Determination of H 2 Densities Over a Wide Range of Temperatures and Pressures Based on the Spectroscopic Characterization of Raman Vibrational Bands

Raman spectroscopy is a powerful method for determining the densities of gas species in fluid inclusions, especially for H 2 -bearing inclusions in which the microthermometry approach is difficult to apply. The relationships between Raman peak position and H 2 density have been recorded in several previous studies. However, systematic discrepancies exist among these studies. In this study, the Raman spectral parameters (peak position, width, and intensity) of the vibrational bands of H 2 (Q 1 (0), Q 1 (1), Q 1 (2), and Q 1 (3)) were systematically measured at temperatures from 25 to 400 °C and pressures up to 150 MPa using a high-pressure optical cell. The variation in each parameter as a function of H 2 density was discussed. Several calibration polynomials derived from the measured peak positions and peak widths of these vibrational bands and the peak intensity ratios of Q 1 (1) to Q 1 ( n = 0, 2, 3) were established to determine H 2 densities up to 0.062 g/cm 3 at 25 °C. For natural fluid inclusions, the peak position of the Q 1 (1) band is the best choice for density determination mainly because (i) Raman spectra derived from fluid inclusions are not always of applicable qualities and the strongest intensity Q 1 (1) band could be obtained easier than others, and (ii) the peak position is insensitive to instrumental factors. The relationship between the peak position of Q 1 (1) band and density can be represented by ΔQ 1 (1) = 90,246.070 × ρ 4 – 5471.203 × ρ 3 + 770.944 × ρ 2 – 41.038 × ρ (r 2 = 0.999), where ρ is the density of H 2 in g/cm 3 ; ΔQ 1 (1) (cm –1 ) is the difference between the obtained peak position of Q 1 (1) band of H 2 and the known peak position of Q 1 (1) band of H 2 at near-zero density. This polynomial is independent of instrumental factors and can be applied in any laboratory, as long as the peak position of H 2 with a near-zero density is known. The effects of temperature on the relationship between these spectral parameters and H 2 density were also examined.