Refined insight into 40Ar/39Ar progressive crushing technique from K–Cl–Ar correlations in fluid inclusions

Abstract

Hydrothermal fluid activity associated with geological processes can now be dated by 40Ar/39Ar progressive crushing of minerals thanks to recent advances in high precision mass spectrometry. However, methods of data analysis and interpretation for these high-resolution measurements still need to be refined. In this study, we report further insights into using K-Cl-Ar diagrams as tools for interpreting ages of secondary and primary fluids from 40Ar/39Ar stepwise crushing experiments. Three paragenetic mineral pairs of muscovite and wolframites from a southern China tungsten deposit were investigated using the 40Ar/39Ar technique through laser incremental heating and progressive crushing of single minerals. Incremental heating analyses of muscovites yielded flat age spectra defining plateau ages of 156–153 Ma. Progressive crushing experiments of wolframites yielded anomalously old apparent ages due to excess 40Ar. However, the apparent ages decline quickly after the initial steps and stabilize to form age plateaus of 156–153 Ma without excess 40Ar. The plateau age steps interpreted as ages of gases derived from primary fluid inclusions (PFIs), yield well-defined isochrons with ages ranging from 157 to 153 Ma and initial 40Ar/36Ar values that are indistinguishable from the modern atmospheric value. Digitally created 2D- and 3D-diagrams of K–Cl–Ar correlations can easily reveal distribution trends and correlation planes from the crushing steps. Interpretations of these diagrams further reveal the ages of secondary fluid inclusions (SFIs) (87 to 83 Ma) and primary fluid inclusions (PFIs) (159 to 150 Ma). Raw data from SFIs degassed during the initial crushing steps form scatters on inverse isochron plots. However, 2D and 3D K-Cl-Ar diagrams yield SFI ages that agree with 40Ar/39Ar ages of K-feldspar veins cutting the tungsten ore bodies.