At 05:49 PM 24/06/2002 +0100, John D. Clemens wrote:
>... 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. ...
?? Assuming equilibrium at specified P on any univariant
dehydration-melting curve, it seems to me aH2O would be defined and
buffered regardless of the presence or absence of a fluid phase. If a fluid
phase is present, isothermal reaction will adjust its XH2O (and thus its
XCO2 etc) so as to be consistent with the equilibrium value of aH2O.
Whether or not fluid is present, I don't see how the "actual aH2O during
melting" (or crystallization) could be raised to a higher value without
losing one of the solid phases present.
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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