No abstract

CO2 fluid inclusions are popular in the mantle‐derived rock. CO2 Raman densimeter is widely used for estimating depth provenance and magma plumbing system. However, in order to obtain precise CO2 density, we should also measure CO2 temperature simultaneously because the densimeter has temperature dependency. In this study, we measured CO2 Raman spectra with densities of 0.8–1.0 g/cm3 at temperatures of 15, 25, 35, 45, and 55°C using a high‐pressure optical cell. We propose a new equation relating hot bands to the Fermi diad intensity ratio, temperature, and distance between the Fermi diad (delta, cm−1), which has higher accuracy than those of previous studies (±3.9–4.7°C) across all measurement conditions. The change in temperature engenders thermal expansion or shrinkage of mineral, resulting in change in CO2 density of fluid inclusion. Simultaneous measurement of both density and temperature of CO2 will be a probe for elastic property of minerals.

Micro‐Raman spectroscopy can find the carbon isotopic ratio of CO2 fluid from the ratio of intensity or area of a 13CO2 peak to that of a 12CO2 peak. We examined the precisions of carbon isotopic ratios (δ13C) of CO2 at constant room temperature and pressure of 10–150 MPa. Measurement of the intensity ratio has precision of 2.8–8.7‰, which is better than that of the area ratio of 4.5–14.7‰. We also investigated the pressure dependence of the Raman intensity ratios and area ratio by changing fluid pressure. When changing fluid pressure from 10 to 150 MPa, the ratios of intensity and area both show negative correlation with fluid pressure (CO2 density). Pressures of two types affect the Raman spectrum of CO2 peaks, affecting the peak position and peak shape. To evaluate effects on the peak position, we repeatedly measured the intensity ratio at constant CO2 pressure (10 MPa) with movement of the grating center position, which is defined as the center value of the analyzed wave number range. Although we moved the grating center position from 1,248.5 to 1,251.5 cm−1, no significant correlation was observed for either ratio of intensity or area. The pressure effect on the ratios can be corrected by ascertaining the CO2 pressure. Combination with the Raman spectroscopic barometry for CO2 enables analyses of δ13C of CO2 respectively using the intensity ratio and the area ratio of CO2 Raman peaks within 8.7 and 14.7‰.

Dependence of residual pressures of fluid inclusions on their size and host mineral species provides valuable information related to the depth provenance and P–T–t path of the rocks. Although Raman‐based barometry is an effective method for ascertaining the internal pressure of H2O–CO2 fluid inclusions, few studies have elucidated Raman spectral features of CO2 in a system of high‐pressure H2O–CO2. New experiments using a high‐pressure optical cell in this binary system with compositions of 100, 75 ± 2, and 60 ± 2 mol% CO2 were conducted for this study to verify the availability of Raman CO2 barometers for use in assessing the temperature and pressure conditions of approximately 22°C and 17.3–141.4 MPa. Our results demonstrate that the existence of H2O does not affect the relation between Fermi diad splits (Δ, cm−1) and total pressure of pure CO2. These results suggest that the Δ–total pressure relation obtained from pure CO2 is also applicable to H2O–CO2 systems, even at high pressure. However, unlike Δ, because the peak positions of the Fermi diad in the system of H2O–CO2 shift to a higher wavenumber than those of pure CO2 at given pressure higher than 30 MPa, the peak positions are not very suitable for the pressure scale in an H2O–CO2 system. Additionally, we confirmed the availability of bandwidths of CO2 as an indicator of compositions that can identify the presence of very small amounts of H2O (at least 0.3 mol% H2O), even at room temperature.

We measured Raman spectra of N 2 fluids obtained at 0.1-25 MPa at room temperature. The 14 N 15 N peak in the Raman spectrum of a low-pressure N 2 fluid is difficult to detect because of the prevalence of a group of peaks attributed to rotational vibration of 14 N 2 . The Raman peaks of 14 N 15 N and 14 N 2 of N 2 fluid at 25 MPa were acquired at various exposure times. The mean values and standard deviations of the peak height ratios and peak area ones of 14 N 15 N and 14 N 2 were examined for each time. The standard deviations of the peak height ratios and peak area ones were 2.2% and 1.9%, respectively, for 20 spectra acquired with peak height of 1 million counts of 14 N 2 . The uncertainties are about two times higher than those predicted from the noise of a CCD.Improvement of the pixel resolution can enhance the precision of the nitrogen isotope ratios by Raman spectroscopy.