Geoscience Reference
In-Depth Information
Part III
Radial and lateral structure
Descend into the crater of Yocul of
Sneffels, Which the shade of Scartaris
caresses, before the kalends of July,
Audacious traveler, and you will reach
the center of the Earth. I did it.
Arne Saknussemm
of the mantle tend to be more complex. High-
resolution seismic techniques involving reflected
and converted phases show about 10 discontinu-
ities in the mantle, not all of which are easily
explained by solid-solid phase changes. They also
show some deep low-velocity zones that may be
eclogite layers.
It is increasingly clear that the upper
mantle is heterogenous in all parameters at all
scales. The parameters include seismic scatter-
ing potential, anisotropy, mineralogy, major and
trace element chemistry, isotopes, melting point
and temperature. An isothermal homogenous
upper mantle, however, has been the underly-
ing assumption in much of mantle geochemistry
for the past 35 years. Derived parameters such
as degree and depth of melting and the age and
history of mantle 'reservoirs' are based on these
assumptions. There is now evidence for major
element, mineralogical, trace element and iso-
topic heterogeneity, on various scales (grain size
to hemispheric) and for lateral variations in tem-
perature and melting point.
The large-scale features of the upper mantle
are well known from global tomographic studies.
The mantle above 200--300 km depth correlates
very well with known tectonic features. There
are large differences between continents and
oceans, and between cratons, tectonic regions,
back-arc basins and different age ocean basins.
Overview
The Australian seismologist Keith Bullen intro-
duced the nomenclature for the subdivisions of
the Earth's interior. Table 8.1 gives these subdi-
visions. The lower mantle, starting at 1000-km
depth, includes Regions D' and D”. The latter is
the only designation in common use today. Using
his nomenclature, the lithosphere and the low-
velocity zone are in Region B. The 650 km discon-
tinuity is in Region C -- the Transition Region --
rather than being the boundary between the
upper and lower mantles. The transition Region
extends from 410 to 1000 km depth. The upper
boundary is primarily a phase change and the
lower boundary may be a chemical change and a
geodynamic barrier.
Standard geochemical and geodynamic mod-
elsofthemantleinvolveoneortwolargevig-
orously convecting regions. Petrological models
