Chemistry Reference
In-Depth Information
easy cleavage). Further identification is outside of the scope of the present
work but few remarks are appropriate here:
i
Primary inclusions are not the «good», secundary the «bad» inclusions.
A number of perturbations may occur during or after the formation of
a primary inclusions, e.g. cavity caused by sticking of a gas bubble on
a grawing face or leakage, making the inclusion content quite different
from the interstitial fluid phase at the time of the mineral growth.
ii
Conversely, secondary inclusions might be quite representative, if P
T
conditions have not changed substantially since the mineral growth. But
even if this has happened, knowing their existence and characteristics
can be quite significant: a rock is never traversed by percolating fluids
without important petrographical or geochemical consequences.
−
In conclusion, each inclusion category must be identified, then studied as
a single population, applying the only criteria which, in each case, guar-
antee the representativity of the inclusion fluid: comparison of P
T fluid
data with independant mineral estimates (see fluid inclusion literature). As
long as an unambiguous conclusion has not been reached, it is important to
stick to a proper langage: primary versus secondary for inclusions distinctly
related to the host mineral gowth, neutral terms (early verus late) if not.
−
2.7.4 Potential interest of fluid/melt inclusion
studies
With all their limitations in mind (difficult of observation measurement,
mutiple possibilities of perturbations, etc..), it must be recognized that fluid
inclusions are the only possibility to record phases having left the rock sys-
tem. Furthermore, they are also the only objects likely to inform, not only of
the chemical composition, but also on the molar volume (density) of a fluid
system. A variable of state most commonly ignored by petrologists, because
of the small variation for solid minerals in the P-T range of geological inter-
est, but of tremendous importance for fluids: a mole of CO
2
has a volume
of less than 20 cm
3
during granulite metamorphism at depth, against more
than 22000 cm
3
when it reaches the Earth's surface. Once an inclusion is
sealed, its volume as well as the mass of the enclosed fluids are fixed, hence
its molar volume (PVT equation of state), which will no vary, provided that
no further perturbation will occur. Again, a complete investigation is out of
the scope of the present work. But a preliminary examination, which should
be done by any petrologist during routine microscopic observation, should
provide some of the following results:
i Identify «good» samples, namely those containing, at least locally, a
great number of workable inclusions. … examples
