Geoscience Reference
In-Depth Information
atomic number than Fe in the core. There is evi-
dence to suggest that some, but not all, of this
density deficient is due to the presence of sulfur.
The S content of the core may be of the order of
∼
is usually treated as a chemically homogenous
layer but this is unlikely. Denser silicates, possibly
silicon- and iron-rich, also gravitate toward the
lower parts of the mantle. Crustal and shallow
mantle materials were sweated out of the Earth
as it accreted and some were apparently never
in equilibrium with core material. The effect of
pressure on physical properties implies that the
mantle and core probably irreversibly stratified
upon accretion and that only the outer shells of
the mantle participate in surface processes such
as volcanism and plate tectonics.
Geophysical data require rapid acc-
retion of Earth and early formation of
the core
. Until recently this has been at odds
with accretional theory and isotopic data but
now these disciplines are also favoring a con-
tracted time scale. A variety of isotopes have
recently confirmed short time intervals between
the formation of the solar system and plane-
tary differentiation processes. This has bearing
on the age of the inner core and its cooling
history.
There are three quite different mechanisms
for making a planetary core. In the homogenous
accretion hypothesis the silicates and the met-
als accrete together but as the Earth heats up
the heavy metals percolate downwards, eventu-
ally forming large dense accumulations that sink
rapidly toward the center, taking the siderophile
elements with them. In the heterogenous accre-
tion hypothesis the refractory condensates from
a cooling nebula, including iron and nickel, start
to form the nucleus of a planet before the bulk
of the silicates and volatiles are available. The
late veneer contributes low-temperature conden-
sates and gases, including water, from far reaches
of the solar system. Finally, large late impacts
can efficiently and rapidly inject their metallic
cores toward the center of the impacted planet,
and trigger additional separation of iron from
the mantle. The Moon is a byproduct of one of
these late impacts. The material in the core may
therefore have multiple origins and a complex
history. In addition to its age and growth rate,
other issues regarding the inner core involve den-
sity, temperature, texture and internal energy
sources.
2 weight%, based on meteorites and mantle
chemistry. H, C, O and Si, and very high temper-
ature, are additional candidates for explaining
core density.
Radioactive elements in the core?
Most of the large ion lithophile elements such
as K, U and Th are undoubtedly in the crust and
mantle, and were probably placed there during
accretion. Nevertheless, the presence of radioac-
tive elements in the Earth's core is often sug-
gested in order to power the geodynamo or to
explain where the volatile elements are, in the
Earth. The cosmochemical argument for K in the
core is based on the presence of potassium sulfide
in enstatite chondrites. Enstatite chondrites also
contain other more abundant sulfides, including
CaS, (Mn,Fe)S, and (Mg,Fe)S and substantial con-
centrations of REE.
Although one may wish
to place some K in the core, there are
associated consequences that exclude
this possibility. Likewise it is not
possible that U or Th or both are in the
core,
with our present understanding of crystal
chemistry and solubilities at high temperature
and pressure. The possibility of a nuclear reactor
in the core, however, has been proposed.
Core formation
Density stratification explains the locations and
relative mass of the crust, mantle and core. The
inner core is likely also the result of chemical
stratification although the effect of pressure on
the melting point would generate a solid inner
core even if it were chemically identical to the
outer core. Low-density materials are excluded
when solidification is slow so the inner core may
be purer and denser than the outer core. As the
inner core crystallizes and the outer core cools,
the material held in solution and suspension will
plate out, or settle, at the core--mantle bound-
ary and may be incorporated into the lowermost
mantle. Analogous processes contribute to sed-
imentation in deep ocean basins. The mantle
