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significant melting, giving rise at times to an extensively
molten outer layer: a '
magma ocean
'. Isotopic evidence
from differentiated meteorites (Wood
et al.
, 2006)
suggests that even small precursor planetary bodies
segregated metallic cores within 5 Ma of formation,
and so impactors accreting to the Earth would already
have formed metallic cores. This suggests that core
segregation within the Earth took place continuously,
hand in hand with planetesimal accretion.
The gravitational accumulation of dense metal into
the centre of the molten Earth has two important
implications: large amounts of gravitational potential
energy would have been released, sustaining a par-
tially molten state of the overlying mantle; and the
siderophile elements (Figure 11.4) would have been
efficiently scavenged from the mantle into the core.
The giant impact event proposed for creating the
Moon, around 4522 Ma ago, would have caused a later
phase of extensive (possibly complete) melting of the
Earth and an additional contribution (from the impac-
tor) to the Earth's core.
Many physical properties of the present molten
outer core are consistent with a major element comp-
osition similar to the Fe-Ni alloy found in iron mete-
orites, although the velocity of compressional seismic
waves through the core indicates a lower density than
expected for Fe-Ni under the appropriate load pres-
sure. It follows that a significant proportion (~8%) of
some less dense element(s) must also be present in the
core. Recent research (see Wood
et al
., 2006) points to
a combination of Si, S and O making up this light
component of the core.
Continental crust
Mantle
Oceanic
crust
Lower
Upper
Na +K
Al
Al
Al
Si
Si
Fe
Si
Si
Fe
Mg
Fe
Ca
Ca
Mg
Figure 11.7
Average compositions (oxide percentages) of
the Earth's mantle and crust.
(b) Crystals tend to dump into the melt certain trace
elements whose ions are difficult to accommodate.
The ions of these
incompatible elements
(Box 9.1)
are more easily accommodated in the open, disor-
dered structure of a melt than in a crystal lattice.
The extraction of magma from the mantle to form
crust has progressively displaced these elements from
mantle to crust over geological time. Crustal rocks (e.g.
basalt) consist of lower-melting mineral assemblages
(Figure 11.7) and are enriched in incompatible elem-
ents compared with mantle
peridotite
.
Parts of the mantle have therefore become depleted
in these elements. It has been estimated that 20-25% of
the mantle's original inventory of highly incompatible
elements (K, Rb, U) now resides in the continental
crust. Supposing these elements were uniformly dis-
persed in the primordial mantle, it seems that at least a
third of its volume has been tapped for igneous magma
during the course of geological time. Whether the
mantle was ever homogenous is debatable (although
it seems likely if at one time the mantle was com-
pletely molten), but its present inhomogeneity is
beyond doubt. Figure 10.8 showed how the varying
geochemistry of recent volcanic rocks points to a range
of chemically distinct source regions ('reservoirs') in
the mantle. Most mid-ocean ridge basalts ('MORBs'),
for example, come from a global mantle reservoir
depleted in the incompatible elements (so having an
isotopic composition falling in the 'depleted quadrant'
in Figure 10.8) - an inheritance, most geochemists
believe, of widespread melt extraction during the
course of Earth history.
The mantle
The silicate material surrounding the core - constitut-
ing 70% of the Earth's mass (Figure 11.6) - has, over
geological time, differentiated into the present-day
mantle and crust as a result of igneous activity through-
out the Earth's history. When a partial melt (Box 2.4)
develops in equilibrium with solid rock, elements are
fractionated in two overlapping ways.
(a) The lower-melting major components of the rock
(Fe, Al, Na, Si) enter the melt preferentially, leaving
the residual solids enriched in refractory (Mg-rich)
end-members (Box 2.4).
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