FTIR microspectroscopy and SIMS study of water-poor cordierite from El Hoyazo, Spain: Application to mineral and melt devolatilization

This paper reports the microchemical and microspectroscopic FTIR study of cordierite from a partially melted graphite-bearing granulitic enclave within the dacitic lava dome of El Hoyazo (SE Spain). Optically transparent single-crystals, hand picked from the rock, were oriented using X-ray diffraction and studied by Fouriertransform infrared (FTIR). Single-crystal FTIR spectroscopy shows that the examined cordierite is CO2-rich and almost H2O-free. Two weak and sharp peaks are observed at 3708 and 3595 cm−1, respectively, which are strongly polarised for E // a. These peaks are assigned to combination modes of CO2. Very weak bands due to H2O molecules oriented with the H…H vector // c (type I water) are occasionally observed in certain zones of the grains, associated with absorptions due to hydrated inclusions of alteration products. The very intense bands observed in the 2600–2000 cm−1 region are assigned to CO2 molecules oriented // a; the spectra also show the presence of 13C and 18O, and weak amounts of CO in the sample. Microspectrometric mapping shows that the distribution of C is relatively homogeneous, whereas that of H2O is complicated by a very broad absorption extending from 3700 to 3100 cm−1. High-resolution FTIR imaging, done using a focal-plane array of detectors, shows that this broad absorption is associated with microfractures. SIMS analyses give an average concentration of H2O=0.033±0.007 wt.% and CO2=0.21±0.07 wt.%. On the basis of these data, molar absorption coefficients can be calibrated for CO2: εiCO2 (integrated)=11,000±4000 l/(mol cm−2) and εlCO2 (linear)=800±250 l/(mol cm−1). Due to the extremely low amount of H2O and its inhomogeneous distribution, calibration of absorption εH2O coefficients is unreliable. The very low H2O contents in the El Hoyazo cordierite indicate continued mineral–melt volatile exchange during decompression from ∼5 kbar to significantly shallower levels