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Rapid fluid inclusion data for exploration (decrepitation)

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Microscope observations of decrepitated samples

Sample J276 fresh, compared to heating to 3 temperatures. Grains mounted in RI oil for observation

A sample analysed as J276,  from a gold deposit in Victoria, Australia, has very intense decrepitation at low temperature (200-300 C) which is typical of fluid inclusions with high CO₂ or gas content. Fitting gaussian distributions to the decrepitation result showed the presence of 4 populations of fluid inclusions in the sample. This study attempted to identify the inclusions associated with each population by decrepitating the same sample material to 3 different temperatures (based on the fitted gaussian population temperatures) and microscopically observing the used sample material from each temperature step and comparing these with fresh, un-decrepitated sample.

The 4 analyses J276, J310, J311 & J312 are of the same sample, but  heated to 4 different maximum temperatures

The skew-gaussian fit to analysis J276 shows 4 fluid inclusion populations.

Based on this fit, an analytical cut-off temperature of 350 C was selected to selectively remove inclusions from  population 1, and a cut-off of 490 C to selectively remove the inclusions of populations 1 & 2. Complete decrepitation to 640 C removed all 4 populations of inclusion. Note that population 4 is a function of the quartz weakening as the quartz changes from alpha to beta phase at 573 C (explained here) and is not an actual fluid population event.

The observations were made on crushed grains immersed in refractive-index (RI) oil (tritolyl phosphate, RI 1.555, the RI of quartz is 1.553) to conceal grain boundary effects, using a Nikon petrographic microscope with 40x objective and 10x eyepiece with a graduated scale. All observations were made at room temperature of 30 to 32 C which is close to or above the critical temperature of CO₂ (31 C) and so liquid CO₂ could not occur or be observed.

The fresh sample, labelled J276, but not decrepitated, had numerous fluid inclusions, typically 2-5 microns in size, equant, subhedral with 2 phases visible and  50% vapour or gas. There were less numerous but still abundant large 15-20 micron  irregular inclusions, the majority of which were 2 phase with large vapour %,  but some are empty or gas filled. No damage fractures were present.

Analysis J311: After heating to 490 C: even small inclusions sized at 2.5 microns mostly lack  a visible phase boundary and seem empty. Only rare small 2.5 micron  inclusions have a bubble - maybe 20% vapour or gas. Population 2 is probably caused by decrepitation of small, 2 phase aqueous inclusions with about 50% vapour.




Analysis J312: After heating to 640 C: Absence of any 2 phase inclusions - all decrepitated?  lots of dark (decrepitated?) inclusions - apparently empty. Many damage fractures even around smaller inclusions. Population 3 is not clearly distinguished, but comprises most of the remaining inclusions which decrepitate by 550 C. Above 550 C most of the remaining inclusions decrepitate as the Young's modulus of quartz drops dramatically as the quartz changes from the alpha to beta phase, facilitating the decrepitation of inclusions with even modest internal pressures. This shows as population 4 on the gaussian fit plot, but is probably not actually a separate fluid inclusion population, but rather small, equant inclusions which are strong enough to survive their internal pressure increase until the quartz itself weakens.


A study of fluid inclusion abundance and actual decrepitation counts showed that probably less than 1% of the inclusions in a sample actually generate decrepitation counts.

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