Non-destructive analysis of a mixed H2O-CO2 fluid in experimental noble-metal capsule by means of freezing and high-energy synchrotron X-ray diffraction

High-pressure high-temperature syntheses that involve volatile-bearing aqueous fluids are typically accomplished by enclosing the samples in gas-tight welded shut noble-metal capsules, from which the bulk volatile content must be extracted to be analyzed with mass spectroscopy, hence making the analysis non-replicable. Here we describe a novel non-destructive method that ensures the identification and the quantitative estimate of the volatiles directly in the sealed capsule, focusing on fluid H2O-CO2 mixtures equilibrated with graphite at conditions of geological interest (1 GPa, 800 degrees C). We used a high-energy (77 keV) synchrotron X-ray radiation combined with a cryostat to produce X-ray diffraction patterns and X-ray diffraction microtomographic cross-sections of the volatile-bearing samples down to -180 degrees C, thus encompassing the conditions at which crystalline phases-solid CO2 and clathrate (CO2 hydrate)-form. The uncertainty of the method is < 15 mol%, which reflects the difference between the volatile proportion estimated by both Rietveld refinement of the diffraction data and by image analysis of the microtomograms, and the reference value measured by quadrupole mass spectrometry. Therefore, our method can be reliably applied to the analysis of frozen H2O-CO2 mixtures and, moreover, has the potential to be extended to experimental fluids of geological interest containing other volatiles, such as CH4, SO2 and H2S.

Automated summary

This article describes a novel non-destructive method for the identification and quantitative estimation of volatile substances directly in sealed capsules. The method can be applied to the analysis of frozen water-CO2 mixtures.