Abstract The P-V-X properties of two-component fluid inclusions (FIs) are generally determined from microthermometry data using appropriate thermodynamic models (i.e., VX diagrams) and/or equations of state (EoS). However, some limitations can hamper the applicability of this technique such as the small size, low density or complex composition of the analyzed FI. Raman spectroscopy is known as the best-suited alternative method to microthermometry for the investigation of natural FIs because it can provide simultaneously non-destructive qualitative and possible quantitative analyses after specific calibrations. The present work aims to provide calibration data to directly determine the P-V-X properties of binary or ternary mixtures of CH4, CO2, and N2. The variation of spectral features as a function of composition and pressure (or density) was investigated by using Raman spectroscopy coupled with an improved High-Pressure Optical Cell (HPOC) system and a customized heating-cooling stage. From our experimental data, the relative Raman scattering cross-section (RRSCS) of CH4 (νCH4∗) was demonstrated to be constant at 7.73 ± 0.16 over the investigated range of pressure (5–600 bars) and for any composition. This parameter can thus be used for the determination of composition with an uncertainty of ~0.5 mol%. Several calibration equations were calculated for different PX domains, linking the Fermi diad splitting of CO2 (Δ) or the relative variation of the CH4 peak position (νCH4∗) to the pressure (or density) and composition of CO2-CH4, CH4-N2, and CO2-N2-CH4 mixtures at 22 and 32 °C. The pressure and density of the fluids can henceforth be directly measured from Raman spectra with an uncertainty of ~20 bars and ~0.01 g·cm−3, respectively. Our calibration equations were then validated on natural FIs by comparing the results obtained from Raman and microthermometry. We also interpreted the variation of the peak position of CH4 based on the change of intermolecular interaction. Finally, we discussed the applicability of the obtained calibration data into another laboratory by comparing it with the data of pure CO2 and CH4 published in literature. A small shift between calibration curves implies a systematic error which is perhaps due to the difference in the configuration or the day-to-day deviation of the instruments. Therefore, standards of well-known P-V-X properties should be regularly measured to prevent and to correct any variation or shifting of the instrumental responses.

中文翻译：

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通过 5–600 bar 的拉曼光谱同时测定二元和三元 CO2 - CH4 - N2 气体混合物的 PVX 特性的校准数据：应用于天然流体包裹体

摘要 双组分流体包裹体 (FI) 的 PVX 特性通常是使用适当的热力学模型（即 VX 图）和/或状态方程 (EoS) 从显微测温数据确定的。然而，一些限制可能会阻碍该技术的适用性，例如所分析的 FI 的小尺寸、低密度或复杂组成。拉曼光谱被认为是最适合用于研究天然 FI 的显微测温替代方法，因为它可以在特定校准后同时提供非破坏性定性和可能的​​定量分析。目前的工作旨在提供校准数据，以直接确定 CH4、CO2 和 N2 的二元或三元混合物的 PVX 特性。通过使用拉曼光谱结合改进的高压光学池 (HPOC) 系统和定制的加热-冷却阶段，研究了光谱特征作为成分和压力（或密度）的函数的变化。根据我们的实验数据，CH4 (νCH4*) 的相对拉曼散射截面 (RRSCS) 被证明在所研究的压力范围 (5-600 巴) 和任何成分内恒定在 7.73 ± 0.16。因此，该参数可用于确定不确定度为 ~0.5 mol% 的成分。针对不同的 PX 域计算了几个校准方程，将 CO2 的费米二重分裂 (Δ) 或 CH4 峰位置 (νCH4*) 的相对变化与压力（或密度）和 CO2-CH4、CH4-N2 的组成联系起来和 CO2-N2-CH4 混合物在 22 和 32 °C。流体的压力和密度从此可以直接从拉曼光谱测量，不确定度分别为~20 bar 和~0.01 g·cm-3。然后通过比较从拉曼和显微测温法获得的结果，我们的校准方程在天然 FI 上得到验证。我们还根据分子间相互作用的变化解释了 CH4 峰位置的变化。最后，我们通过与文献中发表的纯 CO2 和 CH4 数据进行比较，讨论了获得的校准数据在另一个实验室中的适用性。校准曲线之间的微小偏移意味着系统误差，这可能是由于仪器的配置差异或日常偏差造成的。所以，