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
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