Metastable phase equilibria in the ice II stability field. A Raman study of synthetic high-density water inclusions...

Microthermometric measurements of a synthetic high-density (984 kg m^-3) water inclusion in quartz revealed that only part of the super-cooled liquid water (L) transforms to solid ice I_h upon ice nucleation (L → ice I_h + L). While ice nucleation occurs in the ice I_h stability field at -41 °C and 28 MPa the pressure increases instantaneously to 315 MPa into the ice II stability field. At this point, both phases, liquid water and ice I_h are metastable. The coexistence of these two phases was confirmed by Raman spectroscopy and could be traced down to -80 °C. The pressure along this low-temperature metastable extension of the ice I_h melting curve was determined by means of the frequency shift of the ice I_h peak position using both the O-H stretching band around 3100 cm^-1 and the lattice translational band around 220 cm^-1. At -80 °C and 466 MPa the super-cooled ice I_h melting curve encounters the homogeneous nucleation limit (T_H) and the remaining liquid water transformed either to metastable ice IV (ice I_h + L → ice I_h + ice IV) or occasionally to metastable ice III (ice I_h + L → ice I_h + ice III). The nucleation of ice IV resulted in a pressure drop of about 180 MPa. Upon subsequent heating the pressure develops along a slightly negatively sloped ice I_h-ice IV equilibrium line terminating in a triple point at -32.7 °C and 273 MPa, where ice IV melts to liquid water (ice I_h + ice IV → ice I_h + L). Hitherto existing experimental data of the ice IV melting curve (ice IV → L) were found to be in line with the observed ice I_h-ice IV-liquid triple point. If, on the other hand, ice III nucleated at -80 °C (instead of ice IV) the associated pressure drop was about 260 MPa. The ice I_h-ice III-liquid triple point was determined at -22.0 °C and 207 MPa (ice I_h + ice III → ice I_h + L), which is in agreement with previous experimental data.