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Charlie and all,

Sorry, I am not poetic tonight, or ever!

I think one reason the Maine rocks behave so well is that the fluids appear
to be controlled (buffered?) in a relatively simple way by the universal
presence of graphite, low hematite component in the ilmenite, no magnetite
and relatively small amounts of externally derived CO2.  (I think this
excludes the more oxidized Ordovician rocks Charlie talks about).  The
result is that graphite reacts with dehydration H2O, buffers (?) the O2, and
controls the CO2 and CH4 in nearly equimolar amounts.  I suspect that in
many cases where the biotite doesn't behave predictably between isograds
there are variations in O2, H2O, and CO2 such that the divariant reactions
can't vary smoothly between isograds. I suppose it's possible in cases where
the biotite remains constant in composition between isograds that large
percentages of biotite of a given composition may actually be having a
buffering (?) effect on the fluids and other minerals in the absence of
graphite.

Of course, Mn and Ca in garnet must be taken into account, and their
presence may make the biotite LOOK like it is not behaving properly, unless
these effects are included.

Mike Holdaway