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is negatively buoyant (Trampert et al.
2004
). In
the upper part of the lower mantle, thermal and
chemical buoyancies are equally important, so
that the LLSVPs have neutral buoyancy. Finally,
Trampert and colleagues found scarce evidence
of correlation between the subducting slabs and
plumes hypothesized in whole-mantle convection
models and positively or negatively buoyant re-
gions of the lower mantle.
Curiously, some succeeding studies reinter-
preted the LLSVPs in terms of whole-mantle
convection. For example, Burke et al. (
2008
)yet
acknowledging that LLSVPs do not represent
regions of upwelling, proposed a geometrical cor-
relation between the locations of major hotspot
volcanoes and LIPs and narrow belts surrounding
the LLSVPs close to the core-mantle boundary
(CMB). To this purpose, the LIPs were moved
from their present day location to the position
that they occupied at the time of eruption with
respect to the Earth's rotation axis. These authors
claimed that hot spots and reconstructed LIPs
form two clusters lying vertically above the pe-
ripheries of the superplumes, which were thereby
viewed as long-lived “plume generation zones”
(PGZ) for at least the last
300 Myrs, while the
remaining of the lower mantle was considered to
be the “graveyard” of slabs.
In summary, the controversy between whole-
mantle convection models, supported by inter-
preted seismic tomography data, and alternative
theories of layered convection or, possibly, con-
vection limited to the upper mantle, is still alive,
and it is likely that a new quantitative approach
to the analysis of seismic tomography data will
be necessary to establish which model is correct.
Furthermore, more data from the lowermost man-
tle could also contribute to assess the hypoth-
esis that this region forms a thermal boundary
layer hosting hot spots. In fact, the 250-350 km
thick layer just above the CMB represents an
enigmatic region where complex transformations
occur. This region, which is known as the
D
”
layer
, presents an anomalously low gradient of
shear-wave velocity accompanied by seismic dis-
continuity and scattering. Furthermore, lateral
variations of the depth to the
S
-wave velocity
discontinuity suggests a phase transition of the
perovskite to a polymorph called
post
-
perovskite
.
Finally, the observation of isolated pockets of ul-
tralow seismic velocity may indicate the presence
of magma chambers just above the CMB (e.g.,
Garnero and McNamara
2008
).
Problems
1. Determine analytically the melt temperature
at surface for a wet peridotite having 0.05 %
water content;
2. Solve Eq. (
1.8
) assuming a linear decrease of
the coefficient of thermal expansion ' with the
depth
z
;
3. Determine the thickness of new oceanic crust
formed from a 100 km wide melting regime,
assuming that the degree of melting increases
linearly from nearly zero at
z
D
50 km to
24 % close to the Earth's surface and that the
fraction of retained melt is 0.1 %/km;
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