Geoscience Reference
In-Depth Information
and the extreme concentration of the large-ion,
magmaphile elements into the crust suggest
that differentiation has been extremely efficient . The
lack of a thick plagioclase-rich crust on Earth
has been used as an argument that early Earth,
in contrast to the early Moon, did not have a
magma ocean.
There is an easy solution to this apparent
paradox. At pressures corresponding to depths
of the order of 50 km, the low-density miner-
als of the crust convert to a mineral assemblage
denser than olivine and orthopyroxene. Most
oftheoriginalcrustthereforeisunstableand
sinks into the mantle. Any magma below 200--
400kmmaysufferthesamefate.Between50and
500 km the Al 2 O 3 --CaO--Na 2 O-rich materials crys-
tallize as clinopyroxene and garnet, a dense
eclogite assemblage that is denser than peri-
dotite. Eclogite transforms to a garnet solid solu-
tion, which is still denser and which is stable
between about 500 and 800 km, depending on
temperature. Peridotite also undergoes a series of
phase changes that prevent most eclogites from
sinking deeper than about 650 km. There is likely
to be a midmantle garnet-rich layer in the man-
tle. This would explain both the absence of a
thickcrust,andthepresenceofanolivine-rich
shallow mantle. Mars may also have a perched
garnet-rich layer. However, when the planets are
much smaller than Mars-size, eclogite could sink
to the core--mantle boundary.
Subsequent cooling and crystallization of the
Earth introduces additional complications. A
chemically stratified mantle cools more slowly
than a homogenous Earth. Phase change bound-
aries are both temperature and pressure depen-
dent, and these migrate as the Earth cools. The
initial crust of the Earth, or at least its deeper
portions, can become unstable and plunge into
the mantle. This is an effective way to cool the
mantle and to displace lighter and hotter mate-
rial to the shallow mantle where it can melt
by pressure release -- adiabatic decompres-
sion melting -- providing a continuous mech-
anism for bringing melts to the surface. This
mechanism can also, on the smaller planets, cool
the core and, perhaps, start a dynamo.
The separation of melts and crystals is a
process of differentiation. Convection is often
thought of as a homogenization process, tanta-
mount to stirring. Differentiation, however, can
be irreversible. Melts that are separated from the
mantle when the Earth was smaller, or from the
present upper mantle, crystallize to assemblages
that have different phase relations than the resid-
ual crystals or original mantle material. If these
rocks are returned to the mantle, they will not
in general have neutral buoyancy, nor are they
necessarily denser than ''normal” mantle at all
depths. Eclogite, for example, is denser than peri-
dotite when the latter is in the olivine,
β
-spinel
and
-spinel fields but is less dense than the
deeper mantle.
Removal of crystals from a crystallizing
magma ocean and drainage of melt from a cool-
ing crystal mush (also, technically a magma) are
very much faster processes than cooling and crys-
tallization times. Therefore, an expected result
of early planetary differentiation is a stratified
composition. Because of the combined effects of
temperature and pressure on physical properties,
shallow stratification may be reversible -- leading
to plate tectonics -- while deep dense layers may
be trapped at depth.
γ
Search WWH ::




Custom Search