Exploring new protocols for bulk off-line fluid inclusion extraction for the analysis of δ13C-CH4 using a Cavity Ring-Down Spectroscopy (CRDS) analyzer.

For decades, the search for terrestrial abiotic CH₄ has been a central quest in geology and astrobiology. Most of this research has focused on crustal fluid samples collected at surface seeps, hydrothermal vents, and wells. Nonetheless, in such open systems processes like mixing with shallow biotic CH₄, oxidation, and diffusion can lead to large uncertainties in the initial composition of deeply originated CH₄¹. Extracting natural CH₄ from fluid inclusions entrapped in minerals could overcome the effects of shallow contamination occurring in natural open systems and shed light on the origin of deep CH₄ in a wide variety of geodynamic settings^2,3.  

In this abstract, we present a novel approach for bulk off-line fluid inclusion extraction for the analysis of δ¹³C-CH₄ using a Cavity Ring-Down Spectroscopy (CRDS) analyzer (Picarro G2201-i). Two fluid extraction techniques were compared: ball milling in ZrO₂ jars and sample crushing in a stainless-steel sealed tube under a piston. The accuracy and precision of the different protocols was evaluated with blanks, CH₄ isotopic labelling and interlaboratory comparisons.   

Blanks and isotopically labelled tests with the ball milling technique suggested that milling speed, and duration, and sample mass and type may strongly affect the CH₄ concentrations and isotopic compositions measured by the CRDS analyser. These effects are mainly ruled by the blank production of CH₄ –demonstrated by gas chromatography analyses– and potentially other molecules induced by frictional heating. The blank gases may cause interference effects on the absorption bands detected by the CRDS analyser. This effect was marked by a large offset in the δ¹³C-CH₄ measured in the high-range analytical modes of the Picarro G2201-i. The magnitude of the interference was inversely correlated to the CH₄ concentration.Other processes such as CH₄ diffusion and adsorption may play a role in the observed changes in CH₄ concentrations and isotopic compositions. Experimental conditions involving high CH₄ concentrations and sample mass could well reproduce CH₄ isotopic composition. 

Crushing in a sealed stainless-steel tube produces lower blank levels compared to ball milling, nevertheless the crushing efficiency and CH₄ release is lower due to smaller rock sample size.   

The presented ball milling protocol provides a simple and fast way to extract and accurately analyse CH₄ hosted in fluid inclusions even if great care should be considered for samples with low CH₄ concentrations where contamination and interferences can potentially alter the CH₄ isotopic signature of natural samples.   

 

References 

• 1 - Young et al. (2017). 10.1016/j.gca.2016.12.041 
• 2 - Klein et al. (2019). 10.1073/pnas.1907871116 
• 3 - Grozeva et al. (2020). 10.1098/rsta.2018.0431 

 

This work is part of project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 864045).