Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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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
J276 step decrepitations
The skew-gaussian fit to analysis J276 shows 4 fluid inclusion populations.
Gaussian fit to J276

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.

J276 - not decrepitated

J276 not decrepitatedJ276 not decrepitated
The above 3 photographs of J276, not decrepitated, show the high abundance of inclusions and the presence of very irregular shaped inclusions with large vapour bubbles or gas filled.

Analysis J310: After heating to 350 C: large inclusions now lack bubbles. Smaller inclusions up to 5 microns can contain 2 phases. There is visible damage around large inclusions. The low temperature population 1 is caused by large, irregular, gas-rich inclusions which decrepitate because of high internal pressure which increases quickly and linearly with temperature according the the gas law, pV=nRT. (where p, V and T are the pressure, volume and temperature respectively; n is the amount of substance; and R is the ideal gas constant.) These inclusions also decrepitate easily because their irregular shape has stress points which facilitate decrepitation. Note that the gas type (CH4 or CO2 or N2) is irrelevant as they all follow the gas law with only minor variations from ideality)

J310 heated to 350 C

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.

J311 heated to 490 C

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.
J312 heated to 640 C

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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