Geoscience Reference
In-Depth Information
The situation for the core is far more difficult: no one has ever had a sample
of the core to analyse. The closest approach to sampling the core is to consider
the abundance of elements in the Sun and in meteorites. The Earth is believed to
have formed from an accretion of meteoritic material. Meteorites are classified
into two types:
stony
and
iron
(Sections 6.8 and 6.10). The stony meteorites are
similar to the mantle in composition, whereas the iron meteorites may be similar
to the core. If so, the core should be rich in iron with a small proportion of nickel
(
5%). The major problem with theories of core composition is that they depend
on theories of the origin of the Earth and its chemical and thermal evolution,
which are also poorly understood. Solar abundances of iron are slightly higher
than that in stony meteorites. If the solar model is taken, the lower mantle may
have as much as 15% FeO. The core is very iron-rich and may in bulk be roughly
Fe
2
Oincomposition.
More direct evidence for the composition of the core can be inferred from
its seismic velocity and density structure. Pressures appropriate for the core can
be attained in experiments using shock waves or diamond anvils. Thus, lab-
oratory measurements can be made on test samples at core pressures. When
corrections for temperature are made, such laboratory velocity measurements
can be compared with seismic models. Figure 8.12(a) shows the square root of
the seismic parameter
∼
φ
(Eq. (8.15)) plotted against density for a number of
metals. The ranges of values appropriate for the mantle and core, indicated by
the seismic-velocity and density models, indicate that, although magnesium and
aluminium are possible candidates for a major proportion of the mantle, such
low-atomic-number metals are quite inappropriate for the core. All the evidence
on the properties of iron at high temperatures and pressures points unequivocably
to a core that is predominantly composed of iron.
The outer core is probably an iron alloy: iron with a small percentage, 10% by
weight, of lighter elements. Amongst the favoured candidates for the minor alloy-
ing element(s) are nickel, oxygen, sulphur, hydrogen, silicon and carbon. Figure
8.12(b) is a plot of density against pressure (obtained from shock-wave experi-
ments) for pure iron (molten and solid) and possible iron compounds compared
with the
in situ
values for the core. This plot shows that the outer core cannot be
composed of either pure iron or the nickel-iron compound found in meteorites:
both of these materials are too dense. Each possible lighter alloying element has
its advantages and disadvantages, with sulphur and oxygen the strongest candi-
dates. Cosmochemical evidence suggests that a maximum of 7% sulphur may be
present in the core, but, since this amount of sulphur is insufficient to account for
the density of the outer core, there must be additional light element(s).
Iron is the presumed constituent of the inner core. The data for the inner
core indicate that it may well be virtually pure iron. There are several possible
crystalline forms for iron in the inner core: the body-centred cubic (b.c.c.) phase
and the face-centred cubic (f.c.c.) phase are probably unstable under inner-core
conditions, but the hexagonal close-packed (h.c.p.) phase should be stable. If
