High spatial resolution analysis of H2O in silicate glass using attenuated total reflection FTIR spectroscopy coupled with a focal plane array detector

Abstract

High resolution spectroscopic techniques that are used to determine volatile concentration in geological glasses are on the forefront for determining small scale degassing or hydration processes. Several spectroscopic methods are used for this purpose but they mostly require complicated sample preparation. Here, these methods are extended by the use of attenuated total reflectance (ATR) FTIR spectroscopy coupled with a focal plane array detector (FPA). ATR-FTIR spectroscopy requires only singly polished samples. The coupling to an FPA detector, which is novel for the analysis of volatile bearing glasses, enables spatial resolution on a micrometer scale. An ATR-FPA calibration to quantify H2O concentration in peralkaline rhyolitic glass is presented, which is comparable to conventional ATR calibrations using a single element detector. The results confirm that calibrations for different glass compositions are comparable after correction for glass density as suggested by a previous study. A method is developed to compare calibrations obtained with ATR objectives that have different angles of incidence. Despite the larger errors that come with the use of an FPA detector, the method allows the detection of H2O concentration gradients on a micrometer scale.The ATR-FPA calibration was applied to experimentally vesiculated melts that were quenched to glasses. It was possible to resolve only several μm-sized H2O resorption halos around vesicles that formed during isobaric cooling. Besides vesiculated glasses, the new method may enable the analysis of small melt inclusions and partially crystallized glasses and may allow the spatially resolved analysis of CO2 dissolved in silicate glasses.