large uncertainty. Considering that the 10% core
density deficit is reconciled with 1.2 wt% H (18),
TCMB could be as low as 3100 K (Fig. 4).
Seismological analysis shows the pairs of
positive and negative velocity discontinuities in
the lowermost mantle, which have been attributed
to the double-crossing of the post-perovskite
phase transition boundary by the geotherm (4).
Earlier experimental work (7) demonstrated that
perovskite is formed above 3500 T 150 K at the
CMB pressure in a natural mantle material. Considering the upper bound of TCMB (3570 T 200 K)
found in this study, the stability of perovskite at
the base of the mantle is therefore marginal.
Alternatively, the low TCMB suggests that post-perovskite occurs not only in cold subducted
slabs but also in wider regions of the lowermost
The temperature extrapolated along an ad-
iabat from the transition zone is 2500 to 2800 K
at the CMB (5), indicating a temperature jump
of 300 to 1100 K across the TBL developed near
the base of the mantle. The thermal conductivity of
the lowermost mantle, a mixture of (Mg,Fe)SiO3
post-perovskite and (Mg,Fe)O ferropericlase, has
been estimated to be 7.4 to 11.0 W/m/K at 136 GPa
and 3600 K (19, 20). Recent theoretical predic-
tions of high thermal conductivity of the core sug-
gest the adiabatic heat flux of 14 to 20 TW at the
top of the core (21, 22), which should be com-
parable to the heat flux across the CMB unless
the topmost core is thermally stratified in a wide
depth range (22). In this case, the TBL thickness
is limited to less than 130 km (fig. S2). Such thin
TBL gives an alternative explanation for the seis-
mic velocity reduction observed ~100 km above
the CMB (23).
We also calculated the temperature at the
ICB (TICB) from TCMB and the isentropic gradient
across the outer core given by (dln T/dln r) = g,
where r is density and g is the Grüneisen parameter. With uniform g = 1.51 in the core (24) and
~3100 K < TCMB < 3570 T 200 K, we obtain
~4200 K < TICB < 4860 T 270 K (Fig. 4). The
core must be fully molten at both the CMB and
the ICB. Recent density and sound velocity measurements proposed Fe90O0.5S9.5 (in weight ratio)
as a liquid core composition (25). However, the
liquidus temperature of Fe90O0.5S9.5 should be
higher than that of Fe85O2S13 (3600 K at the
CMB and 5630 K at the ICB) (26) and thus exceeds the outer core temperatures estimated above.
On the other hand, geochemical models based on
Si isotope data suggest 6 wt Si in the core (27).
Nevertheless, even the eutectic (solidus) temperatures of Fe90.35Si7.35S2.3 and Fe80.07Si10.36S2.57
were reported to be 3750 and 3600 K at the
CMB, respectively (28), and their liquidus temperatures should therefore be higher than TCMB.
Recent DAC experiments revised the melting
temperature of pure Fe upward to be 4190 and
6230 K at CMB and ICB pressures, respectively
(3), which is consistent with both ab initio calculations and shock-wave experiments. The upper
bounds of TCMB and TICB found in this study
require that the liquidus temperature of the outer
core alloy is depressed by >600 K at 136 GPa
and >1400 K at 329 GPa from that of pure Fe
(Fig. 4). Such a large depression would be impossible without H in the core as discussed above
(2, 17). Given that the incorporation of 6 wt Si
contributes to diminishing the liquidus temperature by ~150 K (29) and the density by ~5% (30),
Fe93.4H0.6Si6 (in weight ratio) may be compatible
with the low TCMB as well as the 10% core density deficit, although a more precise estimate requires knowledge about liquid density and melting
behavior in the Fe-H-Si system. A large amount
of H may have been incorporated into metals
from a hydrous magma ocean at the time of core
References and Notes
1. R. Boehler, Annu. Rev. Earth Planet. Sci. 24, 15–40 (1996).
2. D. Alfè, M. J. Gillan, G. D. Price, Contemp. Phys. 48,
3. S. Anzellini, A. Dewaele, M. Mezouar, P. Loubeyre,
G. Morard, Science 340, 464–466 (2013).
4. J. W. Hernlund, C. Thomas, P. J. Tackley, Nature 434,
5. T. Lay, J. Hernlund, B. Buffett, Nat. Geosci. 1, 25–32 (2008).
6. A. Zerr, A. Diegeler, R. Boehler, Science 281, 243–246
7. G. Fiquet et al., Science 329, 1516–1518 (2010).
8. D. Andrault et al., Earth Planet. Sci. Lett. 304, 251–259
Fig. 3. Solidus curve of
a pyrolitic lower mantle. Solid and open circles
indicate the presence and
the absence of partial melt,
respectively, based on x-ray
tomography. The melting
curve is based on the
Simon and Glatzel equation. The experimental
temperature in this study
represents the maximum
value, whereas pressure
uncertainty is T10% (9).
Light green (7) and purple
(8) curves with squares
show the solidus temperature of primitive mantle
determined by previous
in situ measurements in
a DAC. The orange band
illustrates a possible range
of pyrolite solidus reported
by (6). The results of earlier
multi-anvil experiments are shown by crosses (above solidus, red; subsolidus, blue) (12, 13).
Fig. 4. Temperaturepro-file (geotherm) in the
lower mantle and the
outer core. Dark blue curve,
solidus of pyrolite (this
study); light green curves,
liquidus and solidus of pyrolite (7). Melting (liquidus)
temperatures of pure Fe
(3), Fe-O-S alloy (26), and
FeH (17) are shown by
black, pink, and blue lines,
respectively. The dashed
curves represent extrapolations of experimental data.