IR spectroscopy: Quantitative determination of the mineralogy and bulk composition of fluid microinclusions in diamonds

Infrared (IR) absorption was used to measure the concentration of secondary phases trapped in microinclusion-bearing diamonds. Using KBr pellets with known concentrations, we determined the IR absorptivities (Ε, AU{bullet operator}l/mol{bullet operator}cm) of calcite, dolomite, magnesite, apatite, quartz, phlogopite, olivine and garnet: Ε_calcite1433cm^-1=739, Ε_dolomite1442cm^-1=921, Ε_magnesite1451cm^-1=563, Ε_apatite1049cm^-1=2590 (Ε_apatite605cm^-1=800), Ε_phlogopite1006cm^-1=861, Ε_quartz1084cm^-1=418(Ε_quartz800cm^-1=102), Ε_garnet967cm^-1=1049, Ε_olivine894cm^-1=418. Using the conversion factors for a diamond matrix, the concentrations of carbonates, silicates and apatite were calculated in 13 microinclusion-bearing diamonds that were previously analyzed by electron probe micro analyses (EPMA) and IR and carry carbonatitic to silicic high density fluids (HDFs). Combining the relative abundance in the microinclusions with the composition of each mineral, we calculated the concentration of SiO₂, Al₂O₃, P₂O₅ and MgO+CaO+FeO+BaO+Na₂O of the bulk HDF. Good agreement exists between the IR calculations and the EPMA data for the same diamond. Combining our data with the absorption coefficient of water and correcting for the effect of salinity, the IR spectra show that most of the low-Mg carbonatitic to silicic HDFs have ~20wt.% water while the high-Mg carbonatitic ones carry only ~10%. Our results show that IR spectroscopy alone can be used as a semi-quantitative method to determine carbonatitic to silicic HDF compositions in microinclusion-bearing diamonds. In combination with EPMA, a full characterization of the major constituents is achieved, including carbonate and water content.