Dear geo-metamorphists and fluid-inclusionists,
I'm enjoying the increasing activity and the nice discussions in our
mailing list (which make it somewhat more interesting than discussion at
meetings), and I wish to add my contribution to the debate raised by John
Clemens on "dehydration melting".
I'm sending this mail also to "fluid-inclusion": those who are interested
in the whole "dehydration melting" discussion can find it at:
http://www.mailbase.ac.uk/lists-f-j/geo-metamorphism/2000-08/subject.html
Actually, my comment has more to do with back reactions and the behaviour
of fluid in graphitic migmatites/granulites, a point touched in the last
sentence of Clemens' message:
"..However, on crystallization of any melt formed,
nearly pure H2O would be expelled, and this the graphite would not
enjoy at all! Maybe we have here the origin of some of the
high-density CO2-rich fluid inclusions in some granulites."
In order to avoid that the "Maybe we have" in the cautious suggestion of
this sentence is interpreted or read by some (or many) as "We have", I wish
to explain why I think such origin CANNOT be invoked when we are in the
case of "tight rocks" of frank Spear, i.e. those where the system is closed
and fluid composition is internally controlled.
A first, naive reason why H2O plus graphite cannot make (dense) CO2-rich FI
is mass balance: there's hydrogen missing, which would mostly form CH4 at
reasonable P-T conditions.
A sounder explanation can be found, among others, in the papers by French
(1966, Rev Geophys 4:223-253) and Ohmoto & Kerrick (1977, Am. J. Sci.,
277:1013-1044), recently revisited by Jamie Connolly and myself for
theoretical purposes (JMG, 11: 379-388 (1993); CMP 119: 94-116 (1995)) with
some applications to fluid inclusions (CMP, 122; 25-33 (1995)) and partial
melting studies (CMP 135: 41-52 (1999)).
The main conclusion for the present issue is that at reasonable P-T for
migmatite/granulite development and cooling, the fluid resulting from H2O
expulsion in a graphitic rock has > 50 mol% H2O, the remaining amount being
equally distributed in H-bearing (mostly CH4) and O-bearing (mostly CO2)
species.
I wish to stress that the above conclusions hold for "tight rocks" (in
which, for example, H escape cannot be invoked) and that in this case
graphite is a constraint on fluid composition, much stronger and much more
realistic than QFM (plus or minus some log unit).
Regards,
Bernardo
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Bernardo Cesare
Dipartimento di Mineralogia e Petrologia
Corso Garibaldi, 37, I-35137 PADOVA ITALY
Tel: ++39-49-8272020, 8272019 Fax: ++39-49-8272010
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http://dmp.unipd.it/bernardo/bernardo.html
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