Print

Print


Frank, Bas and all,

Whomever first developed the concept of the metamorphic
"pseudosection", and regardless of who first called it that, there's
little doubt that the ultimate inspiration for "pseudo-binary
(-ternary, etc.)"  metamorphic diagrams is from the use of these in
the ceramics and igneous experimental petrology literatures.  This
was probably implicit in both Frank's and Bas' messages, but I
thought it was worth explicitly noting.  From my work with Alan
Thompson at Harvard in 1974-77, I'm pretty sure that's where he got
his inspiration for the 1976 paper's approach to AFM sections at
fixed Al (undoubtedly with a further inspirational component from
JBT!).  These liquidus diagrams obviously go back well before 1960,
and many particularly good examples can be found in the Geophysical
Lab Annual Reports in the Carnegie Institution Yearbooks through the
50's and 60's.  You can see them as well in Whitney's and Stormer's
papers in the 1970's on granitic magma crystallization and in many
other igneous or experimental petrology papers.  Given Bas Hensen's
Geophysical Lab connection, I wouldn't be surprised if that's where
the concept first implanted itself on him, and he certainly deserves
great credit for translating it to a metamorphic context, whatever he
did or didn't call it.

I suspect that lots of younger petrologists (both igneous and
especially metamorphic) are not fully aware of the value of such
pseudobinaries.  The key concept in them is that pseudobinaries are
not true self-contained compositional joins, and that normal
geometric-topologic rules for true binary phase diagrams don't apply,
since these diagrams involve crystallization (or reaction) of phases
that don't lie within the join.  Also, as Bas Hensen notes, the
assumption of a fixed bulk composition is critical.  Especially
confusing (but potentially powerful interpretive tools) are the
possibilities of non-vertical paths through fields on a T-X or that a
delta-T path for a particular bulk composition may not actually
intersect a phase-field boundary but just cross it passively, much as
Bas notes as a scarcity of intersections of single rocks with
univariant equilibria along a metamorphic P-T path.

Regarding the comments at the end of Frank's message, I guess I would
respond that the "pseudo-" prefix could be taken to have another,
more benign, meaning, i.e., something like "similar to, but behaving
differently from, something else", in addition to its usual
connotation of "false, spurious or sham" that Frank noted.  How about
the optical terms pseudo-isotropic or pseudo-uniaxial?  (I just
finished teaching an optical lab!)  I guess these could be
interpreted to imply the "false" meaning, as in "falsely appearing to
be isotropic".  But it certainly seems to me that Frank has a good
point  (reinforced by Bas) in that we should recognize the essential
value and utility of "pseudo-sections" or whatever we call them,
especially now that computer applications and thermodynamic databases
make them fully quantitative and available for a wide range of
compositions.  However, the old name still seems pretty good to me,
despite the negative implications of the "pseudo-", as it does to Bas.

Bob T.
--
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dr. Robert J. Tracy
Professor of Geological Sciences
Virginia Tech
Blacksburg VA 24061-0420

540-231-5980
[log in to unmask]
(FAX: 540-231-3386)

"We can't solve problems by using
the same kind of thinking we used
when we created them."
-- Albert Einstein