(1) Carbonate Cluster Isotopes
Carbonate cluster isotopes (Δ47) are measured using a MAT253 mass spectrometer. Carbonate minerals are converted to pure CO2 gas, which is carried by helium into the ion source, where it is transformed into charged ions under the action of electron bombardment, and then accelerated by an electric field and then subjected to a magnetic field, which is deflected at different radii according to the magnitude of the mass-to-charge ratio (m/z), thus realizing the separation of homogeneous molecules with different mass numbers, and finally, the different Finally, the charged ions with different mass-to-charge ratios are received and counted by receiving rings at different positions, and the isotopic composition is given by calculation.
The abundance of 13C18O16O in carbonate minerals is sensitive to temperature independent of the whole-rock isotopes of the minerals and the nature of the fluids during the period of mineral formation, and is therefore used as a paleothermometer to obtain the growth temperature of minerals. Carbonate cluster isotope thermometry is widely used in climate evolution, paleoenvironmental reconstruction, and diagenesis.
(2) Methane cluster isotopes
Methane cluster isotopes (Δ18) were measured using TILDAS with two continuous-wave quantum cascade lasers tuned to 8.6 μm and 7.5 μm, respectively, for the detection of the 13CH3D, 12CH3D, and 12CH4 absorption spectra (1169 cm-1), and the 13CH4 and 12CH4 absorption spectra (1331 cm-1). An astigmatic Heliotropic absorption cell with an effective optical range of 76 m was used to ensure that the 13CH3D abundance of 10 ml of pure methane gas was measured with an accuracy of 0.2‰, with similar sensitivity and accuracy to that of the mass spectrometer, and with a corresponding temperature deviation of 7°C (25°C) and 20°C (200°C).
Methane cluster isotopes have opened up a new field of methane geochemical research, deepening the identification of methane formation mechanisms. When methane is formed under internal thermal equilibrium conditions, it provides a direct record of the absolute temperature at which methane is formed, providing valuable information for identifying different methane sources. In many geologic environments, methane cluster isotopes indicate the formation temperatures of inorganogenic, thermogenic, and biogenic methane, facilitating its further development as a geologic thermometer and tracer of natural gas sources.