Geoscience Reference
In-Depth Information
fertile and radioactive layers, even after the onset
of plate tectonics.
The RAZOR process sets the initial stage for
mantle evolution, including the distribution of
radioactive elements. This step is often over-
looked in geochemical and geodynamic models;
it is usually assumed that most of the radioactive
elements are still in the deep mantle. The initial
temperatures may have been forgotten but the
stratification of major and radioactive elements
may be permanent.
CFB
OIB
MORB
C.C.
O.C.
BASALT
MELT
FRACTION
PLUME
SOURCE
PERIDOTITE
PRIMITIVE
MANTLE
LIL
MORBS
RESIDUE
ECLOGITE
DEPLETION-
ENRICHMENT
EVENTS
PRIMARY
DIFFERENTIATION
DEPLETED
LOWER
MANTLE
COOLING AND
GRAVITATIONAL
STRATIFICATION
1
2
3
4
5
Fig. 1.4
A model for the early evolution of the mantle.
Primitive mantle (1) is partially molten either during accretion
or by subsequent whole-mantle convection, which brings the
entire mantle across the solidus at shallow depths. Large-ion
lithophile (LIL) elements are concentrated in the melt. The
deep magma ocean (2) fractionates into a thin plagioclase-rich
surface layer and deeper olivine-rich and garnet-rich cumulate
layers (3). Late-stage melts in the eclogite-rich cumulate are
removed (4) to form the continental crust (C.C.), enrich the
shallow peridotite layer and deplete MORBs, the source
region of oceanic crust (O.C.) and lower oceanic lithosphere.
Partial melting of PLUME -- or Primary Layer of Upper Mantle
Enrichment -- in the upper mantle (5) generates continental
flood basalts (CFB), ocean-island basalts (IOB) and other
enriched magmas, leaving a depleted residue (harzburgite)
layer -- perisphere -- that stays in the upper mantle because of
its buoyancy. Enriched or hot-spot magmas (EMORB, OIB,
CFB) may be from a shallow part of the mantle and may
represent delaminated C.C. Most of the mantle has been
processed through the melting zone and is depleted in the
heat-producing elements such as U and Th, which are now in
the crust and upper mantle.
Evolution of a planet
Isotopic studies indicate that distinct geochem-
ical components formed in the mantle early in
its history.
Zone refining during accretion
and crys-
tallization of a deep magma ocean are possible
ways of establishing a chemically zoned planet
(Figure 1.4). At low pressures basaltic melts are
less dense than the residual refractory crystals,
and they rise to the surface, taking with them
many of the trace elements. The refractory crys-
tals themselves are also less dense than undiffer-
entiated mantle and tend to concentrate in the
shallow mantle.
As the Earth accretes and grows, the crustal
elements are continuously concentrated into the
melts and rise to the surface. When these melts
freeze, they form the crustal minerals that are
rich in silicon, calcium, aluminum, potassium
and the large-ion lithophile (LIL) elements. Melts
generally are also rich in FeO compared to prim-
itive material. This plus the high compressibil-
ity of melts means that the densities of melts
and residual crystals converge, or even cross, as
the pressure increases. They cross again as phase
changes increase the density of the solids. Melt
separation is therefore difficult at depth, and
melts may even drain downward at very high
pressure, until the silicate matrix undergoes a
phase change. During accretion the majority of
the melt-crystal separation occurs at low pres-
sure. All of the material in the deep interior
has passed through this low-pressure melting
stage in a sort of continuous zone refining. The
magnesium-rich minerals, Mg
2
SiO
4
and MgSiO
3
,
have
through the melting zone into the interior. Even
if the accreting material is completely melted
during assembly of the Earth, these minerals will
be the first to freeze, and they will still separate
from the remaining melt. The downward separa-
tion of iron-rich melts, along with nickel, cobalt,
sulfur and the trace siderophile elements, strips
these elements out of the crust and mantle.
The aluminum, calcium, titanium and
sodium contents in chondritic and solar mate-
rial, restrict the amount of basalt that can be
formed, but are adequate to form a crust some
200 km thick. The absence of such a massive
crust on the Earth might suggest that the Earth
has not experienced a very efficient differenti-
ation. On the other hand, the size of the core
high
melting
temperatures
and
are
fed
