Yves Krüger

Femtosecond lasers in fluid-inclusion analysis: overcoming metastable phase states

We have developed a new method to overcome metastable phase states that prevent microthermometric measurements in fluid inclusions. We use tightly focused femtosecond laser pulses to induce bubble nucleation in one-phase (all liquid) inclusions, nucleation of salt crystals in supersaturated brines, and nucleation of ice and salt hydrates. The threshold laser intensity necessary for phase nucleation was determined to be of the order of 10 TW/cm 2 . To avoid potential damage to the fluid inclusion, the threshold for producing ablation of quartz visible under the microscope was also determined and found to be about 25 % higher than the threshold for halite nucleation and 100 % higher than that for bubble nucleation. The experimental setup allows us to induce phase nucleation in selected fluid inclusions at different temperatures under microscopic observation. Subsequent microthermometric measurements can be performed in the same setup, making it suitable for routine applications.

Determining gypsum growth temperatures using monophase fluid inclusions—Application to the giant gypsum crystals of Naica, Mexico

Determining the formation temperature of minerals using fluid inclusions is a crucial step in understanding rock-forming scenarios. Unfortunately, fluid inclusions in minerals formed at low temperature, such as gypsum, are commonly in a metastable monophase liquid state. To overcome this problem, ultra-short laser pulses can be used to induce vapor bubble nucleation, thus creating a stable two-phase fluid inclusion appropriate for subsequent measurements of the liquid-vapor homogenization temperature, T-h. In this study we evaluate the applicability of T-h data to accurately determine gypsum formation temperatures. We used fluid inclusions in synthetic gypsum crystals grown in the laboratory at different temperatures between 40 degrees C and 80 degrees C under atmospheric pressure conditions. We found an asymmetric distribution of the T-h values, which are systematically lower than the actual crystal growth temperatures, T-g; this is due to (1) the effect of surface tension on liquid-vapor homogenization, and (2) plastic deformation of the inclusion walls due to internal tensile stress occurring in the metastable state of the inclusions. Based on this understanding, we have determined growth temperatures of natural giant gypsum crystals from Naica (Mexico), yielding 47 +/- 1.5 degrees C for crystals grown in the Cave of Swords (120 m below surface) and 54.5 +/- 2 degrees C for giant crystals grown in the Cave of Crystals (290 m below surface). These results support the earlier hypothesis that the population and the size of the Naica crystals were controlled by temperature. In addition, this experimental method opens a door to determining the growth temperature of minerals forming in low-temperature environments.

Exploration of the phase diagram of liquid water in the low-temperature metastable region using synthetic fluid inclusions

We present new experimental data of the low-temperature metastable region of liquid water derived from high-density synthetic fluid inclusions (996-916 kg m-3) in quartz. Microthermometric measurements include: (i) prograde (upon heating) and retrograde (upon cooling) liquid-vapour homogenisation. We used single ultrashort laser pulses to stimulate vapour bubble nucleation in initially monophase liquid inclusions. Water densities were calculated based on prograde homogenisation temperatures using the IAPWS-95 formulation. We found retrograde liquid-vapour homogenisation temperatures in excellent agreement with IAPWS-95. (ii) Retrograde ice nucleation. Raman spectroscopy was used to determine the nucleation of ice in the absence of the vapour bubble. Our ice nucleation data in the doubly metastable region are inconsistent with the low-temperature trend of the spinodal predicted by IAPWS-95, as liquid water with a density of 921 kg m-3 remains in a homogeneous state during cooling down to a temperature of -30.5 °C, where it is transformed into ice whose density corresponds to zero pressure. (iii) Ice melting. Ice melting temperatures of up to 6.8 °C were measured in the absence of the vapour bubble, i.e. in the negative pressure region. (iv) Spontaneous retrograde and, for the first time, prograde vapour bubble nucleation. Prograde bubble nucleation occurred upon heating at temperatures above ice melting. The occurrence of prograde and retrograde vapour bubble nucleation in the same inclusions indicates a maximum of the bubble nucleation curve in the ϱ-T plane at around 40 °C. The new experimental data represent valuable benchmarks to evaluate and further improve theoretical models describing the p-V-T properties of metastable water in the low-temperature region.

Unexpected behaviour of fluid inclusions synthesized from silver oxalate and an aqueous NaCl solution

Fluid inclusions were synthesized in quartz from silver oxalate (Ag2C2O4) and a 6-wt.% NaCl aqueous solution to produce a CO2–H2O–NaCl fluid, following the method of previous workers. Synthesis conditions of 500–650°C and 1500–4500 bar were chosen with the intention of trapping a homogeneous, ternary fluid. The resulting inclusions were analyzed by microthermometry and by Raman spectroscopy. Solid residues in the experimental capsules were analyzed after the syntheses by XRD and SEM-EDS. Several properties of the synthetic inclusions were unexpected: TmCla and Thtot vary over significant ranges and they correlate negatively; the calculated total molar volumes deviate significantly from data on the same bulk composition derived by Gehrig [Gehrig, M., 1980. Phasengleichgewichte und pVT-Daten ternarer Mischungen aus Wasser, Kohlendioxid und Natriumchlorid bis 3 kbar und 500°C. Hochschul Sammlung Naturwissenschaft, Chemie, Band 1. Hochschul Verlag, Freiburg] in an optical cell, and from data derived from our fluid inclusions synthesized for comparison using the gas-loading technique of Frantz et al. [Frantz, J.D., Zhang, Y.-G., Hickmott, D.D., Hoering, T.C., 1989. Hydrothermal reactions involving equilibrium between minerals and mixed volatiles: 1. Techniques for experimentally loading and analyzing gases and their application to synthetic fluid inclusions. Chem. Geol. 76, 57–70]; all fluid inclusions contain a chlorargyrite (AgCl) daughter crystal; and HCO3− was detected in the aqueous phase of the inclusions by Raman spectroscopy, presumably reflecting high concentrations of NaHCO3(aq). All these anomalous features can be traced back to effects caused by precipitation of insoluble AgCl and simultaneous formation of sodium oxalate upon loading the experimental capsules with silver oxalate and NaCl solution. Reconstruction of the chemical reactions that occurred during the hydrothermal syntheses suggests that incomplete redissolution of the AgCl during progressive quartz precipitation and fluid trapping yields inclusions with variable bulk properties. Nevertheless, quite accurate total homogenisation temperatures can be recovered by regressing the Thtot–TmCla correlations, but considerable effort is required. We conclude that silver oxalate should be used in combination with chloride solutions only with extreme caution, whereas the gas-loading technique provides an excellent alternative for the synthesis of CO2–H2O–NaCl fluid inclusions.

Ultra-short pulse lasers in geological fluid inclusion analysis

This paper demonstrates second harmonic generation microscopy using an ultrashort pulse laser can be used to three-dimensionally image and determine the volume of fluid inclusions of arbitrary shape. Because of the phase matching conditions in a tightly focused beam, second harmonic light is only generated at the quartz-fluid inclusion interface, directly providing an image of the inclusion walls. A of a fluid inclusion reconstructed from a second harmonic generation image stack is reported.