explanation of the baro-acoustic decrepitation method, mineral exploration, fluid inclusions

An explanation of the baro-acoustic decrepitation method

Presented by K. Burlinson at the ACROFI-2 conference, Kharagpur, India, 2008

The Acoustic decrepitation method was first used in Canada in the late 1940's. As microthermometric methods improved, the decrepitation method lost favour because the microthermometric results were more precise. However, the decrepitation method was not well understood at this time because the thermodynamics of fluids in inclusions was not known and, critically, there was little or no understanding of the importance of CO₂ rich fluids within mineralising hydrothermal fluid systems. With our better current understanding of hydrothermal fluids and thermodynamics, it is now apparent that the baro-acoustic decrepitation method is very useful as a mineral exploration method. Despite being a little less accurate, decrepitation can provide rapid and cheap analyses on large numbers of samples much more easily than using microthermometric methods and provides fluid information which is very useful during mineral exploration.

The acoustic decrepitation analysis procedure does not require the preparation of a thin section or use of a microscope and consists of these steps:

Crush and sieve the sample - NO THIN SECTION!
Heat the sample at 20 C per minute
The inclusions develop high internal pressures and burst (decrepitate)
Detect the pressure pulses
Process and count the decrepitation events in the attached computer
Plot the results as a histogram of inclusion counts versus temperature

No microscope work is required!

In mineral exploration, the understanding of the CO₂ content of hydrothermal fluids is very useful to locate economically interesting targets. The decrepitation method is a rapid way to measure the CO₂ contents of samples.

Gas rich inclusions give prominent low temperature decrepitation counts
This decrepitation is independent of the types of gases present (CO₂, CH₄, N₂)
CO₂ is the most common constituent which causes this low temperature decrepitation
Hence acoustic decrepitation is a quick method of determining the presence of CO₂ rich fluid inclusions to target economically interesting mineralized zones.

To explain the low temperature decrepitation of gas rich fluid inclusions, consider this PT diagram
decrepitation PT plot

Consider 2 fluid inclusions formed at 380 C and 1 Kbar pressure (the Formation point on this graph), one inclusion being pure CO₂ and the other pure H₂O.
After cooling to room temperature the CO₂ inclusion will retain a high internal pressure and when heated it will rapidly develop a high internal pressure and decrepitate at about 160 C. Its P-T path is along the magenta line, which is determined by the gas law of thermodynamics.
The H₂O inclusion, when cooled to room temperature, will consist of a liquid with a vapour bubble at negligible internal pressure and will not develop much internal pressure until it is heated above the Homogenisation point (310 C in this example). It will not develop enough internal pressure to cause decrepitation (about 500 bars) until about 340 C, the decrepitation point of aqueous inclusions. Its P-T path is along the blue "vapour-liquid curve for water" and then along the green isochore after homogenisation.

So despite forming at the same pressure and temperature, the CO₂ inclusions will decrepitate at much lower temperature than equivalent aqueous inclusions and this behaviour means that acoustic decrepitation can be used to determine the presence of CO₂ rich fluids.

Mixtures of CO₂ and H₂O are immiscible at low temperatures and will separate into 2 component phases. The gas phase will dominate the internal pressure of the inclusions and gas-water mixtures will behave much like gas rich inclusions but with lower fluid density.

PTX plot

Fluid inclusions containing gas - water mixtures decrepitate much like gas rich fluid inclusions.

co2-water mixture pt

Because the P-T behaviour of gases is given by the gas law and is independent of the gas molecule species, decrepitation is similar for all gases.
These P-T plots are calculated from the equation of state formulae for 3 common constituent gases in fluid inclusions.

differnet gases behave the same

The decrepitation of fluid inclusions is dependent upon the size and morphology of the inclusions and very small inclusions probably do not decrepitate.

size effects on decrepitation

Mass spectrometer analyses of gases released during thermal decrepitation show large amounts of CH₄ and CO₂ released at low temperature, corresponding with the low temperature baro-acoustic decrepitation peak.

mass spec on released gases

The decrepitation counts are only caused by pressure pulses as fluid inclusions burst open and are not caused by crystallographic effects, as shown by the absence of decrepitation on a sample which is heated for a second time. There is no response at the quartz alpha-beta phase transition temperature of 573 C in the reheated sample despite the fact that this phase change cannot be quenched.

quartz and rerun

Decrepitation counts occur at the quartz phase transition temperature in fresh samples because the strength of quartz, as measured by its Young's modulus, decreases dramatically at the phase transition temperature and this allows small inclusions to decrepitate because of the weakening of the quartz host mineral. Parallel and perpendicular refer to the direction of measurements relative to the c axis of the quartz.

youns modulus of qtz with temperature

Two different instruments have been used to analyse the same quartz standard sample and both instruments give the same analytical result.

