The quantitative determination of H and C in cordierite from FTIR + SIMS

Cordierite, a microporous mineral stable under geological conditions spanning from the amphibolite facies to UHT metamorphism, is able to incorporate significant H2O and CO2 within its structural channels. As the volatile content of cordierite significantly affects its stability and phase relationships (Harley et al. 2002), their quantitative evaluation is crucial in studies where cordierite is used as a P-T tracer. We present here a calibration for the spectroscopic microanalysis of hydrogen and carbon in cordierite, by combining SIMS and FTIR. The sample used for this study is a transparent single-crystal extracted from a partially melted metapelitic enclave at El Hoyazo (SE Spain). Two fragments were oriented // (010) and (001), doubly polished to 140 μm and analysed using polarized IR radiation. The spectra show that in the examined sample the H2O molecules are oriented with the H…H vector // a (type I water). The absorptions observed in the C-O stretching region can be assigned to both CO2 and CO molecules oriented // a; the spectra also show the presence of 13C in the sample. H and C contents of the cordierite have been measured by SIMS using secondary ion ratios of 1H/28Si and 12C/28Si, under operating conditions of 8 nA and 10 kV for the primary beam of O- ions, 4500 V for the secondary beam, and ion ratios measured at an energy offset of 75 V. The resulting H and C contents allow calibration of the following molar absorption coefficients for H and C in cordierite: H2Oε = 200 ± 40 L/(mol∙cm) linear or H2Oεi = 1500 ± 300 L/(mol∙cm-2) integrated; CO2ε = 800 ± 250 L/(mol∙cm) linear or CO2εi = 11000 ± 4000 l/(mol∙cm-2) integrated. Harley SL, Thompson P, Hensen BJ and Buick IS (2002) J. Metam. Geol., 20, 71-86.