Hydrothermal Diamond-Anvil Cell
The hydrothermal diamond-anvil cell (HDAC) can be applied to study fluids and solids and particularly aqueous samples in situ at high pressures and temperatures. Although such cells are usually used at P-T conditions of less than 3 GPa and 850 °C, experiments with aqueous fluids to 23 GPa and 1025 °C have been reported in the literature. Because diamonds are transparent over a large range of the electromagnetic spectrum, samples in the HDAC can in principle be studied using any “photon-in – photon-out” method including optical techniques (microscopy/microthermometry, and infrared absorption, Raman, Brillouin, or luminescence spectroscopy) or X-ray techniques such as diffraction, fluorescence, absorption and inelastic scattering. These cells permit to obtain information on physical and chemical properties of materials that are altered upon quench, e.g., density, viscosity, sound velocity, electrical conductivity, phase transitions, complexation and speciation, solubility and partitioning, or the kinetics of mineral-fluid and fluid-fluid interaction.
The currently used hydrothermal diamond-anvil cells are modifications of the original design by Bassett et al. (1993), which has the advantage of a small temperature gradient in the sample chamber. Accuracy and reproducibility of the temperature measurements are ± 0.1 to 0.2 °C for temperatures between -25 to 25 °C and better than ± 1 °C at temperatures between 100 and 600 °C. Pressure is determined from microthermometric measurements of phase equilibrium temperatures or from the shift in the wavenumber or wavelength of Raman or fluorescence lines of an optical pressure sensor.
Schematic diagram of the central portion (not to scale) of a HDAC.
Upper and lower platen of a HDAC.
Albite, silicate melt, and aqueous fluid in the sample chamber (diameter about 400 µm) of a HDAC at 600 °C, 1.06 GPa. Bulk composition H2O+49 mass% NaAlSi3O8. A small chip of synthetic zircon was used as Raman spectroscopic pressure sensor.
Raman spectra of quartz at 23 °C, 0.1 MPa and 23 °C, 2130 MPa. The shift in the wavenumber of the ν464 Raman line can be used for pressure determination in a hydrothermal diamond-anvil cell to about 3 GPa at 600 °C and to about 10 GPa at room temperature, based on equations (2) and (3) given by Schmidt and Ziemann (2000). A better resolution can be achieved at temperatures close to 23 °C and pressures less than 2.5 GPa from the shift in the wavenumber of the ν206 Raman line (e.g., Schmidt and Ziemann (2000), eq. (5) therein). The random error of the pressure determination is about 10 MPa in the case of the ν206 Raman line, or about 25 MPa in the case of the ν464 Raman line. Prerequisites are a high-resolution Raman spectrometer, a good signal to noise ratio in the spectra, and calibration of the wavenumber (e.g., using plasma lines of the laser or a neon lamp).
Typical pressure-temperature path of the sample in a HDAC followed in the course an experiment. Modified from Schmidt and Ziemann (2000). For isochoric behavior, the pressure at the experimental temperature (point 4) can be calculated from the liquid-vapor homogenization temperature of the pressure medium (point 6) and an appropriate equation of state. At higher densities, other phase transitions may be applicable.
Phase diagram of water showing the principle of pressure determination from the temperature of an univariant phase equilibrium (here the ice VI melting curve).