The BGS model 105 decrepitometer
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 closed.
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 analysed.