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Rapid fluid inclusion data
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The decrepigrams shown here were produced by heating 0.3 to 1.0 gm of crushed, sieved sample at a constant rate of 20o C per minute. The pressure pulses produced as the fluid inclusions burst were detected, processed , counted and summed over 10o C temperature intervals. The 4 graphs shown here illustrate the range of behaviour of quartz samples.
Ordinary quartz 'A' begins to decrepitate at 350-400o C and only shows 2 peaks, one frequently being at 580o C. Between 580 and 800o C quartz does not decrepitate at all, as seen on the graph up to 750o C. The peak at 580o C is related to but not caused by the alpha to beta phase transition of quartz at 573o C. At this temperature the Young's modulus of quartz decreases dramatically. This weakening of the quartz crystal strength enables the decrepitation of any fluid inclusions still present. When the same quartz sample is heated twice, the second analysis gives no counts above instrumental background (see the re-used quartz graph). The absence of any peak at 580o C on many quartz samples (such as quartz 'B' shown) and on re-used quartz confirms that the counts are not caused by the phase transition alone. The process causing the counts is clearly irreversible and not due to physical or thermal expansion effects. There has been an assertion that this decrepitation peak is due to annealing of Dauphine twins in the quartz. However sounds from this annealing process cannot be detected in this instrument as they are shear waves and the instrument ONLY detects pressure pulses transmitted through an air column, which cannot transmit shear waves.
Secondary inclusions do not give rise to decrepitation and most quartz shows no decrepitation below about 350o C despite the abundance of secondary inclusions below this temperature. This is considered to be because the secondary inclusions generate only extremely weak pressure pulses which are below the instrumental detection limit. Note the lack of low temperature decrepitation on both quartz A and B samples.
When quartz decrepitates at low temperatures it is due to the
presence
of CO2 rich fluid inclusions, which
decrepitate at low temperatures because of their commonly
high
internal
pressures and the linear increase of pressure with temperature
during
heating. Confirmation of this was provided in research by Mavrogenes
et al. (1995) and by Gaboury
using mass spectrometric analyses of gases released during thermal
decrepitation of quartz.
The decrepitation peaks between 350 and 550o C are
probably
related to primary inclusion decrepitation. These peaks are at
higher
temperatures
than the actual homogenization temperatures by some 100o C
but
have, in several cases, been systematically correlated with
homogenization
temperatures of primary inclusions. However, most decrepitation
data is
used for rapid sample classification and discrimination rather
than the
determination of
homogenization
or formation temperatures, because absolute temperatures are not
useful
in mineral exploration. Knowledge of precise absolute temperatures
of
formation is only useful in genetic studies of ore deposits after
they
have already been found.
The variation of Young's modulus of quartz with
temperature shows a dramatic decrease in strength at the
alpha-beta
phase transition temperature (573 C). This weakening of the quartz
enables the decrepitation of inclusions which were unable to
overcome
the normal crystal strength of the quartz and gives rise to the
prominent decrepitation peak at this phase transition temperature.
The decrepitation of fluid inclusions depends upon their internal
pressure, size, shape, distance from a surface or plane of
weakness and
on the strength of the host mineral. When quartz is heated it
undergoes a
crystal re-structuring at 573 C, changing from the alpha to the
beta
crystalline form. This change is reversible and essentially
instantaneous with no metastability. As the quartz is heated and
approaches 573 C, the strength of the quartz decreases
dramatically,
and inclusions which might have been unable to generate enough
internal
pressure to decrepitate the quartz may now be able to do so as the
quartz
changes crystal phase, because of the very low strength of the
quartz
during this phase transition at 573 C. The observation of
decrepitation
at this temperature is merely a function of the quartz weakening
and
does not imply that the fluids were actually near this temperature
when
the fluid inclusions were formed.
Perpendicular and parallel refer to the strength measured in a
direction relative to the c axis of the quartz crystal structure.