The bowels of the Earth - New Theory of the Earth - page 116

Chapter 10

The bowels of the Earth

The lower mantle

not be at fixed depths. These clarifications are
needed because of the controversy about whether
slabs penetrate into the lower mantle or whether
they just push down a discontinuity, and where
the boundary of the lower mantle really is.
The midmantle ,or mesosphere , extends from
1000 to 2000 km. This is the blandest part of
the mantle. Estimates of its composition range
from pure MgSiO 3 perovskite to chondritic -- in
major refractory elements -- to pyrolite, within
the uncertainties. It can be Si- and Fe-rich com-
pared with the upper mantle, or identical to
the upper mantle. Geochemical models assume
that it is undegassed and unfractionated cosmic
material, but there is no support for this con-
jecture. The lower mantle must be quite differ-
ent from that which appears in the standard
model of mantle geochemistry ,andthe
geodynamic models that are based on it.

I must be getting somewhere near the
centre of the earth. Let me see: that
would be four thousand miles down.
I think-

Alice

The traditional lower mantle starts near 800--
1000 km where the radial gradient of the seismic
velocities becomes small and smooth. This is
Bullen's Region D. The 1000-km depth region
appears to be a fundamental geodynamic inter-
face, perhaps a major-element chemical and a
viscosity interface. Some authors take the lower
mantle to start just below the major mantle dis-
continuity near 650 km. The depth of this dis-
continuity varies, perhaps by as much as 40 km
and is variously referred to as the '660 km dis-
continuity' or '670 km discontinuity'; the average
depth is 650 km. In detailed Earth models there
isaregionofhighvelocitygradientforanother
50--100 km below the discontinuity. This is prob-
ably due to phase changes, but it could represent
a chemical gradient. Plate reconstructions show
that past subduction zones correlate with high-
velocity regions of the mantle near 800--1000 km
depth. The 'lower mantle proper' therefore does
not start until a depth well below the 650 km
boundary, more in agreement with the classical
definition. Below this depth the lower mantle is
relatively homogenous until about 300 km above
the core-mantle boundary. If there are chemical
discontinuities in the mantle the boundaries will

Composition of the lower mantle

The probable mineralogy of the deep mantle is
known from high-pressure mineral physics --
squeezing -- experiments. The mineralogy is sim-
ple in comparison to that of the crust and
upper mantle, consisting of (Mg,Fe)SiO 3 --Al 2 O 3
orthorhombic perovskite , CaSiO 3 cubic perovskite ,
and (Mg,Fe)O magnesiowustite. No Al-rich phases,
except for perovskites , are considered to exist in
the main part of lower mantle. The total basaltic
content of the mantle is only about 6% and most
of this is probably in the upper mantle and tran-
sition region. If most of the crustal and basaltic