Geoscience Reference
In-Depth Information
silicate melts. This process has been called redox
melting (Taylor & Green, 1988; Ballhaus & Frost,
1994; Foley, 2011).
Similar to water, CO
2
is degassing at mid-ocean
ridges, at hotspot volcanoes and is recycled back
into the mantle in subduction zones, with man-
tle residence times in the order of several Ga
(Dasgupta & Hirschmann, 2010). However, while
hydrous minerals mostly dehydrate in subduction
zones, carbonates remain stable and carbonate
recycling into the mantle is therefore very ef-
ficient (e.g. Yaxley & Green, 1994; Molina &
Poli, 2000; Connolly, 2005). This may have been
different during the Archean when mantle tem-
peratures were generally hotter, leading to effi-
cient devolatilization of subducting slabs, which
may have stabilized higher CO
2
contents in the
atmosphere. However, there are still many uncer-
tainties concerning the Earth's early carbon cycle
(Dasgupta & Hirschmann 2010).
ion during deep melting events. Obviously, this
could imply incomplete degassing of nitrogen
from the mantle; these effects, however, are
not yet well studied. A higher nitrogen partial
pressure in the Earth's early atmosphere could
have contributed to greenhouse warming by
broadening the spectral absorption lines of green-
house gases (Goldblatt
et al
., 2009). Such a model
would imply that a large fraction of the nitrogen
presently residing in the mantle is recycled.
1.5.2 Sulfur
Most of the sulfur in the mantle is contained
in a sulfide phase, which may be exhausted dur-
ing MORB generation (Saal
et al
., 2002). In the
lower mantle, the sulfide may be molten; H
2
S
in a reducing fluid phase could also be a ma-
jor host of sulfur in the lower mantle, but the
high-pressure thermodynamic properties of H
2
S
are poorly known (Frost & McCammon, 2008).
In silicate melts, sulfur occurs primarily in two
oxidation states, as S
2
−
under reducing condi-
tions and as S
6
+
under oxidizing conditions. At
low pressures (
<
0.3 GPa) the change between
the two oxidation states occurs rather sharply
about 1 log unit in oxygen fugacity above the
FMQ (fayalite quartz magnetite) buffer (Carroll
& Rutherford, 1988). At crustal pressures, sulfur
strongly partitions in favor of a hydrous fluid in
equilibrium with a silicate melt. For felsic melts,
fluid/melt partition coefficients range from about
50 for oxidizing conditions to about 500 for reduc-
ing conditions (Keppler, 1999, 2010; Webster &
Botcharnikov, 2011). This strong partitioning into
a fluid phase is probably the physical reason for
the ''sulfur excess'' observed in many explosive
volcanic eruptions (Keppler, 1999). A common
observation is that the amount of sulfur detected
in the eruption cloud by remote sensing may
be orders of magnitude higher than the amount
of sulfur contained in the erupted magma. The
excess sulfur was likely contained in an excess
fluid phase released during the eruption, which
had extracted sulfur out of a much larger magma
reservoir in the magma chamber below the vol-
cano. While it was widely believed that sulfur is
1.5
Other Volatiles
1.5.1 Nitrogen
The behavior of nitrogen is strongly dependent
on oxygen fugacity. Under reducing conditions
and in the presence of hydrogen, the ammonium
ion NH
4
+
readily substitutes for K
+
clinopy-
roxene, phengite and many high-pressure phases
(Watenphul
et al
., 2009, 2010). The substitution
of the ammonium ion for K
+
in sheet silicates
provides an obvious means for recycling nitro-
gen into the mantle (Zhang & Zindler, 1993;
Sadofsky & Bebout, 2000; Marty & Dauphas,
2003). In hydrous silicate melts under reducing
conditions, nitrogen probably dissolves as NH
4
+
ion, possibly also as molecular NH
3
.Under
oxidizing conditions, molecular N
2
is likely to be
a major species, but other species may also occur
(Roskosz
et al
., 2006). The strong dependence of
nitrogen speciation on oxidation state may imply
that its behavior in the deep, reducing mantle
could be very different from the shallow mantle,
with nitrogen being highly incompatible and
atmophile during the generation of MORB, but
being retained in mantle minerals as ammonium
