Diana,
I think that is very tricky. The problem is that one cannot
generally obtain the composition of the melt phase and whether it was
saturated in fluid. Larry Edwards and I used equilibria involving
the fugacity of H2O - fugacity of CO2 at selected P and T without
assuming the presence of a fluid phase (Edwards, R.L. and Essene,
E.J., 1988, Pressure, temperature and C-O-H fluid fugacities across
the amphibolite - granulite facies transition, NW Adirondack Mtns.,
NY., J. Petrol. 29, 39-73). This approach works best for C-O-H
fluids if graphite is present. In the absence of graphite, one can
apply limits, for instance the upper limit of CH4 in the absence of
graphite, etc. The calculations are strongly dependent on the T that
is selected, although one can often make general conclusions by doing
calculations at different T. One could try plots of various
fugacities vs. T. Here Bob Newton would interject that the activity
of NaCl, KCl, etc. may also be important. These can be calculated in
some cases if the Cl, F of biotite or perhaps scapolite is analyzed.
eric
>I'am working with rocks on the granulite-amphibolite interphase, closely
>associated with migmatites. I am trying to calculated the P-T conditions
>using multi-equilibrium analisys, using the TWQEEU. As I don't have direct
>information on the H2O and CO2 activities, I have some doubts on have to
>proceed, and wich criterias can be usefult to correcly select this
>activities. If you can help me with references or some useful criterias, I
>will be really greatful.
>Thanks
>
>Diana
