ABSTRACT
Water-rock interactions determine how the geochemical cycles revolve from the Earth's surface to the deep interior (large T-P intervals). The underlying mechanisms interweave the fluxes of matter, time, and reactivity between fluid phases and solids. The deformation processes of crustal rocks are also known to be significantly affected by the presence or absence of water, typically with the hydrolytic weakening of quartz, olivine, and other silicate minerals. In fact, fluid-rock interactions mechanistically unfold along their interfaces, developing over a certain thickness within the two phases. Diffraction-limited mid-infrared microspectroscopy was employed to monitor the thermodynamic characteristics of liquid water along a quartz boundary. The hyperspectral Fourier transform infrared data set displayed a very strong distance-dependent signature for water over a 1 ± 0.5 µm thickness, while quartz appears unmodified, which is consistent with recent studies. This unexpected thick interface is tested against the geometry of the inclusion, the chemistry of the occluded liquid (especially pH), and the thermal conditions ranging from room temperature to 155 °C. Throughout this range of physicochemical conditions, the micrometer-thick interface is characterized by a ubiquitous, significant shift in the Gibbs free energy of water inside the interfacial layer. This conclusion suggests that the interface-imprinting phenomenon driving this microthick layer has thermodynamic roots that give rise to specific properties along the quartz-water interface. This finding questions the systematic use of the bulk phase data sets to evaluate how water-rock interactions progress in porous media.
ABSTRACT
Mid-infrared microspectroscopy is a non-invasive tool for identifying the molecular structure and chemical composition at the scale of the probe, i.e. at the scale of the beam. Consequently, investigating small objects or domains (commensurable to the wavelength) requires high-resolution measurements, even down to the diffraction limit. Herein, different protocols and machines allowing high-resolution measurements in transmission mode (aperture size (i.e., beam size) from 15 × 15 µm to 3 × 3 µm) are tested using the same sample. The model sample is a closed cavity containing a water-air assemblage buried in a quartz fragment (fluid inclusion). The spectral range covers the water stretching band (3000-3800 cm-1), whose variations are followed as a function of the distance to the cavity wall. The experiments compare the performance of one focal plane array (FPA) detector associated with a Globar source with respect to a single-element mercury cadmium telluride (MCT) detector associated with a supercontinuum laser (SCL) or a synchrotron radiation source (SRS). This work also outlines the importance of post-experimental data processing, including interference fringe removal and Mie scattering correction, to ensure that the observed spectral signatures are not related to optical aberrations. We show that the SCL and the SRS-based setups detect specific spectral features along the quartz boundary (solid surface), invisible to the FPA imaging microscope. Additionally, the broadband SCL thus has the potential to substitute at the laboratory scale the SRS for conducting diffraction-limited high-resolution measurements.
