Environmental Engineering Reference
In-Depth Information
provides a nearly complete time series along an entire volcanic arc for Br and
Cl release from palaeo-Plinian eruptions during the last 450 kyr, and can therefore
be used to constrain global halogen budgets from large explosive eruptions. Such
extrapolations, however, rely on the representativity of Cl emissions calculated
for CAVA eruptions. Data on degassing during volcanic eruptions worldwide, as
reviewed by Shinohara (
2013
), show large variations in Cl emissions determined
by the petrologic method. The concentration differences between pre-eruptive melt
inclusions and matrix glasses range mostly between 500 and 1500 ppm of
degassed Cl. This overlaps well with the average degassed Cl of 1070 ppm for
the CAVA eruptions (Kutterolf
et al
.,
2013a
), which we use as a typical value
for Plinian eruptions worldwide.
All matrix glasses of the CAVA tephras are degassed and have low Br concen-
trations between 0.4 and 1.3 ppm, independent of their chemical composition
(basalt to rhyolite). These values are comparable to the 1 to 3 ppm Br found in
bulk rock-powder samples from the 1902 Santa Maria eruption determined by
to the Laacher See eruption (Textor
et al
.,
2003a
). Because degassing during
eruption is highly effective, the concentrations of residual Br in the matrix glasses
are similar for all eruptions investigated (Kutterolf
et al
.,
2013a
). According to
Kutterolf
et al
. (unpublished), the melt inclusions, which should preserve the
halogen contents sampled from the respective magma chambers, if they did not
leak after their formation, in contrast have maximum Br concentrations ranging
between 0.9 and 17.9 ppm, and are strongly bromine-enriched compared to the
corresponding matrix glasses. Since
fluid-oversaturation of the magmas prior to the
eruptions can be demonstrated in the form of
fluid inclusions in phenocrysts
from all eruptions (Kutterolf
et al
.,
2013a
), we consider the highest Br concen-
tration value determined from each sample as our best estimate for the initial
Br abundance in the melts at magma chamber depths before the onset of eruptive
degassing. Consequently, we estimated the Br fraction released during magmatic
degassing as the difference between the maximum Br abundance in the most
undegassed inclusions and the average concentration in matrix glass, and con-
verted these fractions into erupted Br masses, using the respective erupted magma
masses for each eruption (Kutterolf
et al
., unpublished; this study). Erupted
Br masses of the 31 eruptions investigated range from roughly 2 to 1100 kt
(i.e. 2 to 1100
10
6
kg) and Cl masses from 12 to 80 700 kt (
Table 16.1
).
16.6 Discussion
The values for erupted Br masses for a typical CAVA eruption (95 kt/eruption) lie
within the range of present annual anthropogenic and natural Br emissions into
