Spectroscopic FTIR imaging of water species in silicic volcanic glasses and melt inclusions: An example from the Izu-Bonin arc

Abstract

Recent advances in FTIR spectroscopic imaging allow images of elements or molecules such as water to be constructed on samples over a large (sub mm) area. The imaging detector consists of 64 × 64 channels providing 4096 individual spectra simultaneously collected over a spectral range of 4000–900 cm − 1 . The wavelength range includes measurable peaks for water and carbon species, making it ideal for the analysis of volcanic glasses, particularly melt inclusions. Advantages of spectroscopic imaging include high spatial resolution (∼ 5 μm), allowing the analysis of melt inclusions too small to be analysed by other micro-FTIR techniques, and the acquisition of high definition in situ images of the materials analysed. The technique is applied to the analysis of water species concentrations and distribution in silicic volcanic glasses from Sumisu Caldera and Torishima Volcano in the Izu-Bonin arc to test the application of imaging to geologic samples. Results obtained are found to be comparable to analyses by conventional micro-FTIR techniques, with similar precision, but greater spatial resolution. Groundmass glasses from Sumisu Caldera dacites contain 1.2–1.4 wt.% H 2O and an obsidian sample from Torishima Volcano contains 0.2 wt.% H 2O, consistent with water saturation at their respective pressures of eruption. Melt inclusions in honeycomb plagioclase from Sumisu Caldera have higher water contents (1.02–1.38 wt.% H 2O) than inclusions from Torishima Volcano plagioclase (0.22–0.41 wt.% H 2O). Despite evidence for many inclusions being trapped wholly within honeycomb plagioclase, low water contents similar to those of the groundmass glasses suggest that the inclusions underwent water loss by degassing and/or diffusion. Alternatively, if low water contents of the melt inclusions reflect the composition of the magma at the time of entrapment, then the magma can be constrained as having originated from a water-poor source. Despite the ambiguity in initial water contents of the magma, the anhydrous mineral assemblage of all Sumisu Caldera and Torishima silicic rocks indicates an origin from a water-poor source, consistent with models for partial melting of water-poor mafic lower-mid crust, rather than melting of wet lower-mid crust or differentiation from basaltic magmas.