Freezing and melting behaviors of H₂O‐NaCl‐CaCl₂ solutions in fused silica capillaries and glass‐sandwiched films: implications for fluid inclusion studies

Abstract

Fluid inclusions of the H2O‐NaCl‐CaCl2 system are notorious for their metastable behavior during cooling and heating processes, which can render microthermometric measurement impossible or difficult and interpretation of the results ambiguous. This study addresses these problems through detailed microscopic examination of synthetic solutions during cooling and warming runs, development of methods to enhance nucleation of hydrates, and comparison of microthermometric results with different degrees of metastability with values predicted for stable conditions. Synthetic H2O‐NaCl‐CaCl2 solutions with different NaCl/(NaCl + CaCl2) ratios were prepared and loaded in fused silica capillaries and glass‐sandwiched films for microthermometric studies; pure solutions were used with the capillaries to simulate fluid inclusions, whereas alumina powder was added in the solutions to facilitate ice and hydrate crystallization in the sandwiched samples. The phase changes observed and the microthermometric data obtained in this study have led to the following conclusions that have important implications for fluid inclusion studies: (i) most H2O‐NaCl‐CaCl2 inclusions that appear to be completely frozen in the first cooling run to −185°C actually contain large amounts of residual solution, as also reported in some previous studies; (ii) inability of H2O‐NaCl‐CaCl2 inclusions to freeze completely may be related to their composition (low NaCl/(NaCl + CaCl2) ratios) and lack of solid particles; (iii) crystallization of hydrates, which is important for cryogenic Raman spectroscopic studies of fluid inclusion composition, can be greatly enhanced by finding an optimum combination of cooling and warming rates and temperatures; and (iv) even if an inclusion is not completely frozen, the melting temperatures of hydrohalite and ice are still valid for estimating the fluid composition.

Quantification of Solute Composition in H2O-NaCl-CaCl2 Solutions Using Cryogenic 2D Raman Mapping

“…In addition, three procedures may be taken to increase the success rate. First of all, it is necessary to repeat several runs of heating-freezing process to facilitate the nucleation of hydrates [19,32], and cultivate hydrates by finding an optimum combination of cooling and warming rates and temperatures [19] for each sample. Secondly, reducing the scanning step (i.e., increasing the density of scanning points) is critical to overcome the problem of overlapping of neighboring solid phases.…”

“…The solutions were loaded into silica capillary tubes with an inner diameter of 50 µm, an outer diameter of 300 µm and a length of about 1 cm, and the tubes were then sealed by fusion using the method of Chou et al (2008) [30]. According to the previous studies [17,19], the solutions with the composition at the eutectic point (solution of #10) of the H 2 O-NaCl-CaCl 2 system commonly failed to nucleate crystals with detectable hydrates peaks. Therefore, alumina powder (0.05 µm) was added to solution #10, as the alumina powder will facilitate the nucleation of hydrate crystals [17].…”

“…A Linkam THM600 heating-freezing stage mounted on an Olympus BX 51 microscope (Olympus, Melville, NY, USA) was connected with a Raman spectroscope. To achieve complete freezing of the solution, various cooling/heating rates and cyclic cooling-heating processes were used [19]. The capillary samples were initially cooled rapidly to −185 • C at a rate of 30 • C/min, heated to −100 • C at a rate of 30 • C/min, then to around −60 • C at 2 • C/min, and finally to −185 • C again at a rate of 30 • C/min.…”

“…We froze a series of H 2 O-NaCl-CaCl 2 solutions with known compositions and randomly selected a few spots for Raman spectroscopic analyses, with the aim to establish a quantitative relationship between the intensity of the peaks and the composition of the solutions [17]. Although a quantitative relationship was successfully established, its applicability is compromised by three problems: (1) the disequilibrium behaviors (especially the failure of nucleation of hydrates) is common during freezing [11,17,19], which is a barrier for the application of the cryogenic Raman method to natural fluid inclusions; (2) errors and bias may be introduced in the process of baseline truncation due to the presence of residual interstitial solution between ice and hydrate crystals, and (3) the distribution of hydrates and ice is typically inhomogeneous across the fluid inclusions [17,19]. Although these problems may be alleviated by averaging multiple point analyses [17,18,20], a more robust method with less artificial interference is desirable.…”

Common Problems and Pitfalls in Fluid Inclusion Study: A Review and Discussion

“…However, it is often ignored that the eutectic temperatures only apply to equilibrium situations, even though metastable phase changes are far more common than equilibrium ones with respect to first melting. In fact, experiments with synthetic fluid inclusions of the H 2 O-NaCl-CaCl 2 system that couple the heating-freezing stage with Raman spectroscopy reveal that the majority of the fluid inclusions contain significant amounts of liquid along with some solids when cooled to -185 • C, and this situation persists regardless of the cooling rate and duration [32][33][34]. This means that many fluid inclusions that appear frozen are not completely frozen, and therefore what may appear to be first melting during warming is actually some other process, such as coarsening of crystal grains [32,34,35].…”