Geoscience Reference
In-Depth Information
Table 1.1
Composition of volcanic gases (in mol%).
Mt. St. Helens
with often very high silicate content, and highly
saline brines (Navon
et al
., 1988; Schrauder &
Navon, 1994; Izraeli
et al
., 2001). Methane and
hydrocarbon-bearing inclusions have also been re-
ported from xenoliths in kimberlites (Tomilenko
et al
., 2009).
Although volatiles are only minor or trace con-
stituents of the Earth's interior, they control
many aspects of the evolution of our planet. This
is for several reasons: (1) Volatiles, particularly
water and carbon dioxide, strongly reduce melt-
ing temperatures; melting in subduction zones,
in the seismic low velocity zone and in deeper
parts of the mantle cannot be understood with-
out considering the effect of water and carbon
dioxide (e.g. Tuttle & Bowen, 1958; Kushiro, 1969;
Kushiro, 1972; Tatsumi, 1989; Mierdel
et al
.,
2007; Hirschmann, 2010). (2) Even trace amount
of water dissolved in major mantle minerals such
as olivine can reduce their mechanical strength
and therefore the viscosity of the mantle by or-
ders of magnitude (Mackwell
et al
., 1985; Karato
& Jung, 1998; Kohlstedt, 2006). Mantle convec-
tion and all associated phenomena, such as plate
movements on the Earth's surface, are there-
fore intimately linked to the storage of water
in the mantle. (3) Hydrous fluids and carbon-
atite melts only occur in trace amounts in the
Earth's interior. Nevertheless they are respon-
sible for chemical transport processes on local
and on global scales (e.g. Tatsumi, 1989; Iwamori
et al
., 2010). (4) The formation and evolution
of the oceans and of the atmosphere is directly
linked to the outgassing of the mantle and to
the recycling (''ingassing'') of volatiles into the
mantle (e.g. McGovern & Schubert, 1989; R upke
et al
., 2006; Karato, 2011).
Kilauea
Kilauea
Etna
1980
1918
1983
2000
H
2
O
91.6
37.1
79.8
92
H
2
0.85
0.49
0.90
CO
2
6.94
48.9
3.15
7.3
CO
0.06
1.51
0.06
SO
2
0.21
11.84
14.9
1.0
H
2
S 0.35 0.04 0.62
HCl 0.08 0.1 0.1
HF 0.19 0.07
Source
: Data from Symonds
et al
. (1994) except for Etna (from
Allard
et al
., 2005).
compounds, particularly SO
2
,H
2
S, HCl, and HF.
Noble gases are only trace constituents of vol-
canic gases, but they carry important information
on the origins and history of the reservoirs they
are coming from (Graham, 2002; Hilton
et al
.,
2002). Nitrogen is a particular case. Volcanic
gas analyses sometimes include nitrogen, but
it is often very difficult to distinguish primary
nitrogen from contamination by air during
the sampling process. The most conclusive
evidence for the importance of nitrogen as a
volatile component in the Earth's interior is
the occurrence of N
2
-filled fluid inclusions in
eclogites and granulites (Andersen
et al
., 1993).
Ammonium (NH
4
+
) appears to be a common
constituent in metamorphic micas, which may
therefore recycle nitrogen into the mantle in
subduction zones (Sadofsky & Bebout, 2000).
Generally, the composition of fluids trapped as
fluid inclusions in magmatic and metamorphic
rocks of the Earth's crust is similar to volcanic
gases. Water and carbon dioxide prevail; hydrous
fluid inclusions often contain abundant dissolved
salts. Methane (CH
4
) containing inclusions are
also sometimes found, particularly in low-grade
metamorphic rocks of sedimentary origin and in
sediments containing organic matter (Roedder,
1984). Fluid inclusions in diamonds are an impor-
tant window to fluid compositions in the man-
tle. Observed types include CO
2
-rich inclusions,
carbonatitic compositions, water-rich inclusions
1.2
Earth's Volatile Budget
The Earth very likely formed by accretion of
chondritic material that resembles the bulk
composition of the solar system. In principle,
it should therefore be possible to estimate the
Earth's volatile budget by considering the volatile
content of chondritic meteorites (e.g. Morbidelli
