Mixed CH4-CO2 fluid inclusions in quartz from the South Wales Coalfield as suitable natural calibration standards for microthermometry and Raman spectroscopy

Laser Raman and microthermometric analysis have been used independently to calculate the molar proportions of mixed CH4-CO2 fluid inclusions in quartz from the South Wales Coalfield, UK. The respective ratios of the mean values obtained are 84 : 16 and 85 : 15 mol%. Therefore, the fluid inclusions are suitable for independently checking the validity of LRM and microthermometric data, and for determining the instrumental efficiency coefficient ξ of Raman microprobes. Individual fluid inclusions show consistent and reproducible phase transitions on heating and cooling between room temperature and − 196 °C. LRM tracking of these phase changes shows that on cooling CO2 is strongly partitioned into a solid phase, thus purifying the residual CH4 liquid down to − 196 °C at which point it freezes. Reheating results in the melting of the CH4 solid to form co-existing liquid + vapour + solid (L + V + S) CH4 at the triple point of − 182.5 (± 1) °C. In these samples, this phase transition is exceptionally clear and reproducible making the inclusions suitable as a low temperature calibration standard for microthermometric studies. The Raman spectra for L, V and S methane are distinguishable, with a prominent peak at 2906 (± 1) cm− 1 for CH4 (S) at − 190 °C, compared with peak positions for CH4 (V) and CH4 (L) at − 105 °C at 2917 and 2910 (± 1) cm− 1, respectively. Similar patterns apply to the CO2 spectra. With change of aggregation the peak position shifts by up to 7 wave numbers. These data show that the Raman peak positions in natural inclusions depend not only on temperature and pressure as previously reported, but also on the state of matter in which the component occurs: L, V or S.