The decrepitometer measures the characteristics of the complete
fluid inclusion population in a sample by heating the sample at a
constant rate, usually 20 C / minute, and counting the number of
pressure pulses caused by the explosion of fluid inclusions as they
develop internal pressures which exceed the confining strength of
the host mineral grains and also recording the temperature at which
the inclusions explode. The result is a histogram of fluid inclusion
counts versus temperature.
The instrument uses digital (not analogue) electronics for all of
its functions and is completely automated, being controlled by a
Personal Computer and dedicated software. This ensures that the
analysis is completely objective and free of all operator bias. The
result is printed and / or stored as a data file for subsequent
processing and interpretation.
The model 105 instrument connects to the Personal Computer through
the specially designed interface card.
This card has Analogue to Digital and Digital to Analogue converters
and counters and so that the computer can continuously monitor the
furnace temperature, change the furnace power level to maintain
constant heating rate and set the counter sensitivity. The interface
card for this model only fits in an older PC, so a new system, model 205, has been developed.
An external unit provides the controlled furnace power. The furnace
accepts a 1" silica test tube which contains the temperature sensor
and can be used up to 800 C. The furnace is mounted in an acoustic
isolation enclosure to eliminate external interference.
In operation, the sample (0.5 to 3 grams of crushed and sized
grains) is placed into the silica tube and the pressure sensor is
attached at the other end of the tube which is then inserted into
the furnace aperture and the acoustic isolation enclosure door is
After entering the sample information and analysis temperature
limits into the computer program, the entire analysis is completed
without any operator intervention. An analysis takes about half an
hour, depending on the settings in use.
The pressure sensor only detects pulses from
exploding fluid inclusions and does not detect events caused
by mechanical or crystallographic changes in the sample as such
events generate S waves which are not detected by the pressure
sensor. Secondary inclusions are also not detected, probably because
they leak or generate only very low pressure pulses which are below
the threshold setting of the instrument. Consequently the
decrepitation data is a record of the fluid inclusion population in
the sample, free of unwanted interferences.
For quality control of the decrepitation results, a standard sample
is analysed each day and compared with the archive of previous
results. A very large quantity of crushed and sieved quartz from a
proterozoic gold deposit near Darwin was prepared many years ago to
use as this long term calibration standard. The sample was selected
because it contains a modest level of CO2 rich fluid
inclusions which decrepitate near 300 C. In addition unusual
samples are occasionally analysed twice, and background blank levels
are checked by re-analysing material which has already been