Geoscience Reference
In-Depth Information
According to their study, increasing the amount
of Si in liquid Fe decreases the bulk modulus
slightly, by only 0.5 GPa per weight percent of Si.
This result indicates a negligible effect of Si on
the bulk modulus of liquid Fe, which is consistent
with similarity in the local structure in liquid Fe
and liquid Fe-Si alloys. Therefore, a small effect
of Si on the bulk modulus of liquid iron, and a
decrease in density of liquid iron by addition of
Si would be consistent with the effect of the light
element in the outer core. These results indicate
that Si might be a plausible light element in the
outer core; i.e., Si reduces the density of liquid
iron, whereas it increases the V
p
of liquid iron.
On the other hand, S and C seem to reduce both
the V
p
and density of liquid iron in the pressure
range around 4 GPa as was discussed by Sanloup
et al
. (2004).
Sanloup
et al
. (2000, 2002) reported the effects
of S on the density and compressibility of liq-
uid iron at high pressure. Their results indicate
that S decreases the density and sound velocity
of liquid iron. Thus, S might not be the light el-
ement candidate in the outer core. Nishida
et al
.
(2011a) determined the density of FeS melt up to
6 GPa, and showed a significant increase in the
liquid density in the low pressure range, suggest-
ing that the liquid Fe-S alloy has a small bulk
modulus. On the other hand, Morard
et al
. (2007)
suggested the existence of a structural change of
the Fe-S liquid at high pressure, and the liquid
might become less compressible than at low pres-
sure. Nishida
et al
. (2011a) measured the sound
velocity of an Fe-S liquid at high pressure and
temperature up to 6 GPa by using the ultrasonic
method and showed that it is lower than the
sound velocity of pure liquid Fe, but increases
rapidly and becomes higher than for pure liquid
Fe above 10 GPa by extrapolation of the velocity
to higher pressures. Therefore, these experimen-
tal results suggest that S cannot be ruled out as a
light element candidate in the core. Huang
et al
.
(2011) measured the bulk sound velocity and den-
sity of liquid iron alloys with Fe
92.5
O
2.2
S
5.3
and
Fe
90
O
8
S
2
compositions up to the pressure of the
outer core by using shock experiments. Their
results indicate that adding O and S increases the
bulk sound velocity of iron at the same density,
but O and S have different effects on the bulk
sound velocity; i.e., the bulk sound velocities of
Fe
90
O
8
S
2
are higher than those of Fe
92.5
O
2.2
S
5.3
,
indicating that O has a stronger effect on the bulk
sound velocities of iron than S. Thus, they con-
cluded that O can be ruled out as a major light
element in the liquid outer core because adding O
into liquid iron would not reproduce the observed
density and sound velocity profiles of the outer
core simultaneously.
The density of the Fe-C melt was measured by
Terasaki
et al
. (2010) using an X-ray absorption
imaging technique up to 9.5 GPa and 1973 K. The
density of liquid Fe
3
C increases (6.94-7
.
38 g
/
cm
3
)
with pressure (in the range 3.6-9.5 GPa). The bulk
modulus of liquid Fe
3
C was determined to be
50
±
7 GPa at 1973 K, which is smaller than that
of liquid iron. The effect of C on the bulk mod-
ulus of liquid iron is similar to that of S, and
it may be explained by modification in the local
structure of liquid Fe derived from the intersti-
tial occupancy of C. Because dissolution of C
causes reduction in
ρ
and K
0T
,andV
p
,itmay
not be a major light element in the Earth's outer
core. Sanloup
et al
. (2011) measured the density
of the liquid Fe-5.7 wt% C up to 8 GPa by us-
ing an X-ray absorption method and proposed the
existence of compressibility increase (softening)
at around 6 GPa, suggesting a liquid-liquid tran-
sition at this pressure. They argued that such a
structure change in the liquid causes a change
of the partitioning coefficient of Ni, Co, and W
between metallic iron melt and silicate melt.
The existence of a liquid-liquid transition causes
miscibility of the Fe and Fe-C melts at pressures
above 5 GPa. These experimental results strongly
suggest that S and C could increase the sound
velocity at high pressure. Further density mea-
surements at higher pressure are needed to clarify
the light elements in the outer core. Therefore,
we cannot rule out the possibility that S and C
could also increase the sound velocity similar to
Si in the molten iron at higher pressures.
Figures 8.17 (a, b) shows the compositional de-
pendence of the density and excess volume of the
Fe-S and Fe-Si liquids at 4 GPa (Nishida
et al
.,
