Volatiles under High Pressure - Physics and Chemistry of the Deep Earth - page 26

contained in magmatic-hydrothermal fluids and
in volcanic gases primarily as H 2 SorasSO 2 ,
recent spectroscopic studies suggest that already
at about 2 log units in oxygen fugacity above
the Ni-NiO buffer, a large fraction of the sul-
fur may be present as S 6 + , probably as H 2 SO 4
(Binder & Keppler, 2011). The injection of sulfur
dioxide into the stratosphere during major explo-
sive volcanic eruptions causes the formation of
sulfate aerosols that backscatter sunlight. This is
the main reason for the cooling often observed
after major eruptions (McCormick et al ., 1995;
Robock, 2000).

could become less incompatible or even compat-
ible at deep mantle pressures, and accordingly,
degassing of the deep mantle could be limited.
However, Schmidt & Keppler (2002) could not re-
produce this effect in multi anvil experiments and
suggested that it may perhaps be due to degassing
of the very small samples in laser-heated dia-
mond cells during quenching. Shcheka & Keppler
(2010), on the other hand, reported that argon, but
not xenon, is highly soluble in silicate perovskite
at lower mantle pressures, probably because of
the abundant oxygen vacancies in this phase.
This could imply that noble gases have been frac-
tionated between the mantle and the atmosphere
during the crystallization of a magma ocean.
Recycling of noble gases into the mantle dur-
ing subduction appears to be possible, although
the precise mechanism is not well understood
(Kendrick et al ., 2011).

1.5.3 Noble gases

Noble gases are widely believed to be exceedingly
incompatible in mantle minerals, although some
alternative views have been proposed (Watson
et al ., 2007). Table 1.4 compiles mineral/melt par-
tition coefficients for noble gases as reported by
Heber et al . (2007). Due to their highly incompat-
ible behavior, noble gases have been widely used
to develop models of mantle degassing (e.g. Marty
& Yokochi, 2006). In silicate melts, noble gases
are quite soluble and solubilities systematically
decrease from He to Xe and from polymerized to
depolymerized melts (Carroll & Webster, 1994).
Chamorro-Perez et al . (1998) proposed that the
solubility of noble gases in silicate melts abruptly
decreases at deep mantle pressures, because of a
compaction of pore space in the melt that makes
them unable to dissolve any noble gases. This
effect could potentially change mineral/melt par-
tition coefficients in such a way that noble gases

1.5.4 Halogens

Halogens are among the most incompatible ele-
ments during mantle melting (Saal et al ., 2002;
Dalou et al ., 2011). Chlorine and particularly flu-
orine may substitute for OH in hydrous mantle
minerals and in particular the substitution by flu-
orine could potentially extend the stability range
of these phases, as compared to the stability lim-
its shown in Figure 1.1 (above). However, the
very low abundance of fluorine in the mantle
makes it rather unlikely that this effect will be
significant. Subduction zone fluids are expected
to contain elevated concentrations of recycled
chloride (Manning, 2004; Bali et al ., 2011), con-
sistent with the high Cl / H 2 O ratios in primitive
arc magmas (e.g. Cervantes & Wallace, 2003).
Chloride-rich brines have been found in inclu-
sions in diamonds (Izraeli et al ., 2001). The phase
relationships between silicate melts, carbonatites
and chloride melts have been explored by Safonov
et al . (2007). Under crustal pressures, chlorine (but
not fluorine) strongly fractionates into a hydrous
fluid coexisting with a silicate melt (Webster,
1992; Carroll & Webster, 1994). The release of
chlorine during major flood basalt eruptions may

Table 1.4 Mineral/melt partition coefficients of noble
gases.

D olivine/melt D clinopyroxene/melt

He 0.00017 (13) 0.0002 (2)

Ne 0.00007 (7) 0.00041 (35)

Ar 0.0011 (6) 0.0011 (7)

Kr 0.00026 (16) 0.0002 (2)

Xe 0.0006 ( − 6 / + 9) 0.0002 (2)

All data are for 0.1 GPa
Source : after Heber et al . (2007). Reproduced with permission of
Elsevier.