Other
Scientific paper
Mar 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54..553g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 3, pp.553-558
Other
Scientific paper
A quadrupole mass spectrometer and a solid probe which can be inserted directly into the ionization chamber have been used to analyze gas compositions of fluid inclusions. The probe holds a solid sample which can be heated continuously or stepwise from 30 to 750°C using variable heating rates. The decrepitated gas is released directly into the spectrometer source, thus reducing contamination. A Single Ion Monitoring mode program is used for the analysis, which is capable of detecting 26 preselected gases separately, and gases at picogram levels have been analyzed with this method. Each single burst of inclusions is detected and analyzed separately using a surface area integrator, and the computer program automatically traces the baseline above the background. Gas ratios are calculated for single bursts, or bursts over different ranges of temperature, as well as the sum of the total range. Routine petrographic and microthermometric analyses are used to determine the different generations of inclusions and their decrepitation temperatures. Then tiny pieces of the doubly polished section containing representative fluid inclusions or inclusions targeted for analysis are cut and introduced into the solid probe and heated accordingly. The principal gas species which have been analyzed are CO 2 , CO---N 2 (not discriminated), H 2 O, H 2 S, and nearly all light hydrocarbons. Fluid inclusions from different host minerals such as quartz, fluorite, barite, and sulfides have also been analyzed. The most important advantage of this method is that analytical results can be correlated with specific inclusion types since a small amount of sample material is required which makes it easier to choose specific areas from doubly polished sections. Another advantage is its capability to match the presence of gases in inclusions in quartz with those in associated sulfides, thereby confirming or denying that similar fluids were trapped by both the minerals. More tests are underway to examine the use of fluid inclusion data from sulfides. Preliminary tests on samples from an Archean gold deposit indicate the potential of this method, not only to detect the presence of different gases in the fluid, but also to determine gas ratios of fluid inclusions rapidly and fairly accurately. It has previously been established through alteration assemblage studies that CO 2 ---H 2 O ratios of the fluid decreased outwards from the gold-bearing zones and the new fluid inclusion data confirm this. This method also detected the presence of other gases such as CO / N 2 , CH 4 , C 2 H 6 , and H 2 S, indicating a multi-component C-O-N-H-S system. This method can be useful for both reconnaissance and detailed investigations where gas compositions of fluid inclusions are important for the understanding of fluid evolution processes.
Gagnon Michel
Guha Jayanta
Lu Huan-Zhang
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