mass-spectroscopy on fluid inclusion gases

Comparison of decrepitation with mass spectrometric gas determinations during heating

An additional verification that low temperature baro-acoustic decrepitation events are caused by gas rich fluid inclusions is provided by the work done by professor Damien Gaboury of the Universite du Quebec at Chicoutimi. He and his co workers have built an instrument to analyse gases by quadrupole mass spectrometer as they are released during heating of a sample in vacuum. This instrument slowly heats a sample while monitoring the vacuum pressure to quantify the amount of gas released and passes that gas into a sensitive mass spectrometer to identify the gases.

A quantity of the calibration standard baro-acoustic decrepitation sample used for many years at Burlinson Geochemical Services was analysed by professor Gaboury. This showed a complex mixture of gases, with CO₂ and H₂O being dominant. In particular, the CO₂ was preferentially released at low temperatures, commencing at 200 C, which corresponds to the low temperature baro-acoustic decrepitation peak. This clearly shows a direct relationship between the low temperature baro-acoustic decrepitation peak and the decrepitation of CO₂ rich fluid inclusions, in accordance with the theoretical explanations presented herein. In addition, Mavrogenes et. al. used mass spectrometry to analyse gas emissions during thermal decrepitation of a sample from the Cowra creek goldfield, NSW. Their work also confirmed that CO₂ and CH₄ are the dominant fluids released from the low temperature decrepitations

mass-spec gas and decrep

The calibration standard sample can be represented by 3 or 4 overlapping gaussian populations of inclusions.

h2203calib-4

Sub population parameters for the calibration standard sample

Sample #		Peak 1 Temp	Peak 2 Temp	Peak 3 Temp	Peak 4 temp
h2203	Calibration Sample	333	419	494	593
h1430	Calibration Sample	316	_	486	592

Summary

Low temperature baro-acoustic decrepitation peaks are indeed caused by the decrepitation of CO₂ rich (gas rich) fluid inclusions and the baro-acoustic decrepitation method can be used to recognise such gas-rich fluid systems in an exploration programme without resort to tedious and/or expensive micro analytical work.

In this sample, the first population peak, centered at 333 C , corresponds with the strong release of CO₂. The intense peak centered at about 490 C is still associated with CO₂ release, but is most likely caused by the decrepitation of predominantly aqueous inclusions. The fourth peak at 593 C is related to the quartz alpha-beta transition as discussed here.

References

Mass Spectrometric analysis of volatiles in fluid inclusions decrepitated by controlled heating under vacuum. Damien Gaboury, Moussa Keita, Jayanta Guha and Huan-Zhang Lu, Economic Geology v 103, pp 439-443, March-April 2008.

Comparison of decrepitation with mass spectrometric gas determinations during heating

Sub population parameters for the calibration standard sample

Summary

References

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