Environmental Engineering Reference
In-Depth Information
chemical composition and
flux of gases emitted at the surface. Such monitoring
efforts, now run by many volcano observatories worldwide, have greatly
improved our knowledge of volcanic-gas chemical compositions and
uxes.
This chapter summarises the current state-of-the-art of volcanic-gas monitoring,
focusing on recent progress in instrumentation techniques for measuring major
volcanic volatiles.
6.2 Chemical composition of volcanic-gas emissions:
clues from direct sampling techniques
Most information on volcanic-gas chemical composition comes from conven-
tional laboratory analyses of gas samples that are directly collected from
fumaroles using evacuated bottles and caustic solutions (Giggenbach,
1975
;
Figure 6.1a
). Direct sampling makes it possible to characterise undiluted and
uncooled gas samples, and therefore detect a large number of chemicals down
to the parts-per-billion level (Symonds
et al
.,
1987
)in
fluids that are most
representative of quenched high-temperature magma-gas equilibria (Symonds
et al
.,
1994
). Direct sampling also allows the isotope composition of
fluids,
to be characterised, and therefore to fully constrain their origin (Hilton
et al
.,
2002
).
Direct sampling has demonstrated that the major components of volcanic gases
comprise molecular combinations of H, O, C, S and halogens, with H
2
O, CO
2
and either SO
2
or H
2
S being the dominant molecular species (Symonds
et al
.,
1994
). The compositions of a representative selection of high-temperature, directly
sampled magmatic gases are illustrated in
Figure 6.2
. The figure illustrates the
considerable intervolcano variability of gas composition. Two dominant source
processes, the mantle-inherited hydrous composition of source magmas (Wallace,
2005
) and the extensive hydrothermal interactions (Giggenbach,
1996
), contribute
to the volcanic gases released by arc volcanoes being richer in H
2
O than their
counterparts in within-plate/divergent margin settings (
Figure 6.2
). Scrubbing of
water-soluble magmatic gases at depth (Symonds
et al
.,
2001
), combined with
the addition of meteoric
fluids, result in a higher H
2
O content of low-temperature
hydrothermal gases (
Figure 6.2
).
Interpretation of fumarolic gas data has provided invaluable insights into the
time-dependent mixing relations between magmatic and hydrothermal
fluids,
which vary with the restlessness of the volcano (Chiodini
et al
.,
1993
,
2012
).
Hydrothermal interactions also control geothermal gas equilibria. These equilibria,
pioneered by Giggenbach (
1987
) and Chiodini and Marini (
1998
), have allowed
the
P
compositional properties of hydrothermal systems to be characterised,
which is key to interpreting gas time series collected at restless volcanoes.
For instance, both models and observations indicate the CO
2
/CH
4
-
T
-
ratio as a
