An equation for determining methane densities in fluid inclusions with Raman shifts

Abstract

Raman shift of the C-H symmetric stretching band (v 1 ) of methane can be used to calculate the pressure and density of methane in fluid inclusions. However, previous numerical functions for Raman band shifts and methane density are only suitable for methane with densities lower than 0.3 g/cm 3 . In this study, Raman shifts for pure CH 4 were systematically measured at 25 ºC and pressures up to 150 MPa. Parameters of the long range attractive forces were fitted, and Raman shifts of CH 4 were calculated with hard sphere model for density up to 0.55 g/cm 3 . Based on experimental data and theoretical calculation, a unified equation was established to calculate the density of methane gas from Raman shifts over a wide density ranges up to 0.55 g/cm 3 :Where represents the density of methane in g/cm 3 ;is the difference between the meaured 23 peak position of methane in the fluid inclusion ( ) and the known peak position of methane at quite 24 low density ( ). The equation will work regardless of which machine the measurements of Raman 25 shifts are done on, as long as the user knows or finds the zero-density peak position. 26 Hansen et al., 2001; Lin et al., 2007; Lu et al., 2007). 40 Lu et al. (2007) established a unified equation for calculating methane pressures in the 41 CH 4 -H 2 O system from Raman shifts for the C-H symmetric stretching band of methane. However, 42 the pressure range of most previous measurements is just from 0 to 70 MPa, and the corresponding 43 methane densities are lower than 0.3 g/cm 3 , so their equations do not apply to the inclusions with 44 density higher than 0.3 g/cm 3 . 45In this study, we measured Raman shifts of pure methane at pressures up to 150 MPa at 25 ºC. 46 With the new data and theoretical calculation, we established an equation which could be also 47 applied in other laboratories for determining the density of methane in pure CH 4 fluid inclusions. The 48 new equation was effectively applied to determine the density of four typical pure CH 4 inclusions in 49 two samples obtained from a MVT-Pb-Zn deposit in mid-south China. 50 2. Methods 51 2.1 Experimental apparatus and procedures 52 4 The apparatus and procedures are similar to those used in the study of Lu et al. (2007) (Fig. 1). 53Experiments were carried out with a capillary high-pressure optical cell (HPOC; Chou et al., 1990 Chou et al., , 54 2005. After the capillary cell and pipeline were evacuated, methane (99.99%, Air Products) was 55 loaded into the cell and Raman spectra were collected under high pressures gradually up to 150 MPa. 56The methane pressure in the optical cell was adjusted by a high pressure generator and monitored by 57 an Omega PX91N0-50KSV digital pressure transducer (on a full scale of 400 MPa, accurate to ± 0.5 58 %). The temperature in the cell was controlled by a Linkam CAP 500 heating-cooling stage with an 59 accuracy of ± 0.1 ºC at 25 ºC (Lu et al., 2013). 60 Spectra collection and calibration 61Raman spectra of gas were collected with a JY/Horiba LabRam HR 800 system provided with 62...

Quantitative Raman spectroscopic study of the H2─CH4 gaseous system

“…In the CH 4 ‐bearing fluids, including pure CH 4 , CH 4 ─H 2 O, CH 4 ─N 2 , CH 4 ─C 2 H 6 , and CH 4 ─NaCl─H 2 O systems, the Raman shift (peak position) of the ʋ 1 band of CH 4 is an effective pressure (density) indicator . This is mainly because the peak position of the ʋ 1 band of CH 4 is very sensitive to pressure, and the magnitudes of its pressure‐dependent shifts are sufficiently large enough for pressure determinations.…”

“…This is mainly because the peak position of the ʋ 1 band of CH 4 is very sensitive to pressure, and the magnitudes of its pressure‐dependent shifts are sufficiently large enough for pressure determinations. For pressure determinations of pure CH 4 , readers can refer to Zhang et al, in which a unified equation was established to calculate the pressure (density) of pure CH 4 fluids. For H 2 ─CH 4 gaseous mixtures, where CH 4 proportions are high enough to induce sufficient pressure‐dependent ʋ 1 band shifts (e.g., 1:10, 1:5, and 1:1 H 2 ─CH 4 gaseous mixtures), the peak position of the ʋ 1 band of CH 4 is a better pressure indicator than other Raman spectral parameters.…”

Carbon and mineral transformations in seafloor serpentinization systems

“…for quantitative CH 4 measurements were acquired using a 473 nm laser, 1800 mm -1 grating, and 100 µm slit width. CH 4 vapor densities were then calculated using the calibration of Zhang et al (2016), and CH 4 pressures were calculated using the calibration of Lu et al (2007). Detection limits for Raman spectroscopy are instrument-specific and depend on the measurement conditions, which can vary from sample to sample and even within individual mineral grains.…”

“…A summary of the composition of secondary fluid inclusions is presented in (Zhang et al, 2016) and pressures of ≤11 MPa (Lu et al, 2007). Olivine in all Zambales samples hosts brucite and serpentine minerals, including chrysotile, lizardite, and antigorite.…”

Raman spectroscopy of methane (CH4) to 165 GPa: Effect of structural changes on Raman spectra

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