IR spectroscopy: Quantitative determination of the mineralogy and bulk composition of fluid microinclusions in diamonds Y. Weiss a, ⁎, I. Kiflawi a,b , O. Navon a a The Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmund J. Safra Campus, Israel b Department of Physics, King's College, London, UK abstract article info Article history: Received 6 January 2010 Received in revised form 7 April 2010 Accepted 10 April 2010 Editor: R.L. Rudnick Keywords: IR absorption Conversion factor Microinclusion-bearing diamonds High density fluid (HDF) EPMA 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∙l/ mol∙ cm) of calcite, dolomite, magnesite, apatite, quartz, phlogopite, olivine and garnet: ε calcite 1433 cm −1 = 739, ε dolomite 1442 cm −1 = 921, ε magnesite 1451 cm −1 = 563, ε apatite 1049 cm −1 =2590 (ε apatite 605 cm −1 = 800), ε phlogopite 1006 cm −1 = 861, ε quartz 1084 cm − 1 = 418(ε quartz 800 cm −1 = 102), ε garnet 967 cm − 1 =1049, ε olivine 894 cm −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 SiO 2 , Al 2 O 3 ,P 2 O 5 and MgO+CaO+FeO+BaO+Na 2 O 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 ∼ 20 wt.% 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 char- acterization of the major constituents is achieved, including carbonate and water content. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Infrared spectroscopy (IR) is commonly used in mineralogy, both for the identification of phases and the study of their structure (Farmer, 1974). Its use as a quantitative tool in the determination of mineral abundance is rare (Vagenas et al., 2003; Namduri and Nasrazadani, 2008). Navon et al. (1988) have shown that in the case of fibrous diamonds, where high density fluids (HDFs) were trapped in myriads of microinclusions, IR can be used to quantify the amount of water and carbonates. This was substantiated by the good negative correlation found between the CO 2 /(CO 2 +H 2 O) ratio determined by IR (CO 2 in carbonates) and the silica content of the HDFs as measured by electron probe micro analyses (EPMA, Schrauder and Navon, 1994). However, the translation of IR absorption to concentration was based on the absorption of pure water (Thompson, 1965) and a single determination of the absorption of calcite powder in four KBr pellets. Shiryaev et al. (2005) used the absorption coefficients of apatite and clay minerals to estimate the contents of phosphate and sheet silicates and to constrain the proportions of the various groups (water, silicate, carbonate and phosphate) in the microinclusions. Klein-BenDavid et al. (2007, 2009) and Weiss et al. (2009) have tried to estimate these proportions based on the carbonate/(water + carbonate) ratio, as measured by IR, and the bulk composition of oxides and chlorine, as determined by EPMA. The majority of microinclusions consist of secondary minerals and a residual low-density hydrous solution (Lang and Walmsley, 1983; Guthrie et al., 1991; Klein-BenDavid et al., 2006, 2007). Using IR spectroscopy, Chrenko et al. (1967) were the first to detect the absorption bands of water and carbonates in the infrared (IR) spectrum of the coat of a coated diamond. Later, Navon et al. (1988) and Navon (1991) correlated between calcite, dolomite or magnesite, apatite, mica, and quartz and the low energy IR absorption peaks in the range of 550–900 cm −1 , usually observed in microinclusion- bearing diamonds. Recently, the higher energy absorption bands at 1002, 1060 and 1100 cm − 1 were related to the presence of phlogopite, apatite and quartz, respectively (Tomlinson et al., 2007; Klein-BenDavid et al., 2009; Weiss et al., 2009). Secondary-ion mass spectrometry (SIMS) and EPMA revealed that the major-element compositions of these submicrometer inclusions vary between 4 end-members: high- and low-Mg carbonatitic, silicic and highly saline (Navon et al. 1988; Schrauder and Navon, 1994; Izraeli et al., 2001, 2004; Wang et al., 2003; Shiryaev et al., 2005; Chemical Geology 275 (2010) 26–34 ⁎ Corresponding author. E-mail address: yakov.weiss@mail.huji.ac.il (Y. Weiss). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.04.010 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo