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has negative buoyancy because is colder than
the surrounding mantle, will be subject to phase
transition from ringwoodite to perovskite and
magnesiowüstite at a depth greater than 670 km,
and only after this transformation its density will
increase by
9%(Fig.
1.14
). Therefore, in this
instance the anomalous lower mantle region will
be partially formed by less dense transition region
minerals, which will give a
positive
contribution
to the total buoyancy. If such a positive contri-
bution balances the negative thermal buoyancy,
then the body will stop sinking. Consequently, the
effect of the 670-km phase transition on buoy-
ancy can potentially prevent deep penetration
of subducting slabs and constitutes at least an
obstacle to whole mantle convection.
1.7
Lower Mantle
The 670-km discontinuity potentially represents
a
thermal barrier
separating the
lower mantle
from the outer shells of the Earth. In the previous
section, we have shown that this discontinuity
hinders the downward injection of cold material
coming from the upper mantle. Following a sim-
ilar reasoning, it is easy to show that it may also
impede the upwelling of hot (less dense) material
from the lower mantle. As a consequence, it is
conceivable that convective motions and large-
scale circulation within the upper mantle proceed
separately and independently from any potential
lower mantle convection, so that the interaction
between the two layers would occur essentially
through conductive transfer of heat across the
670-km discontinuity. In general, the possibility
of layered convection depends on whether or not
the magnitude of positive buoyancy associated
with the endothermic phase transition is greater
than the negative thermal buoyancy of a slab
reaching the base of the transition zone. There-
fore, the possibility of layered convection within
the mantle cannot be affirmed on the basis of
simple qualitative estimates. For example, Chris-
tensen and Yuen (
1985
) showed that the critical
value of the Clapeyron slope for triggering lay-
ered convection depends from the Rayleigh num-
ber,
Ra
, a dimensionless geodynamic parameter
Fig. 1.14
Buoyancy of negative thermal anomalies in
proximity of the mantle transition zone boundaries.
Black
arrows
indicate buoyancy associated with phase transi-
tions,
red arrows
indicate buoyancy arising from thermal
anomalies
discontinuity will be subject to phase transition
to wadsleyite at shallower depth, and its density
will increase by
6 % after transformation. The
region will acquire further negative buoyancy
with respect to the surrounding asthenosphere, in
addition to the originary negative buoyancy as-
sociated with its negative thermal anomaly. Con-
sequently, the region will accelerate sinking until
it is assimilated by the transition zone. Although
the effect of the exothermic release of latent heat
is to
increase
the temperature by
100 K, so
that it would oppose sinking, if this increment
of temperature also occurs in the surrounding
mantle then the contribution of the latent heat to
the forces balance is negligible.
Let us consider now a region characterized
by negative thermal anomaly, which has trav-
elled across the upper mantle and is now lo-
cated just below the 670-km discontinuity. In
this instance, the phase transition has negative ”
and is endothermic. The downgoing blob, which
