Geology Reference
In-Depth Information
Box 4.6 Fluid inclusions in minerals
During the growth of a crystal from a hydrothermal fluid, it
is common for the growing lattice to enclose and trap a
minute volume of fluid, accidentally preserving a small
sample of it for subsequent microscopic examination.
Such
fluid inclusions
(Figure 4.6.1), which vary in size from
less than 1
μ
m to more than 100
μ
m, can be used to esti-
mate the temperature and composition of the original fluid.
The post-enclosure evolution of a saline fluid inclusion
is shown in Figure 4.6.2, which resembles the phase dia-
gram for pure water in Box 2.2. The saline fluid occupies a
constant-volume enclosure (neglecting thermal contrac-
tion of the crystal) and therefore cooling causes the fluid
pressure to fall from A to B along a constant-volume path
called an
isochore
. At B the fluid becomes saturated with
vapour, and after some supercooling a bubble nucleates at
B
1
. As the inclusion cools along the liquid/vapour phase
boundary to room temperature (R), thermal contraction of
the liquid allows the bubble to grow. The liquid may become
saturated with one of its solutes (at D, say), so that a
daughter crystal
of halite (Figure 4.6.1) or some other salt
nucleates at D
1
and grows during subsequent cooling.
Using a microscope whose stage is specially equipped
to heat the sample, the geologist can recreate and observe
the cooling process in reverse, and the temperature at
which the bubble just disappears indicates approximately
the temperature at which the crystal and its inclusions
were formed. The 'homogenization temperature' meas-
ured is actually
T
B
, but this is a useful minimum estimate
of
T
A
. Measurement of the original pressure by other
means allows
T
A
to be determined more accurately.
Like the salt used to melt ice on the roads in winter, the
salinity of the fluid depresses the freezing point relative to
that of pure water. The phase relations of the system
q
h
b
Figure 4.6.1
Thin section view of a fluid inclusion (about
30
μ
m across) from a quartz vein in the Dartmoor Granite,
Devon, England. h = halite crystal, b = gas bubble, q = host
quartz crystal. The gas bubble boundary appears more
prominent (in optical terms, it has higher
relief
) because
of the high contrast in refractive index between the gas
and the surrounding liquid. (Source: Photo courtesy of
Dr D. Alderton.)
water-salt resemble those of Di-An (Figure 2.4). Modern
microscope 'heating stages' are also equipped to cool
samples as far as −180 °C, making it possible to deter-
mine the temperature at which ice crystals first appear
upon cooling or disappear upon warming up, from which
the salinity of the fluid can be estimated.
Fluid inclusions provide a very powerful means of explor-
ing the physical and chemical properties of hydrothermal
systems. More information can be found in Rankin (2005)
and the topic by Samson
et al
. (2003).
The lower diagonal line marks the onset of reducing
conditions powerful enough to reduce water to
hydrogen:
Between these two diagonal boundaries lies the
'water window' that embraces all natural aquatic
environments.
It is useful to compare Figure 4.1a with Figure 4.1b,
which summarizes the
Eh
-pH ranges of waters from
a variety of aquatic environments. The dots show the
range of the large number of water analyses compiled
by Baas Becking
et al.
(1960). Waters that circulate in con-
tact with oxygen in the atmosphere and remain well
+
−
−
HOHeHOH
2
++ +
2
(4.34)
2
The occurrence of reduced hydrogen H
2
in aqueous
systems is very rare, so this line approximates to the
lower
boundary for
Eh
in the hydrosphere.
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