Geothermometry of calcite spar at 10-50 °C.

Carbonate geothermometry is a fundamental tool for quantitative assessment of the geothermal and geochemical evolution of diagenetic and hydrothermal systems, but it remains difficult to obtain accurate and precise formation temperatures of low-temperature calcite samples (below ~ 40 to 60 °C). Here, we apply three geothermometry methods (∆47-thermometry, nucleation-assisted fluid inclusion microthermometry-hereafter NA-FIM-and oxygen isotope thermometry) to slow-growing subaqueous calcite spar samples to cross-validate these methods down to 10 °C. Temperatures derived by NA-FIM and Δ47-thermometry agree within the 95% confidence interval, except for one sample. Regression analyses suggest that the real uncertainty of ∆47-thermometry exceeds the 1 SE analytical uncertainty and is around ± 6.6 °C for calcite spar that formed at 10-50 °C. The application of δ18O thermometry was limited to a few samples that contained sufficient primary fluid inclusions. It yielded broadly consistent results for two samples with two other geothermometers, and showed higher temperature for the third spar. We also found that calcite with steep rhombohedral morphologies is characteristic of low temperatures (11-13 °C), whereas blunt rhombohedra prevail in the 10-29 °C domain, and the scalenohedral habit dominates > 30 °C. This suggests that the calcite crystal morphology can be used to qualitatively distinguish between low- and higher-temperature calcite.

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

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 Ih upon ice nucleation (L → ice Ih + L). While ice nucleation occurs in the ice Ih 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 Ih 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 Ih melting curve was determined by means of the frequency shift of the ice Ih 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 Ih melting curve encounters the homogeneous nucleation limit (TH) and the remaining liquid water transformed either to metastable ice IV (ice Ih + L → ice Ih + ice IV) or occasionally to metastable ice III (ice Ih + L → ice Ih + 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 Ih-ice IV equilibrium line terminating in a triple point at -32.7 °C and 273 MPa, where ice IV melts to liquid water (ice Ih + ice IV → ice Ih + L). Hitherto existing experimental data of the ice IV melting curve (ice IV → L) were found to be in line with the observed ice Ih-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 Ih-ice III-liquid triple point was determined at -22.0 °C and 207 MPa (ice Ih + ice III → ice Ih + L), which is in agreement with previous experimental data.