Jökull


Jökull - 01.12.1985, Side 7

Jökull - 01.12.1985, Side 7
al- (1980) have identified an acidity layer in the Camp Century ice core, with up to 5 11 equiv, H+/kg. They equated the layer, which they dated as 1390 B.C., with the Santorini eruption, and estimated a total acid fallout °f 1-25X1014 g (H2S04+HX) or more than an order of magnitude higher than the petrologic estimate. The correlation of this ice-core layer with the Santorini eruption is problematic, both in terms of age dis- crepancy and acid yield. More recently, Hammer (pers. comm., 1984) has indicated that the 1390 B.C. acidity layer may not correlate with the Minoan eruption of Santorini, and that the acidity signal from that eruption may in fact be hidden in the missing section from approximately 1500 B.C. to 2500 B.C. in the Camp Century ice-core. Judging from the petrologic estimate, the acidity signal from the Santorini volcanic aerosol was probably so low that it may not be detectable in the Greenland ice-sheet. discussion The examples presented here illustrate that the petro- logic estimates of volcanic volatile mass from several eruptions are closely comparable to estimates of volca- nic aerosol based on ice-core acidity layers and direct satellite-based aerosol extinction measurements. This correspondence is encouraging since the petrologic esti- mates do not take into account such factors as degassing of non-erupted magma, crystal and lithics content of tephra deposits and sources of volatiles other than degassing of the silicate melt (e.g. decomposition of sulfur- or halogen-bearing minerals). To our know- ledge, the only documented example of the latter is the E1 Chichon 1982 eruption, where the breakdown of anhydrite phenocrysts is inferred to be the only viable source of over 90% of the 2xl013 g sulfur-rich aerosol mass, whereas degassing of the magma accounts for only 7xl010 g H2S04 (Devine et al. 1984). The mineral- ogy of the anhydrite-bearing E1 Chichon magma is, however, unique and other examples of enhanced aero- sol injection by mineral breakdown are likely to be rare. Another important potential source of error in the petrologic estimates is compositional variation during eruption. Most of the volatile yield we have calculated (Devine et. al. 1984) are based on analysis of a single single sample of tephra, and the resulting mass calcula- tions implicitly assume a homogenous composition. In reality, however, a significant variation in tephra com- position has been observed during several eruptions, with relatively evolved magma followed by more primi- tive magma and we anticipate that the degassing of sulfur and halogens will vary also. Two eruptions of Hekla volcano are a good illustration of this phe- nomenon. During the H—1 (1104 A.D.) and H—3 (2800 B.P.) eruptions the composition of tephra gradually changed from rhyolitic to basaltic andesite in the final stages (Larsen and Thorarinsson 1977). Our petrologic estimates of volcanic volatile mass are based on analysis of samples from the initial rhyolitic phases of these eruptions and are one or two orders of magnitude lower than the aerosol mass indicated by the acidity layers. It is likely that the main aerosol mass was derived from degassing of the unstudied, later-erupted and iron-rich basaltic andesite magma. Thus our preliminary results are clearly inadequate in the study of compositionally varying eruptions, where detailed analysis of stratig- raphic sections will be required to obtain more realistic petrologic estimate of volcanic volatile mass. Another problem in petrologic estimates of volcanic volatile mass involves eruptions of mixed magma, where the two magma components have widely diffe- rent volatile concentrations. It has been proposed (Self and Rampino 1981) that the 1883 Krakatau eruption involved mixed magmas. Our petrologic study of glass inclusions in a single tephra sample (Devine et al. 1984) indicates total volcanic volatiles of 6.7x 1012 g (H2S04+ HCl), or lower than the ice-core estimate (Hammer et al. 1980) by a factor of eight. The tephra also contains crystals with glass inclusions of less-evolved magma with 4470+370 ppm C1 and 186+10 ppm S. Degassing of significant quantities of this mixed magma, which con- tains about twice the chlorine and sulfur of the more evolved magma, could account for the discrepancy in the ice-core and petrologic estimates. Similarly, our petrologic estimate of the volatile mass of degassed sulfur, chlorine and fluorine during the 1963 Agung eruption amounts to 4.17X1012 g total acids (Table 1), whereas estimates of the total volcanic aerosol from this eruption are 2xl013 g. The eruption is reputedly of mixed magma (Rampino and Self 1984) and it is thus possible that our estimate, which is lower than other estimates by a factor of 4, may be low because only the volatile-poor magma component has been studied so far. We emphasize therefore that accurate petrologic estimates of degassing and volcanic volatile mass must take into account heterogeneities in the erupted magma, whether due to magma mixing or eruption from a compositionally zoned reservoir. It is evident from petrologic studies of glass inclusions that the degassing of magmas during volcanic eruptions produces a variety of volatiles and that the sulfur species are not always the dominant component. This was pointed out by Johnston (1980) who showed, on basis of glass inclusion analyses, that the Augustine 1976 erup- tion (Alaska) liberated 5.25X1011 g C1 to the atmos- JÖKULL 35. ÁR 5
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