The method is based on: (i) microthermometric measurement of ice melting, hydrate melting, halite dissolution and sylvite dissolution temperatures in fluid inclusions; (ii) calibration of LA-ICPMS signals of Br by analyzing Na-Br-Cl standard solutions loaded in silica glass capillaries; (iii) calculation of solute (Cl, Na, Ca, K, Mg) concentration based on LA-ICPMS intensities and the Pitzer thermodynamic model (for inclusions of salinity below 23 wt. % equivalent NaCl), or a charge-balance approach (for higher salinity inclusions) and (iv) calculation of uncertainty on Cl, Br and Na concentration and Cl/Br and Na/Br ratios in the fluid.
This method is applied to natural fluid inclusions with variable Cl, Br and Na contents as previously determined by microthermometry and bulk crush-leach coupled with ion chromatography analysis. The studied materials consist of: i) low-salinity inclusions from Alpine quartz veins and ii) high-salinity inclusions from the Trimouns talc deposit and from Athabasca Basin uranium deposits. Molar Cl/Br and Na/Br ratios range from 140 ¡À 50 to 1200 ¡À 400 and 17 ¡À 6 to 880 ¡À 290 in the analyzed inclusions. The calculated uncertainty on Cl/Br and Na/Br ratios in individual fluid inclusions is between 30 and 38 % , regardless of the salinity. It appears that the analysis of individual inclusions coupled with uncertainty calculation allows the identification of significant variation of Cl/Br and Na/Br ratios within individual samples that could not be identified from previous bulk crush-leach and ion chromatography analyses. Because Br is a minor solute (molar Cl/Br is between ~ 100 and ~ 10000 in most crustal fluids), and because of its high first ionization energy, the determination of Br concentration in individual inclusion by combining microthermometry and LA-ICPMS analysis is most readily achievable for large (> 50 ¦Ìm in diameter) inclusions. For smaller inclusions (10 to 50 ¦Ìm in diameter), the applicability of the present method is more sensitive to the Br concentration.