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In the example discussed:
A carbonic inclusion derived from a parent homogenous fluid with X
- CO2 of 0.1 ( X - H2O of 0.9) at 1Kbar
pressure and which is trapped at 200 C (the right hand side red
cross in the solvus diagram) would have X - CO2 of 0.92
(with 0% salinity.)
This is a single carbonic phase when trapped, but when cooled to
100 C this would separate into 2 immiscible phases within the
fluid inclusion These 2 fluid components will have X - CO2
compositions of 0.97 and 0.04 (The intersections of the
solvus with the 100 C axis). In theory you might expect to see
this as a vapour-rich 2 phase fluid inclusion with a small liquid
phase. But the liquid phase is so small, as calculated here, that
it will be invisible.
The lever rule is used to determine the volume ratios of these
two components which will exist within cooled the fluid inclusion.
The aqueous phase proportion will be:
(0.97 - 0.92) / (0.97 - 0.04) =
0.05/0.93 = 0.054
The carbonic phase proportion will
be: (0.92 - 0.04)
/ (0.97 - 0.04) = 0.88/0.93 = 0.946
In a spherical fluid inclusion, this small 5% quantity of aqueous
phase will be invisible and the inclusion will appear to contain
just a single phase carbonic fluid. Despite actually forming from
a 90% aqueous fluid.
Any salinity in the system would further reduce the visibility of
the aqueous phase.
This image from Professor P. Brown's lecture notes shows
that a volume % of 75% gas in a spherical inclusion (bottom right)
gives a barely visible liquid phase. (the gas bubble is black in
this diagram)
With a volume % of 95% CO2 as calculated above the 5%
liquid phase would not be visible at all. Microscope
observations will be more difficult than shown in this careful
diagram because of the substantial refractive index changes
between the fluid phases and the quartz.