Calibration of Infrared Absorption Coefficients for Structural Hydroxyl in Garnet by a Combined Study of Isotope Ratio Spectrometry and Vibrational Spectroscopy

Abstract

An accurate determination of water content in garnet is critical to quantify the transport of water to the deep mantle by the subducted oceanic crust beyond the breakdown of hydrous phases. Fourier transform infrared spectroscopy (FTIR) is the most widely used approach to determine the species and contents of water in garnet. Accurate quantification of OH in garnet requires independent calibration using an external method, as OH absorbance is mineral and composition specific. To obtain the infrared absorption coefficients of structural hydroxyl in garnet, a combined study of spectrometric analyses by FTIR and a method combining a thermal conversion elemental analyser with isotope ratio mass spectrometry (TC/EA‐MS) was carried out for fourteen gem‐quality natural garnet crystals with variable compositions. The obtained molar absorption coefficients were 9322 ± 338 and 240 ± 26 l mol−1 cm−2 for grossular‐ and spessartine‐rich garnet and pyrope‐almandine garnet, respectively. These results are within the range of previous studies. A new molar absorption coefficient of 689 ± 177 l mol−1 cm−2 was obtained for pyrope‐spessartine garnet. The large variation in the absorption coefficient indicates it is controlled by both garnet composition and OH‐absorption bands. The obtained absorption coefficients are only appropriate for certain types of eclogitic garnet, and more studies should be carried out on eclogitic garnets.