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 Fourier-transform infrared (FTIR). Single-crystal FTIR spectroscopy shows that the examined cordierite is CO(2)-rich and almost H(2)O-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 CO(2). Very weak bands due to H(2)O 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 CO(2) molecules oriented // a; the spectra also show the presence of (13)C and (18)O, and weak amounts of CO in the sample. Microspectrometric mapping shows that the distribution of C is relatively homogeneous, whereas that of H(2)O 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 H(2)O=0.033 +/- 0.007 wt.% and CO(2)=0.21 +/- 0.07wt.%. On the basis of these data, molar absorption coefficients can be calibrated for CO(2): epsilon(iCO2) (integrated)= 11,000 +/- 4000 l/(mol cm(-2)) and epsilon(lCO2) (linear)= 800 +/- 250 l/(mol cm(-1)). Due to the extremely low amount of H(2)O and its inhomogeneous distribution, calibration of absorption epsilon(H2O) coefficients is unreliable. The very low H(2)O contents in the El Hoyazo cordierite indicate continued mineral-melt volatile exchange during decompression from similar to 5 kbar to significantly shallower levels. (C) 2009 Elsevier B.V. All rights reserved.