An improved gas extraction model during stepwise crushing: New perspectives on fluid geochronology and geochemistry

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Trapped fossil fluids in mineral-hosted fluid inclusions preserve pivotal information on the evolution of paleo geological systems, particularly the hydrothermal mineralization systems. However, identification of the sources of the extracted gases during bulk analysis of fluid inclusions is challenging because multiple generations, types, and phases of fluid inclusions may exist in the same mineral. To better identify the gas sources during stepwise crushing we report an improved gas extraction model of fluid inclusions. Stepwise extracted gases from fluid inclusions in quartz and cassiterite were measured with a quadrupole mass spectrometer and compared with Raman analysis of different types of secondary (SFIs) and primary (PFIs) fluid inclusions. We found that H2O-poor and CO2-rich gases with elevated CO2/CH4 ratios, H2O and formamide-rich gases, and CH4-rich gases were extracted sequentially during stepwise crushing. Such compositional variations of the released gases suggest that the extraction of fluid inclusions starts from SFIs with high vapor-filling degrees (f) and CO2-bearing tri-phase high-f fluid inclusions, which is followed by large liquid-rich SFIs and mid-sized high-f PFIs that are rich in CO2 and formamide, and ends with small CH4-rich low-f PFIs. These findings suggest that the degassing pattern of hydrothermal minerals during stepwise crushing is mostly governed by the physicochemical characteristics of the fluid inclusions, in particular, their vapor-filling degrees, sizes, compositions (which can lead to different inner pressures and densities), and associations with micro-cracks. This updated model of gas release patterns strengthens the theoretical basis for differentiating and identifying the sources of gases extracted during stepwise crushing experiments, which is key to understanding the fluid geochronology and geochemistry of hydrothermal deposits with multi-stage hydrothermal evolution in general.