| Summary: | The composition of volcanic gas plumes depends largely on the chemistry of the degassing magma, the depth of volatile exsolution, and the level of volcanic activity. The ratios between the most common volcanic gases: C02, H2O, SO2, HCl and HF, as measured at the surface, can be used to provide information on the evolution of the magma body. My research on volcanic gases has centred on the use of open-path Fourier transform IR spectroscopy (Op_FTTR) and correlation spectroscopy (COSPEC). I have also used data collected using other direct and remote-sensing techniques. Remote-sensing techniques rely on the characteristic IR or UV absorbances of natural and/or artificial radiation by different gases. The longer range of these techniques enables the analysis of gases in inaccessible plumes; thus reducing the need for operators to enter hazardous areas. As the instruments do not interact with the analysed gases there is no contamination, condensation or secondary reactions. However, the instruments tend to be heavy, expensive, and complex. Environmental factors can complicate analyses; clouds can dissolve and remove analyte rapidly, and variations in wind speed can result in gas fluxes having high errors. It is also much more difficult to analyse specific gas sources remotely as mixing of gases from different sources can occur. Direct-sampling techniques rely on gases being trapped, dissolved or adsorbed before being analysed by traditional methods, e. g. wet-chemistry, colourimetry, and gas chromatography. The capture of gases is best achieved as close to the source as possible, thus increasing the risk to the operator, and may only be possible during periods of low activity. The physical interaction of gases with instrument and collection vessels can lead to contamination and initiation of secondary reactions. Direct-sampling techniques are labour intensive and thus are capable of only generating a relatively small amount of data compared to the more automated remote-sensing techniques. The suitability of an individual technique therefore depends greatly on: the type of gas to be measured; the location of vent or fumamle; the level of volcanic activity; and the environment in which data are collected. I used OP-FrIR on La Fossa di Vulcano to measure the S02: HCI mass ratios of gases emitted from the rim and central crater fumaroles, -4.3 - 6.1 and 0.9 - 2.6 respectively. I attributed the higher crater rim gas ratios to the interaction of the gases with shallow hydrothermal reservoirs, causing scrubbing of the more soluble HCl- At Mt. Etna, my OP-FTIR analysis of gases emitted from the central craters showed that, in 1994, S02: HC1 mass ratios were -4.9 - 5.8. These values lie between those reported for eruptive degassing, >10, and background degassing, < I, and probably relate to refilling of the magma system prior to the 1997 eruption. A comparative study of lava effusion rates and COSPEC-derived SO2 fluxes for the 1991 - 1993 Etna eruption showed that variations were generally synchronous; small scale differences relating to the drainage of degassed magma from beneath the summit craters into the eruptive fissure. I also conducted OP-FT'IR and COSPEC analyses on Montserrat in June 1996 to show the gas plume to be relatively S02 poor, with S02: HC1 mass ratios of < 0.5. The OP-FTIR technique enabled the first remote measurements of SiF4 in volcanic plumes to be made. I have also used HF-SiF4 ratios to estimate gas equilibrium temperatures at La Fossa and Mt. Etna to be --200°C and -250 - 290°C respectively. I have also investigated the structural evolution of the Masaya Volcanic Complex. The visible complex has formed over -1000 years; with average rates of effusion of -0.2 x 106 m3/y, much lower than those required to provide the estimated volume of caldera infill, -2 x 106 m3/y. Historic activity has centred on the twin massifs of Volcän Masaya and Volcän Santiago and is dominated by pit-crater collapses. I propose that the degassing episodes, which occur with no increase in eruptive activity, are related to the convective overturn of magma beneath the craters.
|
|---|
