Current Research on Fluid Inclusions (ECROFI-XIX)
An updated understanding of Acoustic emission decrepitation
Burlinson Geochemical Services Pty. Ltd.,
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.
Schmidt-Mumm A. (1991) Phys Chem Minerals. 17:545-553.