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light elements in the core. On the other hand,
Si can be alloyed in metallic iron under reducing
and high temperature conditions, as is observed
in enstatite chondrites, which were formed under
reducing conditions (Mason, 1966). Thus, Si in
the core indicates reducing and high temperature
conditions during the accretion and core forma-
tion of the Earth. However, the volatility could be
changed with O fugacity, i.e., S becomes refrac-
tory under very reducing conditions as is shown
by the presence of oldhamite CaS in enstatite
chondrites (Crozaz & Lundberg, 1995).
Both Si and O are plausible candidates for the
light element in the core because they are abun-
dant elements in the Earth (e.g., Ringwood, 1959),
and the mantle of the Earth is depleted in Si
relative to C1 chondritic material (MacDonald
& Knopoff, 1958; Ringwood, 1959). Dissolution
of Si into molten iron has been confirmed ex-
perimentally by several authors; the solubility of
both Si and O in metallic iron coexisting with
silicates increases with increasing pressure and
temperature. The effects of fO
2
, emperature, and
pressure on the solubility of O and Si are shown
in Figure 8.2 (a)-(d) (Takafuji
et al
., 2005; Kawa-
zoe & Ohtani, 2006; Sakai
et al
., 2006; Ozawa
et al
., 2009).
Oxygen fugacity has a significant effect on the
dissolution of light elements such as Si and O into
the core. Kilburn and Wood (1997) showed entry
of Si into the core under reducing conditions,
whereas Rubie
et al
. (2004) argued for the disso-
lution of O into molten iron during formation of
Earth and Mars. Effects of temperature and fO
2
on
the solubility of O and Si in molten iron coexist-
ing with Mg-perovskite were studied by Kawazoe
and Ohtani (2006). They clarified that the solubil-
ity of both O and Si in molten iron coexisting with
Mg-perovskite increases with increasing tempera-
ture. The O solubility increases and the Si solubil-
ity decreases with increasing fO
2
. The reactions
between molten iron and perovskite, and between
molten iron and post-perovskite have been stud-
ied by Takafuji
et al
. (2005) and Sakai
et al
. (2006).
These experiments showed that the solubility of
both Si and O in molten iron increases with in-
creasing pressure. The reaction between molten
iron and the silicate mantle is an important pro-
cess occurring in the deep magma ocean during
core formation. Combining the effects of pres-
sure, temperature, and fO
2
, we can estimate the
Si and O content during the core formation of the
Earth. Figure 8.3 shows the solubility of O and
Si and the temperature profiles during the core
formation in the primitive Earth (Abe & Matsui,
1986). It indicates that the dissolution of O and
Si into molten iron was inevitable during core
formation in the primordial Earth.
Several other elements are proposed for the
light elements in the core, such as H, C, N, and
even Mg (Poirier, 1994). Cosmochemical studies
on iron meteorites have provided evidence for the
existence of 5-15% of Ni in the Earth's core (e.g.,
McDonough & Sun, 1995; Bottke
et al
., 2006).
Li and Fei (2008) pointed out the importance of
the effect of Ni in S-bearing systems, because the
Temperature profile during accretion
(e.g., Abe & Matsui, 1986)
CMB pressure
4000
Fig. 8.3
The solubility of O and Si and a model of the
temperature profiles during core formation in the
primitive Earth (Abe & Matsui, 1986). Solidus and
liquidus temperatures in the Fe-S-O metal (Terasaki
et al
., 2011) and the melting temperature of Fe-Si
alloy (Asanuma
et al
., 2010), and the pressure and
temperature conditions for dissolution of O and Si for
producing the core density deficit (cdd) of 5% and
10% are shown in this figure. The dissolution of O
and Si in molten iron was inevitable during the core
formation in the primordial Earth.
Cdd
=
10%
Cdd
=
5%
3000
2000
Post-Pv phase
transition
0
50
100
150
Pressure, GPa
