A novel method for pressure-density-temperature-composition reconstruction of water-bearing CO2/CH4-rich geofluids based on a high-precision Raman calibration model

Abstract

Water-bearing CO2-rich and CH4-rich fluid inclusions are commonly found in various geological environments. Pressure-density-temperature-composition (P-V-T-x) reconstruction of geofluids from such fluid inclusions is fundamental for understanding fluid events related to hydrothermal, metamorphic, and ore-forming processes. Previous methods for P-V-T-x reconstruction of geofluids, such as microthermometry and Raman analysis, often ignore the small amount of water in CO2-rich and CH4-rich fluid inclusions, resulting in obvious errors in composition-density determination, and thus geofluid P-T reconstruction. In the present study, a novel method for P-V-T-x reconstruction of water-bearing binary CO2-rich and CH4-rich geofluids was proposed based on the measurement of the Raman peak positions of water in the vapor phase. Raman spectra of dissolved H2O in vapor-phase CO2/CH4-H2O fluid systems were collected at the P-T range of 10–120 MPa, 100–300 °C, i.e., 0.0091–0.894 of x (H2O) molar fraction, and quantitative models for composition-density calculation of CO2-rich and CH4-rich systems were developed. As the Raman peak positions of dissolved H2O in the CO2/CH4-rich phase are more sensitive in wave number to density changes than the peak positions in previous methods of the CO2 Fermi diad and C-H stretch mode of CH4 the composition–density calculation using the newly developed Raman calibration models was ten times higher in accuracy. As a demonstration, we determined the composition-density properties of the CO2-rich inclusions in quartz veins collected from the Cape Muroto, Shimanto belt, SW Japan; the P-T field constrained by fluid inclusion indicated the heating effect of the gabbro intrusion event.