Raman Spectra of Aqueous Fluid Inclusions: Effect of Mineral Birefringence and Metastability on Salinity Measurement

Introduction: Crustal fluids play a major role in ore deposits, basin diagenesis and metamorphic reactions, among others. The knowledge of the chemical composition of individual fluid inclusions is essential for the understanding of past fluid transport and circulations. Microthermometry, by the observation of specific phase transitions as a function of temperature, is the general approach to determine salinity in aqueous fluid inclusions [1]. However in some cases, this method cannot be carried out because of, for instance, difficulties encountered in carrying optical observations, metastabil-ity, or if the thermodynamic system is not enough constraint. Besides, some minerals are highly cleavable, particularly carbonates, and consequently fluid inclusions might easily stretch, leak or decrepitate during freezing or heating on the heating-cooling stage, because of the volume expansion of the content with temperature. In replacement of microthermometry, salinity can be determined by Raman spectroscopy using a method based on the change of the shape of the water stretching vibration band with salinity. This change is due to the decrease of the strength of the hydrogen bonds in water by dissolved anions, often chloride [2,3]. An increase of the chloride concentration results in a decrease of the intensity of the OH stretching vibration band of water around 3200 cm-1. Different methods of treatment of the raw spectrum correlate the deformation of the OH stretching vibration band of water to chlorinity [4-9]. The Raman signal of water is also sensitive to the fluid density and the birefringence of the host crystal. Birefringence is critical in quartz and carbonates, and more generally in all highly birefringent minerals. Depending on the crystallographic orientation of the sample , the value obtained for salinity in quartz samples displayed variations higher than 50 % [5,7-9]. It has been observed that the " true " value is measured when the crystal is at its extinction position [6-9]. However, the optical phenomena behind the effect observed on the Raman spectra are not elucidated. Moreover, only the case of quartz was studied. Consequently, there is no general method at this time to determine salinity from the Raman signal of water in birefringent minerals. Met-astability may also affect the OH stretching vibration band of liquid water, the decrease of the fluid density affecting the strength of hydrogen bonds. Objectives: The purpose of the present study is to measure the effect of (i) metastability and (ii) mineral birefringence on the Raman spectra of water in aqueous fluid inclusions, in order to define a protocol to obtain accurate salinity measurement in any minerals, as an alternative to microthermometry.