Hello All,

I'm just writing again to thank all of you, who helped me with the
Grt-Opx-Pl-Qtz barometry and want to give a short summary of what I
received and performed.

There are two kinds of ways to calculate the P-T conditions of a 
reaction. Either you use a conventional barometer or thermometer,
calibrated by experiments especially for this particular reaction or you
take an internally consistent database where all thermodynamic data is
included and you can calculate any reaction you want. Of course the basis
of this data are also experiments.
Some of you, like Ian Fitzsimous, recommended the first way, and that's
what I did before.
Now I calculated the reactions with the TWQ program, which was a bit
difficult because we only have Macs here (hello Perth!). But the program
gave me intresting results. The GOPS reaction for the Fe and the Mg
calculation converged in the Sil stability field. This is satisfying
because there are Sil bearing rocks in the same unit. I also used the
GRIPS barometer by Bohlen & Liotta (1986) and the same equilibrium with
the TWQ program. The results of these two coincide withtin 0.5 kbars and
above all they also coincide with the two GOPS calibrations (Fe and
Mg) calculated with TWQ.
I downloaded the TPF program Vyacheslav Fonarev talked about. I don't have
the results till now, but I wanna say that this program is based on
windows and is more comfortable than the DOS programs. But there is also
no Mac version available, is there?
To return to the conventional geobarometers I want to report their
results. The Lal (1993) calibration produced pressures far beyond all the
others mentioned above. Perkins and Chipera (1985) pressures are above the
Ky-Sil reaction which is probably to high, because there is no Ky in the
whole area. The Bhattacharya et al. (1991) pressures are closest to the
TWQ pressures.
In the end I want to mention the paper of Dan Harlov (2000) in Mineralogy
and Petrolagy 69:11-33, who sent me a reprint. Thank you. He emphasized
the importance of non-ideal mixing in Orthopyroxene of granulite facies
rocks.

Thanks again for the lively discussion

horst

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Horst Marschall
Mineralogisches Instiut
Universitaet Heidelberg
INF 236
69120 Heidelberg
++49-(0)6221-546021
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