Geoscience Reference
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iron-nickel alloys strongly suggest that the core
contains light elements. The density of the outer
core, estimated from the seismic wave velocity
and moment of inertia, is approximately 10%
lower than the density of pure iron under the
outer core conditions (e.g., Birch, 1964). On the
other hand, the density of the inner core is 2.5%
lower than that of pure iron under the inner
core conditions (e.g., Dubrovinsky
et al
., 2000).
Therefore, it is generally believed that the outer
and inner cores contain light elements. Several
light elements have been proposed to date as
candidates for the light elements in the core,
such as S, Si, O, or C (Birch, 1964; Mao
et al
.,
1990; Poirier, 1994).
S in particular has been considered as one of the
plausible light elements, because it is depleted in
the mantle, suggesting that it exists in the Earth's
core, and iron sulfides are observed universally in
iron meteorites, which may be analogues of the
planetary core (e.g., Murthy & Hall, 1970). There
are debates on the S content of the silicate Earth.
It has been considered that the Earth is depleted in
volatiles, and McDonough (2003) estimated that
the S content of the silicate Earth based on the
primitive mantle peridotiteis 250 ppm, which is
significantly lower than that of the C1-chondritic
abundance. The S content in silicate Earth and the
high-pressure partitioning of S between metal and
silicate suggest the estimate that the S content in
the core is at least a few weight percent (e.g., Li
& Agee, 1996; Hillgren
et al
., 2000).
O is also a possible candidate for the light el-
ement in the core. Oxide and metallic melts are
likely to become mutually miscible at high pres-
sures and temperatures. Although the eutectic
composition in the Fe-FeO system contains only
0.16 wt.% O and the liquid immiscibility field
extends across almost the entire Fe-FeO join just
above the eutectic temperature at 1 bar, Ring-
wood (1977) pointed out that at high temperatures
(
>
4000 K), complete miscibility between Fe and
FeO is expected. Furthermore, based on thermo-
dynamic considerations, he argued that O should
become increasingly soluble in liquid Fe metal
with increasing pressure. The miscibility of the
metallic and ionic liquids has been reported in the
Fe-FeO system at high pressure and temperature
(Ohtani & Ringwood, 1984; Kato & Ringwood,
1989). Since then, investigations of phase rela-
tions in the Fe-FeO system have confirmed that
with increasing pressure the eutectic composi-
tion shifts toward an FeO component and that the
region of liquid immiscibility narrows with pres-
sure above 15 GPa (Ohtani & Ringwood, 1984;
Kato & Ringwood, 1989). The models for dissolu-
tion of O into metallic core have been proposed
by Rubie
et al
. (2004) and Corgne
et al
. (2008).
The species and amounts of light elements in
the core depend on the core formation conditions,
such as the O fugacity, temperature of accretion
and core separation, and the degree and depth
of metal-silicate equilibrium in the primitive
Earth. The important characteristic, excess of Ni
in mantle olivine, has been noted by Ringwood,
who called it ''the nickel paradox'' (Ringwood,
1979). The excess of Ni in the mantle has been ex-
plained by the metal-silicate equilibrium at high
pressure (Li & Agee, 1996; Ohtani
et al
., 1997;
Siebert
et al
., 2012). The pressure dependency
of the partition coefficient of Ni between metal
and silicates suggests that the equilibrium at the
bottom of the deep terrestrial magma ocean at
high pressure and temperature, corresponding to a
pressure of about 30-40 GPa, can account for the
abundance of Ni in the mantle (e.g., Li & Agee,
1996; Ohtani
et al
., 1997). The metal-silicate
chemical equilibrium at low pressure has been
suggested by several authors (e.g., Stevenson,
1981; Karato & Murthy, 1997; Rubie
et al
., 2004;
Rubie
et al
., 2011). In this case, Si may not be
partitioned into metal, instead, S and O might
be good candidates, depending on the oxygen
fugacity during core formation (Figure 8.2 (a)-(d)).
The species of light elements strongly depend
on the O fugacity and temperature during the
accretion and core formation of the Earth (see
Figure 8.2 (a), (b)). Some of the light elements,
such as H, S, O, and C, have high volatility in
the O fugacity of the present Earth, and these el-
ements are included in primitive materials such
as carbonaceous chondrites. Therefore, the gener-
ally oxidized (fO
2
higher than IW buffer) and lower
pressure conditions of core formation favor these
