Rapid FTIR sensing of volcanic gases released by Strombolian explosions at Yasur volcano, Vanuatu

We report here the results of a field campaign in which a portable Fourier transform infrared (FTIR) spectrometer was used to measure gas emissions from Yasur volcano, Vanuatu, in January 2005. By collecting FTIR spectra at a high rate (about 1 Hz), we were able to observe a marked difference in the proportions of SO2 and HCl in emissions released during Strombolian eruptions (SO2/HCl molar ratio up to similar to 30 or more) compared with the intervening passive emissions discharged from the magmatic vent (SO2/HCl similar to 2). This contrast can be explained by sourcing gas at different depths with respect to levels at which SO2 and HCl exsolve from the melt: deeper volatile exsolution supplies relatively SO2-rich gas responsible for the ephemeral explosions at the top of the conduit; while degassing of shallow magma, depleted in sulfur but rich in chlorine, contributes to the passive emission.