Decrepitation as an aid to microthermometry
Kingsley BurlinsonSecond biennial Pan-American conference on research on fluid inclusions (PACROFI II), 1989, Virginia polytechnic Institute, Blacksburg, VA.
AbstractThe decrepitation method has traditionally been thought of as an alternative to microthermometry in fluid inclusion studies but its limited accuracy has led to its disuse. However, because the method is quick and simple it can be used in conjunction with microthermometry to provide coverage of larger groups of samples and to assist in selecting the samples most suitable for detailed microscope work.
The method is capable of identifying CO2-rich fluid inclusions because they decrepitate at anomalously low temperatures, as often noted during microthermometric studies. Suites of samples can therefore be quickly tested for the presence of CO2-rich fluid inclusions far more precisely than by manual searches of thin sections. Subsequent microscope work on samples selected on the basis of the decrepitation data can then provide the fine details. During a conventional microthermometric study of some 15 sections at the Enterprise gold mine, Northern Territory, Australia, the presence of CO2-rich inclusions was not recognized and it was not until a decrepitation survey was later undertaken that the importance of CO2-rich fluids in this deposit became apparent.
A study of auriferous quartz veins in West Australia was done using the clear (barren) vug filling quartz from the vein centre, because the quartz at the vein margins (where the Au occurred) was milky and difficult to study microthermometrically. It was assumed that the quartz vein was homogenous across its thickness and thus that the temperature results from the vein centre would reflect the formation temperature of the mineralized vain walls. However the decrepitation analyses of quartz from the vein centre and margin were significantly different, suggesting that the assumption of homogenous deposition is tenuous at best. Decrepitation data can quickly provide a preliminary test for such zonation effects to assist in the design and interpretation of an associated detailed microthermometric study.
Although the relationship between decrepitation and homogenisation temperature is complex, when studying a suite of similar samples a useful relationship can sometimes be derived. By then using microthermometry as a control for a larger suite of decrepitation samples a more thorough study can be accomplished with little more effort. In one such study, the decrepitation relationship having been derived, a large discrepancy between homogenisation temperature and temperature as inferred from decrepitation was observed on several samples. Upon re-examination of the thin sections of those samples it was found that some secondary inclusions had inadvertently been measured. Re-measurement of these samples gave temperatures consistent with the decrepitation data and the use of both techniques in conjunction resulted in more accurate and consistent data.
Although it has been presumed that the decrepitation method would be prone to interference from secondary inclusions, in practice they do not interfere at all as no decrepitation is observed at the low temperatures typical of secondary inclusions unless CO2-rich fluids are present. This fortuitous curiosity is though to be because the secondary inclusions, being small and decrepitating at low temperatures, do not generate a large enough pressure pulse to be detectable in the current instrument (G.Hladky, personal communication).
The major problem in applying decrepitation to date is when the quartz has been recrystallised. In such samples only small inclusions remain, mostly along grain boundaries and these give no decrepitation response; nor are they usable in microthermometric studies.
When used together and applied to a sensibly
suite of samples the decrepitation and microthermometric methods can
useful complementary information to improve the reliability and
of the data. Rather than being competing techniques, they are mutually