Jökull


Jökull - 01.12.1985, Side 8

Jökull - 01.12.1985, Side 8
phere whereas the sulfur yield was negligible. Our results (Devine et al. 1984) support this observation in that they indicate that the mass of chlorine, and in some cases fluorine, may be larger than the sulfur mass degassed during some eruptions. „Volcanic eruptions can be divided into two groups in terms of volatile composition, expressed as H2SO4/HCI ratio. Firstly, the sulfur-rich eruptions, with ratio greater than 20. These include all the Icelandic eruptions studied to date and may be typical of mantle-fed rift-zone volcanism. Secondly, the halogen-rich eruptions, with ratio less than 3, characteristic of the volcanic arc systems. No intermediate types are present in our data set. A third type are the exceptionally sulfur-rich events, where decomposition of anhydrite or other sulfur-bearing minerals dominates the aerosol composition (E1 Chichon-type) and the contribution of degassing from the magma is relatively minor. Although present in high concentration in the near- field eruption plume, the halogens may be fractionated out early and thus their importance in the stratospheric volcanic aerosol is unknown. Direct chemical analyses of the acidity layers in Greenland ice cores indicate, however, that some chlorine compounds may persist. The acidity layer from the Eldgja 934 A.D. eruption in the Crete core contains at least 65% HCl (Hammer 1980) and Herron (1982) has also shown high levels of both C1 and F in the Dye 3 Greenland ice core layer from this eruption. Finally, data from this core show an acidity peak in early 1889, where the concentration of C1 is at least twice that of the sulfate anion (Herron 1982). This acidity peak, which may have originated from the 1888 eruptions of Bandai San or Ritter Islands, is indication that chlorine may be an important compo- nent in some volcanic aerosols. In contrast, studies of modern volcanic aerosols have generally shown that the halogens are absent or present only as trace gases, compared to sulfur. As pointed out above, the Cl' peak in the Greenland ice core acidity layer from the Laki eruption of 1783 does not correspond with a Na peak and thus sea-salts can be excluded. Other direct evidence for volcanoes as an important source of HCl in the stratosphere has recently come from studies of the volcanic aeerosol cloud resulting from the E1 Chichon eruption. Mankin and Coffey (1984) have shown a 40 percent increase in HCl in the stratosphere following the 1982 E1 Chichon eruption, or equivalent to about 4xl010 g HCl. The original suggestion of Stolarski and Cicerone (1974) of direct injection of chlorine into the stratosphere by volcanoes has now gained credibility through the study of the E1 Chichon eruption and as suggested by Mankin and Coffey (1984) should lead to a reassessment of the role of volcanoes in determining the stratospheric che- mistry of chlorine. Accurate petrologic estimates of the mass of sulfur and halogens degassed from magma during volcanic eruptions require a variety of data on the petrology and geologic features of the deposit. Firstly, the mass of erupted material must be determined by geologic mapp- ing of the volcanic deposit. In the case of lava eruptions this is straight-forward and subject to small error, whereas volume-estimates of tephra from explosive eruptions can be in error by an order of magnitude. Secondly, the mass of erupted liquid must be deter- mined by subtraction of the mass of crystals and lithics from the total erupted mass. This requires knowledge of the modal composition of the entire deposit. The com- bined volume of these components is rarely more than 10%, but tephra from some eruptions may contain up to 40% crystals, e.g. Mount St. Helens, 1980 (Carey and Sigurdsson 1984) and up to 60% lithics, e.g. in the 1979 Soufriere eruption (Sigurdsson 1982b). Third, the com- positional variation in erupted magma must be deter- mined, and the proportions of the two magmas estab- lished, in the case of eruptions of mixed magmas or compositionally zoned deposits. Finally, the pre-erup- tion (glass inclusion) and post-eruption (matrix glass) concentration of sulfur and halogens can be determined by electron microprobe analysis. We have found by analysis of standards, that accuracy of sulfur and chlor- ine microprobe analyses is about 1 to 6% of the amount present and precision (lo) about 2 to 7%, with detec- tion limits of 50 ppm (Devine et al. 1984). The microp- robe analysis for fluorine is much more difficult and our detection limit for this element is only 300 ppm, with accuracy and precision of 3 to 5% for silicic glasses. In practice, the largest potential errors in petrologic esti- mates of the mass of sulfur and halogen degassing from magmas do not therefore relate to the analysis of these elements in the glass, but rather are due to inadequate information on the total mass and modal composition of erupted tephra. Very few deposits have yet been stu- died in sufficient detail to permit such a rigorous analy- sis as outlined above, and all published petrologic esti- mates of volcanic volatile mass must therefore be regarded as preliminary. We find the correspondence between our estimates and those determined by other methods encouraging, however, indicating that the potential errors should not prevent the application of this method in further examination of the link between volcanism and climate changes in the geologic record. 6 JÖKULL 35. ÁR
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