Geoscience Reference
In-Depth Information
Ni
Ni
13.8
14.26
0.25
0.25
14.09
0.2
13.4
0.5
0.5
13.92
0.2
13.75
13.6
13.0
Fe
Si
13.42
0.5
0.1
13.3
13.08
12.88
0.1
0
0
14.09
0
0.1
0.2
0.25
Si
13.83
Fe
Si content, at. frac.
Fig. 8.22
The density of hcp-iron alloys with various compositions determined in this and previous works at
330 GPa and 300 K. The density was calculated based on the Pt pressure scale by Fei
et al
. (2007). The open circles
indicate the density values for Fe
0.93
Si
0.07
and Fe
0.83
Ni
0.09
Si
0.08
alloys, 13.49 g
/
cm
3
and 13.61 g
/
cm
3
respectively, as
determined by Asanuma
et al
. (2011); the solid square and a solid triangle indicate the density of pure iron and
Fe
0.8
Ni
0.9
, 14.09 g
/
cm
3
, 14.37 g
/
cm
3
are the density by Mao
et al
. (1990); a solid upside triangle, the density of
Fe
0.84
Si
0.16
alloy, 12.90 g
/
cm
3
by Hirao
et al
. (2004). The densities of these alloys are recalculated using the pressure
scale by Fei
et al
. (2007). The estimated inner core density at 300 K, 13.3-13.6 g
/
cm
3
(Diewonski & Anderson, 1989;
Stacey & Davis, 2004; see the text in detail) locates in the gray shaded area, assuming the Ni content in the inner
core, 4
5
.
4 wt%. The compositional range changes with the pressure scale. The gray lines are the density isocbors
(in g
/
cm
3
) based on the Pt pressure scale by Holmes (Holmes
et al
., 1989). The compositional range explaining the
inner core density by this scale is given as a light gray area. The compositional range estimated by Antonangeli
et al
. (2010) using the same pressure scale (Holmes
et al
., 1989) is shown as a dark gray area. (See Color Plate 6).
∼
the inner core is 4
5.4 wt%. The Ni content of
the core may be similar to that in the chondritic
composition. Allegre
et al
. (1995) estimated the
Ni content of the core to be 4.87 wt% based
on the chondritic Earth model. Whereas Mc-
Donough and Sun (1995) estimated the amount
of Ni in the core about 4.9
∼
by this work is consistent with the previous
geochemical model of the core, 7.35 wt% (Allegre
et al
., 1995).
Sound velocity is the most important among
observable properties. Thus, the experimental
and theoretical studies of iron alloys are per-
formed intensively. Although several theoretical
works using
ab initio
calculations provide
important insight into the Earth's inner core
(e.g., Tsuchiya
et al
., 2009; Vo cadlo
et al
.,
2009; Sha & Cohen, 2010a,2010b), experimental
works are still limited under the inner core
conditions. Sound velocities of iron alloys have
been studied by nuclear resonance inelastic
X-ray scattering spectroscopy (NRIXS) (e.g., Mao
et al
., 2001), inelastic X-ray scattering (IXS) (e.g.,
5.4 wt% based on
geochemical and cosmochemical models. If the
Ni-content of the inner core is in the range from
4 wt% to 5.4 wt%, the present analysis suggests
that the Si content of the inner core would be
in the range from 7.5 to 11 at % (from 3.9 to
5.8 wt%). We applied a model of the ideal mixing
for estimating the density of the Fe-NiSi alloys
in this calculation as was used by Badro
et al
.
(2007). The Si content of the inner core estimated
∼
