Geoscience Reference
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ratios will be slightly different if it is assumed
that the upper mantle extends to 1000-km
depth, which is the classical depth to the top
of the lower mantle. An orthopyroxene-rich com-
position for the bulk of the mantle is expected
with a chondritic model for the major elements,
particularly if the upper mantle is olivine- and
basalt-rich. At low pressure olivine and orthopy-
roxene are refractory phases and are left behind
as basalt is removed. However, at high pressure
the orthopyroxene-rich phases, majorite and per-
ovskite, are both refractory and dense. If melting
during accretion extended to depths greater than
about 350-km, then the melts would be olivine-
rich and separation of olivine from orthopyrox-
ene can be expected.
Figure 13.3 shows the concentrations of the
lithophile elements in the various components,
all normalized to mantle equivalent concentra-
tions. The refractory elements in the upper man-
tle have normalized concentrations of about 3.
Since the upper mantle is about one-third of the
whole mantle, a strongly depleted lower man-
tle is implied. The upper mantle is not necessar-
ily homogenous. The basaltic fraction, as eclog-
ite, may be in a separate layer. Seismic data
are consistent with an eclogite-rich transition
region.
