Gas release systematics of mineral-hosted fluid inclusions during stepwise crushing: implications for 40Ar/39Ar geochronology of hydrothermal fluids

Abstract

Abstract 40Ar/39Ar stepwise crushing technique has become more important for dating hydrothermal fluids trapped in mineral-hosted fluid inclusions. However, understanding the gas release patterns and interpreting the 40Ar/39Ar dating results have remained challenging. Here we use 40Ar/39Ar analyses coupled with quadrupole mass spectrometer (QMS) analyses of gases released by stepwise crushing to reconstruct the hydrothermal activities recorded by fluid inclusions hosted by wolframite, cassiterite, and quartz from the Yaogangxian tungsten deposit in South China. Microscopic observations and laser Raman spectroscopic studies reveal two main groups of fluid inclusions hosted in hydrothermal minerals: small, isolated and CH4-rich primary fluid inclusions (PFIs), and larger, mostly CO2-rich secondary fluid inclusions (SFIs) that distribute along micro-cracks. Formamide and alkane in the vapor phases of large aqueous SFIs are also found. The QMS analyses reveal systematic compositional features for gases released during different crushing stages: (1) gases from the initial crushing stage are H2O-poor, CO2-rich with CO2/CH4 > 1, likely dominated by gases from large vapor-rich SFIs; (2) gases from the final crushing stage are CH4-dominated gas mixtures with CO2/CH4