Jökull - 01.12.1985, Blaðsíða 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