Geoscience Reference
In-Depth Information
higher than their starting temperatures, but
lower than normal mantle temperatures. They
heat up by conduction from the surrounding
mantle (Figure 5.2); they are fertile blobs in the
mantle, in spite of being cold.
Trenches and continents move about on the
Earth's surface, refertilizing the underlying man-
tle. Ridges also migrate across the surface, sam-
pling and entraining whatever is in the mantle
beneath them. Sometimes a migrating ridge will
override a fertile spot in the asthenosphere; a
melting anomaly ensues, an interval of increased
magmatic output. Ridges and trenches also die
and reform elsewhere. What is described is a
form of convection, but it is quite different from
the kind of convection that is usually treated by
geodynamicists. It is more akin to fertilizing and
mowing a lawn. The mantle is not just convect-
ing from a trench to a ridge; the trenches and
ridges are visiting the various regions of the man-
tle. The fertile dense blobs sink, more-or-less ver-
tically into the mantle and come to rest where
they are neutrally buoyant, where they warm up
and eventually melt (Figures 5.3 and 5.4). They
will represent more-or-less fixed points relative to
the overlying plates and plate boundaries. They
are chemically distinct from 'normal' mantle.
Eclogites are not a uniform rock type; they come
in a large variety of flavors and densities and end
up at various depths in the mantle.
ROOT FORMATION
1
ridge
DELAMINATION
2
SPREADING
3
heating
UPWELLING
4
SPREADING
5
Fig. 5.1
The delamination cycle.
return to the surface as melting anomalies, often
at ridges. Large melting anomalies that form on
or near ridges and triple junctions may be due to
the resurfacing of fertile blobs, including delam-
inated continental crust. This is expected to be
especially prevalent around the 'passive' margins
of former supercontinents: Bouvet, Kerguelen,
Broken Ridge, Crozet, Mozambique ridge, Marion,
Bermuda, Jan Mayen, Rio Grande rise and Walvis
ridge may be examples; the isotopic signatures
of these plateaus are expected to reflect lower
crustal components.
Mantle stratigraphy
The densities and shear-velocities of crustal and
mantle minerals and rocks are arranged in order
of increasing density (Figure 5.2). Given enough
time, this is the stratification toward which the
mantle will evolve -- the neutral-density profile of
the mantle. Such density stratification is already
evident in the Earth as a whole, and in the crust
and continental mantle. This stratification, at
least of the upper mantle, is temporary, however,
even if it is achieved. Cold eclogite is below the
melting point but eclogite melts at much lower
temperatures than the surrounding peridotite. As
eclogite warms up, by conduction of heat from
the surrounding mantle, it will melt and become
buoyant,
The eclogite cycle
Cold oceanic crust and warm lower continen-
tal crust are continuously introduced into the
mantle by plate tectonic and delamination pro-
cesses.
These
materials
melt
at
temperatures
creating
a
form
of
yo-yo
tectonics.
