The
premature demise of acoustic emission decrepitation as a fluid
inclusion study technique was mainly because of an inability to
relate
the temperature of onset of massive decrepitation with the
formation
temperature. But much of this problem was in fact due to a lack of
understanding of the importance and behaviour of CO2 in
fluid inclusions. From the gas law, it is clear that CO2
rich fluid inclusions will develop high internal pressures at low
temperatures, resulting in decrepitation well below their
formation
temperatures. Although this behaviour is a hindrance in
determining
formation temperatures it means decrepitation data can easily be
used
to detect CO2 rich inclusion populations, which is very
useful in mineral exploration for Au deposits because of the
commonly
documented association between Au and CO2 rich fluids. (Fig 1)
Other gases such as CH4 behave just like CO2
and
so they contribute to this low temperature decrepitation
effect. Plots
of the equation of state for various gases show that they
all result in
high inclusion pressures and low temperature
decrepitation. Statements
by some authors that CH4 does not cause fluid
inclusion
decrepitation are incorrect and contradict the gas law.
Schmidt-Mumm (1991) asserted that sounds measured in
decrepitation
experiments were dominated by cyrstallographic and grain
boundary
effects. This is incorrect as the instruments used measure
a pressure
pulse in the air column between the sample and sensor.
Changes in
crystal structure or grain boundary movements simply
cannot generate
large enough pressure pulses to be detected. Only the
rupture of fluid
inclusions with subsequent release of high pressure gases
or a steam
explosion from superheated water can generate the
pressures necessary
for detection. And because secondary inclusions leak
gradually or open
at low temperatures, they fail to generate sufficient
pressure to be
detected. Consequently acoustic decrepitation side-steps
the entire
problem of secondary inclusions and their potential
mis-identification
in microthermometric studies.
A comparison of fluid inclusion abundance counts in thin
section with
decrepitation of the same samples shows that only some
0.5% of
inclusions larger than 8 microns across decrepitate and
are detected
during analysis. Despite this, replicate analyses of
aliquots of the
same sample give consistent and reliably reproducible
results.
Acoustic decrepitation has been incorrectly maligned and
although it is
not a high precision method, it gives consistently
reproducible fluid
inclusion population temperatures and an indication of CO2
+
CH4 gas contents. As it is fast and cheap it is
ideal for
use in exploration or for preliminary scanning in
conjunction with
conventional microthermometric studies.
Fig. 1. Quartz from Ballarat decrepitates at low
temperatures between
180 - 300 C as it contains abundant CO2 rich
inclusions,
while quartz from Dongping lacks CO2 rich
inclusions and
does not decrepitate until 370 C, its approximate Tf.
REFERENCES
Schmidt-Mumm A. (1991) Phys Chem Minerals. 17:545-553.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 