ANALYSIS OF GASES IN SINGLE MELT OR FLUID INCLUSIONS BY QUADRUPOLE MASS SPECTROMETRY [QMS] AND ION TRAP - TIME OF FLIGHT - MASS SPECTROMETRY [It-TOF-MS] – APPLICATION TO MINERAL RESOURCE INVESTIGATIONS

The gas composition of single melt or fluid inclusions is essential to understand how magmas and hydrothermal fluids evolve and generate significant ore deposits. The USGS–Denver Inclusion Analysis Laboratory has used QMS and is now evaluating the ability of IT-ToF-MS to determine the mole % H2O, CO2, CH4, CnHm, N2, Ar, H2S, SO2, HCl, HF in single melt or fluid inclusions. To find and open inclusions of interest in polished plates, a Nd-Yag UV [213 nm] laser ablation system with customized optics is utilized. To obtain unbiased data on the inclusion populations in 1-3 mm chips, inclusions are opened by thermal decrepitation. The catastrophic release of gas from a 25 micron aqueous inclusion generates a transient peak about 500 msec wide. Inclusion gases are released into the ion source of dynamically pumped QMS or IT-ToF-MS where gas molecules are ionized by electron impact. In the QMS, ionized gas molecules are directed down the axis of a quadrupole mass filter set to dwell for 10 msec on 5 to 20 user selected masses for gas species of interest with detection by a secondary electron multiplier. The QMS has a dynamic range of 10⁶, a cycle time of 50-250 msec, and only provides information on the masses monitored. In the IT-ToF-MS, ionized gas molecules are directed into an ion trap where they accumulate. At 10-50 msec intervals, the concentrated cloud of ionized gas in the trap is ejected and accelerated down a 1 m flight tube and reflected back to a multichannel plate detector where a mass spectrum of the gas molecules is obtained by their time of flight. The IT-ToF-MS has a dynamic range of 10⁴, a cycle time of about 10-50 msec, and generates a complete mass spectrum of the gas mixture in each inclusion. Successive mass spectra collected by either instrument over such peaks are corrected for background, specific ionization sensitivities, ion fragmentation, ion/molecule reactions [IT-ToF-MS only], and isobaric overlaps using a matrix solution of linear equations. To determine the composition of each inclusion, the peak height of each mass is compared to the peak height of gas standards introduced with a microvolume injection valve. Results demonstrate the pros and cons of each instrument and show how the integration of single inclusion gas data with other information permits study of magma and ore fluid evolution at a sufficient level of detail to benefit mineral resource investigations.