At 11:42 AM 26/06/2002 +0100, Bruce Yardley wrote:
>,,,
>(There are also calculations which, in the absence of carbonate phases,
>nevertheless put upper limits on aCO2, aCH4).
Yes. Using the TWQ software and assuming equilibrium, putting an upper
limit on aCO2 (or XCO2 in a hypothetical gas phase) is one simple step
beyond finding P and T. If the sum of aH2O and the upper limit of aCO2 is
much less than 1, either the hypothetical gas phase was nonexistent or it
must have contained substantial amounts of other volatile species.
>...
>In order to work out what the fluid or fluids were that gave rise to the
>calculated aH2O in your rocks, you need to use techniques other than
>thermodynamic calculations, ie observational science!
True, but in study of the transition from amphibolite to granulite facies,
observational science is less than fully reliable. The silicate liquid
phase, if present, did not quench. The volatile fluid phase, if present,
did not quench. Oxide and silicate phases quenched imperfectly. But
experimental reaction rates are fast enough to support the postulation that
equilibrium was closely approached during the prograde metamorphism and
genesis of the inferred granitoid liquid. Accordingly, some of us oldtimers
still cling to hope that equilibrium thermodynamics is useful in modelling
the reality of metamorphic and anatectic processes.
Cheers, Dugald
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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