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Jökull - 01.12.1985, Page 6

Jökull - 01.12.1985, Page 6
stratospheric plume 650 km downwind from the volcano (.Inn et al. 1982) where HCl was estimated at 10—50 ppbv and methyl chloride (CH3 Cl) up to 5 ppbv, and a HC1/S02 ratio of 0.1 to 0.5. Chlorine is widely reported, both in fumarolic gases issuing from the crater floor (Evans et al. 1981) and air-borne filter samples over 2000 km from the volcano (Sedlacek et al. 1982). Finally, the petrologic chlorine estimate falls in the mid- range of the estimate of Turco et al. (1982) of 1010 to 10n g C1 total emission during the eruption. Data from Mount St. Helens (Inn et al. 1982) and some other small eruptions (Lazrus et al. 1982) indicate, however, that most chlorine is fractionated from the plume before chlorine-bearing compounds stabilize as volcanic aero- sols in the stratosphere. Whether such fractionation occurs in chlorine-rich large explosive eruptions such as Tambora 1815, with a mass eruption rate three orders of magnitude higher than Mount St. Helens, is not known at this time. The fate of the chlorine in the eruption column is highly dependent on its molecular form. Thus the highly soluble HCl may combine with atmospheric water vapor in the troposphere and be rapidly removed by precipitation. Methyl chloride (CH3C1) is, on the other hand, relatively inert in the troposphere and may therefore be transported to the stratosphere where it is dissociated to chlorine radicals by ultraviolet radiation. The results in Table 1 indicate a significant degassing of fluorine during the Mount St. Helens eruption. The fluorine data should be treated with great caution, however, because of the poor detection limit for this element with the microprobe (300 ppm). HF was observed in considerable quantities in fumarole gases emanating from the crater floor (Evans et al. 1981) and fluorine was also present in significant amounts as acid adsorbed on particles in the tephra deposit (Nehring and Johnston 1981) as well as in the down-wind plume (Hobbs et al. 1981). Neither CFC13 nor CF2C12, how- ever, were found to be above normal stratospheric concentration in the distal plume (Inn et al. 1981). By consideration of these observations, we judge that our fluorine yield from this eruption is either an overesti- mate, or that nearly all fluorine was rapidly deposited by adsorption on tephra particles. The latter process was well documented during the 1970 explosive erup- tion of Hekla volcano, where 3xl010 g fluorine were precipitated near-field with the tephra (Oskarsson 1980). SOUFRIERE 1979 A petrologic estimate of the volcanic volatile yield from the eight explosive eruptions of Soufriere volcano (St. Vincent) in 1979 can be made on basis of the data of Devine et al. (1984). The magma had a complex crys- tallization history before eruption, resulting in the trap- ping of a range of liquid compositions in olivine and plagioclase phenocrysts (Devine and Sigurdsson 1983). Late liquids trapped by plagioclase are more representa- tive of the magma which degassed to give rise to the volcanic aerosol and hence are used here in mass esti- mates (anal. 21 and 22 in (Devine et al. 1984)). Scaling to the total erupted mass of 4x 1013 g of juvenile tephra (Sigurdsson 1982b), we calculate a sulfuric acid yield of 1.5xl010 g and HCl yield of 3.5xl010 g to the atmo- sphere, or total yield of 5xl010 g during the eight explosive events. By comparison, the volcanic aerosol mass determined by SAGE satellite measurements for only two of the eruption plumes is 2.3xlO9 g (McCor- mick et al. 1981). It is likely that the satellite-based aerosol extinction measurements include about half of the erupted mass, in which case the petrologic estimate of volatile degassing is higher by an order of magnitude. Both sulfuric acid droplets and chlorine-bearing parti- cles were collected at the periphery of one of the eruption clouds (Woods and Chuan 1982), and high chlorine content reported in both filter samples and tephra (Sedlacek et al. 1982). The petrologic estimate of sulfur yield during the explosive phase of the eruption indicates a yield of 120 ppm sulfur per mass of erupted magma (Devine et al. 1984). Direct measurements of sulfur mass flux emis- sion with COSPEC (Hoff and Gallant 1980) during the subsequent effusive or lava dome growth phase of the eruption (May to October 1979) indicate nearly the same yield. Following the last explosive event on 26 April 1979, volcanic activity continued as magma was extruded, initially at a rate of 1.25x 1012 g/day, to form a lava dome (Huppert et al. 1982). during this period, S02 was vented to the troposphere at a rate of 3.4x 108 g/day (Hoff and Gallant 1980). This sulfur yield of 136 ppm per erupted mass during the effusive phase is therefore closely comparable to the petrologic estimate of 120 ppm sulfur yield during the explosive phase, lending further credence to the petrologic method of the study of volatile release. SANTORINI 1470 B.C. In about 1470 B.C. the Mediterranean volcano San- torini (Thera) produced 6.7X1016 g tephra (Watkins et al. 1977, Sparks and Huang 1980, Wilson 1980). We have estimated a volcanic volatile yield of 3.86X1012 g H2S04 from this explosive eruption, on the basis of glass inclusion petrology. No loss of chlorine is indicated from these data, although the chlorine content of the magma was quite high (Devine et al. 1984). Hammer et 4 JÖKULL 35. ÁR
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