Fused-silica capillary capsules (FSCCs) as reference synthetic aqueous fluid inclusions to determine chlorinity by Raman spectroscopy

Microthermometry is the most common technique used to determine the salinity of aqueous fluid inclusions but, in some cases, this technique cannot be applied. The Raman analysis of water is a good alternative. Instead of preparing fluid inclusions within minerals by the crack-and-heal method, fused-silica capillary capsules (FSCCs) were used as synthetic fluid inclusions to calibrate the Raman signal of water toward chlorinity. The preparation of FSCCs is convenient in the preparation of synthetic fluid inclusions, and the composition of the standard solutions is well controlled all along the preparation procedure. Standard fluid compositions made in FSCCs are stable over time. The treatment of Raman data is reduced to the subtraction of a straight baseline and the measurement of Raman intensity at two wavenumbers, such that it is simple and independent from the operator. The method has been tested by comparison of the results obtained from Raman spectroscopy and with phase transitions observed in microthermometry from natural and synthetic fluid inclusions hosted in quartz. The polarisation effects, due to the birefringence of quartz, are treated by positioning the crystal at the extinction angle or by using an unpolarised laser radiation. The results obtained from Raman spectroscopy are in good agreement with microthermometry, over a wide salinity range (3.3 to 22.8 mass% NaCleq), with a similar precision.