Infrared microthermometry of fluid inclusions in transparent to opaque minerals: challenges and new insights

Infrared (IR) studies of fluid inclusions in opaque minerals provide direct insights into the ore-forming fluids. However, the challenge posed by the “warming effect” of IR light during microthermometry remains unresolved. Here we address this problem and show that the deviation in temperature of phase changes within fluid inclusions caused by IR light is more common than what was thought before. Our results reveal that transparent to translucent minerals (e.g., quartz, sphalerite) also absorb IR energy. Since IR absorption is influenced by the host mineral, the fluid inclusions hosted by different minerals exhibit different degrees of deviation in temperature during microthermometry. The Fourier transform infrared (FTIR) spectra do not display a consistent correlation between the band gap energy of a mineral and its absorption of IR energy. Minerals with low band gap energy, such as pyrite, absorb limited IR energy, resulting in small deviations of fluid inclusion data. In contrast, this deviation could be significant for fluid inclusions hosted in minerals with a relatively high band gap energy, such as iron-rich sphalerite and wolframite. Substitution of iron increases the absorption of IR energy in these minerals. The substitution of trace elements may also play a role. Our quantitative analyses confirm that using the lowest possible IR light intensity with the smallest diaphragm minimizes the “warming effect” of IR light. We also propose an improved cycling method as a better option where high IR light intensity is required.