Geoscience Reference
In-Depth Information
mantle from cooling and crystallizing as rapidly
as a homogenous fluid, radiating to outer space.
On a large body, such as the Earth, the
dense mineral
garnet
forms and sinks into
the interior; on a small body
plagioclase
forms and rises to the surface. In both cases
an aluminum-poor, residual mantle forms, com-
posed of olivine and pyroxene. The chemical strat-
ification that forms during accretion and magma
ocean crystallization may be permanent features
of the planet. The importance of these processes
during the earliest history of the planet cannot
be over-emphasized. No part of the interior is
likely to have escaped extensive heating, melting
and degassing. What happened at high tempera-
tureandrelativelylowpressureisunlikelytobe
reversed.
elements -- such as Os, Ir -- are in the core. Given
these circumstances, it is probable that the man-
tle is also zoned by chemistry and density. Large-
degree melts from primitive mantle can have rel-
atively unfractionated ratios of such elements
as Sm, Nd, Lu and Hf, giving 'chondritic' iso-
tope ratios. This has confused the issue regarding
the possible presence of
primordial unfractionated
reservoirs
.
The assumed starting composition for the
Earth is usually based on
cosmic
or
meteoritic
abundances
. The refractory parts of carbonaceous,
ordinary or enstatite chondrites are the usual
choices. These compositions predict that the
lower mantle has more silicon than the olivine-
rich buoyant shallow mantle and that only a
small fraction of the mantle, or even the upper
mantle, can be basaltic. The volatile components
that are still in the Earth were most likely added
to Earth as a late veneer after most of the mass
had
The 'initial' state of a planet
already
been
added
and
the
planet
had
Partial differential equations require boundary
conditions and initial conditions; so do geody-
namic and evolutionary models. The present sur-
face boundary condition of the Earth is a contin-
uously evolving system of oceanic and continen-
tal plates. The initial condition usually adopted
employs one edge of Occam's razor;
the mantle
started out cold and homogenous and remains homoge-
nous today
. The more probable initial condition
is based on the other edge of Occam's razor.
Although a homogenous mantle with constant
properties is the simplest imaginable assumption
about the
outcome
, it is not consistent with a sim-
ple
process
. No one has simply explained how the
mantle may have arrived at such a state, except
by slow, cold, homogenous accretion. This is an
unstated assumption in the
standard models
of mantle geochemistry
. The accretion of
Earth was more likely to have been a violent
high temperature process that involved repeated
melting and vaporization and the probable end
result was a hot, gravitationally differentiated
body.
That the Earth itself is efficiently differenti-
ated there can be no doubt. Most crustal elements
are in the crust, possibly all the
40
Ar -- depend-
ing on the uncertain potassium content -- is
in the atmosphere and most of the siderophile
cooled
to
the
point
where
it
could
retain
volatiles.
A process of
RAdial ZOne Refining
(RAZOR) during accretion
may remove
incompatible and volatile elements and cause
purified dense materials to sink. Crystallizing
magma oceans at the surface are part of this pro-
cess. The formation of a deep reservoir by
per-
ovskite fractionation
in a magma ocean
is not necessary. The magma ocean may always
have been shallower than the perovskite-phase
boundary -- roughly 650 km depth -- but as the
Earth accretes, the deeper layers will convert to
high-pressure phases. There is no need for mate-
rial in the upper mantle to have been in equi-
librium with the dense phases that now exist at
depth.
Prior to the era of plate tectonics, the Earth
was probably surfaced with thick crustal layers,
which only later became dense enough to sink
into the mantle. But because of the large stabil-
ity field of garnet, there is a
subduction bar-
rier
, currently near 600 km. The great buoy-
ancy of young and thick oceanic crust, partic-
ularly oceanic plateaus, dehydration of recycled
material, the low melting temperature of eclog-
ite, and the subduction barrier to eclogite (and
harzburgite) probably prevents formation of deep
