Analysis of gases in individual fluid inclusions in natural minerals using a dual mass spectrometer system

Abstract

Microscopic samples of the fluids in the vicinity of growing crystals can be trapped in defects called fluid inclusions. These have the potential to provide geologically significant information. Based on experience with an earlier system a new dual mass spectrometer system has been developed for analysing volatiles in individual fluid inclusions. Small mineral samples weighing about 10 mg are heated in a vacuum and as each inclusion decrepitates it produces a burst of gas lasting about 30 ms (for the geometry of the system). The gases are continuously pumped away through a pair of quadrupole mass spectrometers that scan overlapping ranges of 1–70 and 67–140 u in less than 25 ms. Mass spectrometer outputs are digitized, interleaved, combined with temperature data and stored on 9-track tape. Off-line processing locates each individual burst and subtracts the local background to give the inclusion composition. Mass spectra for individual inclusions can be displayed or groups of inclusion data can be combined on ternary diagrams. Release of any volatile components as a function of temperature can be monitored and displayed. These analytical techniques are applicable to a wide range of minerals.