Dear Diana,
Geothermometry may be useful in constraining H2O activity. You
need to start with an assumption on whether the partial melt in the
migmatites roughly approximates haplogranite. The locus of the
haplogranite solidus is reasonably well known as a function of P, T
and aH2O. If the metamorphism (associated with melting) took place at
a known P and T (from independent thermobarometry) then you can just
read off a minimum aH2O from a P-T graph of the granite solidus at
different values of aH2O. If you get low values of T, H2O activity
will be high and you can bet that no appreciable CO2 was around, or
you would not be in the melting field. If aH2O is around .2 to .5,
you are probably dealing with either fluid-absent melting reactions
(highly likely) or you might have a strange H2O-CO2 fluid present
(unlikely). If the melting was, indeed fluid-absent, then the aH2O
that you read off your graph is the actual aH2O during melting, not
just the minimum. You can get such graphs out of the Johannes and
Holtz reference below.
As an example, if your thermometry says 850 degrees C and P is
500 MPa, the aH2O for the fluid-absent case is about 0.3. If this
system were, instead, fluid-present, you would know that this value
(0.3) was the minimum aH2O at the time of melting. Remember that the
simple observation that aH2O < 1 does not tell you that a CO2-bearing
fluid phase was present.
I have no idea what you might do with TWQEEU, but perhaps it
would be best to stay clear of any assumption about the presence or
absence of CO2 in the system. Maybe sticking with solid-solid
reactions would be the safest starting point. We are, of course,
certain that most crustally derived granitic magmas were generated by
fluid-absent melting reactions at granulite grade.
A highly selective and personal list of possible references:
Clemens, J.D., 1993. Experimental evidence against CO2-promoted deep
crustal melting. Nature, 363: 336- 338.
Clemens, J.D. and Watkins, J.M., 2001. The fluid regime of
high-temperature metamorphism during granitoid magma genesis.
Contrib. Mineral. Petrol., 140: 600-606.
Johannes, W. and Holtz, F., 1996. Petrogenesis and Experimental
Petrology of Granitic Rocks. Springer-Verlag, Berlin, 335 pp.
Stevens, G. and Clemens, J.D., 1993. Fluid-absent melting and the
roles of fluids in the lithosphere: a slanted summary? Chem. Geol.,
108: 1-17.
--
John D. Clemens
Professor of Geosciences,
Associate Dean (Research), Faculty of Science
Editor in Chief - Visual Geosciences
http://link.springer.de/link/service/journals/10069/index.htm
School of Earth Sciences and Geography, CEESR, Kingston University,
Penrhyn Rd, Kingston-upon-Thames, Surrey, KT1 2EE, UK
phone: +44 (0)20 8547-7023 fax: +44 (0)20 8547-7497
e-mail: [log in to unmask]
personal web page:
http://www.king.ac.uk/esg/staff/pages/clemens.htm
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Our passions cannot alter the facts, only hide them from us.
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