The upper instrument in this picture is the BGS model 105 decrepitometer as used in Darwin. The lower instrument is operated by professor Xieyihan in Beijing.
The instruments are electronically completely different. The BGS instrument is completely digital and is operated by an attached computer (not shown), while the Beijing instrument used entirely analogue electronics.

Both instruments give the same result on the standard sample despite their vastly different electronics and 2 different instrument operators.

results comparison

Regular re-analyses of the calibration standard sample shows that the instrument reproducibility over time is good.

The instrument ONLY detects PRESSURE changes.

soundpath

Crystallographic changes in the sample generate shear waves which are not detected as they cannot travel through the airpath in the instrument and are not detected by the pressure sensor.
Only steam explosions or gas release as fluid inclusions decrepitate can be detected by the pressure sensor and counted.
Acoustic decrepitation ONLY responds to fluid inclusions.

Updated Understanding:

Gas rich fluid inclusions give distinct low temperature decrepitation responses.
Baro-acoustic decrepitation is caused by fluid inclusions and NOT by crystallographic effects in the host mineral.
CO₂ is released during low temperature thermal decrepitation.
Inclusion decrepitation is facilitated at 573 C in quartz by the drastic reduction in the Young's modulus of quartz as it passes through the alpha-beta phase transition.
The alpha-beta phase transition in quartz does not generate decrepitation counts in the absence of fluid inclusions.
Baro acoustic decrepitation gives reliably reproducible results with different instruments and operators and over extended time intervals.

Inclusion Abundance Measurements

To investigate the proportion of inclusions which decrepitate during heating, inclusion counts were carried out on thin sections of several samples for comparison with the decrepitation curves from the same samples.

The area of thin section counted was a square 50 microns by 50 microns.
Using a 40* objective, the depth of field of view was measured to be 5 microns
This gave a volume counted of 12.5 * 10^-6 cubic millimetres.
Multiply the counts by 1.54 * 10⁷ to get the number of inclusions per half gram, the quantity used in the decrepitation analysis.
No attempt was made to separate primary and secondary inclusions, all visible inclusions were counted.

Sample PCE15

The counts and decrepitation for this sample are:
(interval counts in red: cumulative counts in green)
pce15 cumulateive counts

Just less than 1% of the observed inclusions > 8 microns were detected during decrepitation.

Sample PCE3

The counts and decrepiation for this sample are:
(interval counts in red: cumulative counts in green)
pce3 cum counts

Only 0.4% of the inclusions > 8 microns were detected during decrepitation.

Sample PCB4

The counts and decrepiation for this sample are:
(interval counts in red: cumulative counts in green)
pcb4 cum counts

Only 0.5% of the inclusions > 10 microns were detected during decrepitation.

Most fluid inclusions do not decrepitate and quartz can apparently withstand fluid inclusion with internal pressures of several kilobars.

Although the decrepitation method detects only a small proportion of the total number of inclusions in a samples, the results are consistently reproducible on multiple analyses of the same sample. And decrepiation still detects and counts a vastly greater number of inclusions per sample than is ever achieved during microthermometric studies, so the decrepitation data is far more statistically robust.

In addition to the effect of inclusion size upon decrepitation, the stress regime in the host mineral will control which inclusions are able to decrepitate. These comparison photographs of the same sample are before and after thermal cycling shows that an inclusion close to a plane of weakness in the host mineral has decrepitated, but nearby larger inclusions have not decrepitated because they were distant from a plane of weakness in the quartz host mineral.

dec and undec sample

Conclusions:

Decrepitation counts are caused by pressure pulses as gas is released from fluid inclusions during heating.
Decrepitation is not caused by crystallographic changes in the host mineral
Decrepitation easily detects gas rich inclusions regardless of the actual gas species present.
Decrepitation counts an enormous number of inclusions per sample, giving statistically robust and valid results.
Analyses on different instruments with different operators are consistent and the method is robust and stable.
Decrepitation gives objective and consistently reproducible results. It is independent of bias during microscope observations.
This is despite the rather low decrepitation efficiencies when compared to the total abundance of inclusions in the samples.

Why use baro-acoustic decrepitation? Because it is the most appropriate fluid inclusion technique for use in mineral exploration.

Different procedures are appropriate for different outcomes. Microthermometry is appropriate for mineral deposit genesis study, but too slow and cumbersome for mineral exploration. Baro-acoustic decrepitation gives rapid and reliable data with quick sample turn-around and is the preferred method to apply fluid inclusion measurements to mineral exploration programmes.