Friends,
A day or so after I sent the note regarding the T and P of the Mt.
Moosilauke rocks I discovered an inconsistency in my GASP program. I have
found and corrected an error in the programming of the garnet ternary
Margules for GASP. This has the effect of lowering P for Mn-rich garnets
and slightly increasing P for Mn-poor garnets. So, the Mt. Moosilauke rocks
are: 7 rocks at T from 551-589 deg C, and 3 rocks at P from 3.48-4.23 kbar.
My estimated error is 0.75 kbar absolute and 0.6 kbar relative, and for T it
is 25 deg absolute and 15 deg relative.
This helps to answer Frank Spear's question. However, I suspect the rocks
might have experienced some P increase along with T increase. The
andalusite (in addition to the sillimanite) was forming from staurolite
breakdown, and this probably required a T of at least 540 deg C in the
andalusite field. If the rocks were strictly isobaric, it is likely that no
andalusite would have been forming while the rocks were in the andalusite
field, because staurolite would still have been stable. To form both Al
silicates, the rocks would have been tending to move along (and across) the
staurolite breakdown curve with increasing P and T together. I would be a
little skeptical about arguing the triple point conditions on the basis of
these rocks, regardless of whether or not they were isobaric.
I hope your summers are cooler than mine here in Dallas!
Mike
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|