Whoa - Lets not lose site of the fact that we are talking thermodynamics
here, not reality!
Thermodynamic calculations give a good estimate of aH2O. Period.
(There are also calculations which, in the absence of carbonate phases,
nevertheless put upper limits on aCO2, aCH4).
A particular value of aH2O may be achieved in a melt with a particular
water content, a mixed volatile fluid such as H2O-CO2-N2 etc, or a brine,
as well as in any number of less likely but theoretical possibilities such
as metastable oil and vinegar dressing.
Two or more of these fluids with the same value of aH2O may be able to
coexist immiscibly, eg melt + CO2-H2O(-salt) + H2O-salt - Kirill Shmulovich
has a nice diagram of this in a paper in a book I edited with him:
Shmulovich, Yardley & Gonchar "Fluids in the Crust" - Chapman & Hall (now
Kluwer) 1995
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!
Bruce Yardley
At 16:05 25/06/02 -0400, you wrote:
>At 10:43 AM 25/06/2002 +0100, JC wrote:
>>... In the fluid-present case (with aH2O externally
>>controlled) T would then be governed by the power of the heat source,
>>thermal diffusivities, the heats of dissolution reactions
>>(progressive mineral dissolution in the melt) and any advective heat
>>loss. We could then have melt at any imposed fluid composition that
>>allowed us to be above the solidus. In contrast, the fluid-absent
>>case nails aH2O to a fixed value for any given T, at constant P. The
>>H2O budget is internally fixed. This also fixes the amount of melt in
>>the rocks. There's the difference.
>
>I would argue for general prevalence of an intermediate case, in which a
>mixed-volatile fluid phase is present but not in sufficient quantity to
>overcome the aH2O-buffering capacity of the phase assemblages present in
>the zone of granitoid magma-genesis. This fluid phase would necessarily
>contain volatile species such as CO2 that are only sparingly soluble in the
>liquid phase, but its aH2O (and XH2O) would be no less nailed to a fixed
>value at any T,P than in the fluid-absent case (which - let's face it - can
>prevail only if the system is less than saturated with CO2 and other
>sparingly soluble volatile species).
>
>Cheers, Dugald
>
--------------------------------------------
Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK
Tel. 0113 233 5227 Fax 0113 233 5259
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GEOFLUIDS now exists! http://www.blackwell-science.com/gfl
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