots of great maps and field photos and geochemical data from
the Tuolumne Intrusive Suite in Yosemite, a classic example of a normally
zoned intrusive system. Problem is, the rocks in the center are about 5
million years younger than the ones at the rim, so it is flatly impossible
that the system evolved by crystal fractionation at the observed level. My
student Walt Gray's geochemical, isotopic, and mineral-chemical data also
show that you can't get from the rim to the core by crystal fractionation,
but why even bother testing this when the physics of the system don't allow
it?

The M&M exercise is great for illustrating controls on bulk composition and
how concentrations and absolute amounts are not the same thing, and I intend
to use it this spring. Sparingly.

Great discussion, and I really enjoyed reliving the field trip with Kurt's
site!

Allen

================================
Prof. Allen F. Glazner
Department of Geological Sciences, CB# 3315
University of North Carolina
Chapel Hill, NC 27599-3315  USA
919-962-0689, fax 966-4519
http://www.geosci.unc.edu/faculty/glazner/glazner.html