page_title: Geology applications of Fourier transform infrared spectroscopy
section_title: Evaluate the explosive potential volcanic dome
hierarchical_section_title: Geology applications of Fourier transform infrared spectroscopy / Applications in geology / Volatiles diagnosis / Evaluate the explosive potential volcanic dome
caption_attribution_description: English: Schematic diagram of Water concentration profile across a pumice-obsidian sample. The shape of the profile can be translate into a diffusion timescale.
context_page_description: Fourier transform infrared spectroscopy is a spectroscopic technique that has been used for analyzing the fundamental molecular structure of geological samples in recent decades. As in other infrared spectroscopy, the molecules in the sample are excited to a higher energy state due to the absorption of infrared radiation emitted from the IR source in the instrument, which results in vibrations of molecular bonds. The intrinsic physicochemical property of each particular molecule determines its corresponding IR absorbance peak, and therefore can provide characteristic fingerprints of functional groups.
  In geosciences research, FTIR is applied extensively in the following applications:
  Analysing the trace amount of water content in Nominally anhydrous minerals
  Measuring volatile inclusions in glass and minerals
  Estimating the explosion potential in volcanic setting.
  Analysing chemotaxonomy of early life on earth
  Linking biological affinities of both microfossils and macrofossils
  These applications are discussed in details in the later sections. Most of the geology applications of FTIR focus on the mid-infrared range, which is approximately 4000 to 400 cm⁻¹.
context_section_description: Vesiculation, i.e. the nucleation and growth of bubbles commonly initiates eruptions in volcanic domes. The evolution of vesiculation can be summarized in these steps:
  The magma becomes progressively saturated with volatiles when water and carbon dioxide dissolves in it. Nucleation of bubbles start when then magma is supersaturated with these volatiles.
  Bubbles continue to grow by diffusive transfer of water gases from the magma. Stresses buildup inside the volcanic dome.
  The bubbles expand in consequence to the decompression of magma and explosions occur eventually. This terminates the vesiculation.
  In order to understand the eruption process and evaluate the explosive potential, FTIR spectromicroscopy is used to measure millimeter-scale variations in H₂O on obsidian samples near the pumice outcrop. The diffusive transfer of water from the magma host has already completed in the highly vesicular pumice which volatiles escapes during explosion. On the other hand, water diffusion has not yet completed in the glassy obsidian formed from cooling lava and therefore the evolution of volatiles diffusion is recorded within these samples. The H₂O concentration in obsidian measured by FTIR across the samples increase away from the vesicular pumice boundary. The shape of the curve in the water concentration profile represent a volatile-diffusion timescale. The vesiculation initiation and termination is thus recorded in the obsidian sample.  The diffusion rate of H₂O can be estimated based on the following 1D diffusion equation.
  D(C, T, P): the Diffusivity of H₂O in melt, which has an Arrhenian dependence on Temperature (T), Pressure (P) and H₂O Content (C).
  When generating the diffusion model with the diffusion equation, the temperature and pressure can be fixed to a high-temperature and low-pressure condition which resemble the lava dome eruption environment. The maximum H₂O content measured from FTIR spectrometer is substituted into the diffusion equation as the initial value that resembles a volatile supersaturated condition. The duration of the vesiculation event can be controlled by the decrease of water content across a distance in the sample as the volatiles escape into the bubbles. The more gradual change of the water content curve represents a longer vesiculation event. Therefore, the explosive potential of volcanic dome can be estimated from the water content profile derived from the diffusive model.
caption: Schematic diagram of Water concentration profile across a pumice-obsidian sample. The shape of the profile can be translate into a diffusion timescale.
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