Page 44 - Global Tectonics

THE INTERIOR OF THE EARTH 31

of the transition zone. There is a further velocity discon- bined into the crust. The latter, so called “incompatible”
tinuity at a depth of 660 km, the base of the transition elements, include the heat producing elements K, Th,
zone. and U. It is clear from the composition of mid-ocean
Within the lower mantle velocities increase slowly ridge basalts (MORB), however, that the mantle from
with depth until the basal 200–300 km where gradients which they are derived by partial fusion is relatively
decrease and low velocities are present. This lower- depleted in these elements. So much so that, if the
most layer, at the core–mantle boundary, is known as whole mantle had this composition, it would only

Layer D″ (Section 12.8.4) (Knittle & Jeanloz, 1991). account for a small fraction of the heat flow at the
Seismic studies have detected strong lateral heteroge- Earth’s surface emanating from the mantle (Hofmann,
neities and the presence of thin (5–50 km thick) ultra- 1997). This, and other lines of geochemical evidence,
low velocity zones at the base of Layer D″ (Garnero have led geochemists to conclude that all or most of the
et al., 1998). lower mantle must be more enriched in incompatible
elements than the upper mantle and that it is typically
not involved in producing melts that reach the surface.
2.8.3 Mantle composition However, seismological evidence relating to the fate of
subducted oceanic lithosphere (Sections 9.4, 12.8.2) and
The fact that much of the oceanic crust is made up of the lateral heterogeneity of Layer D″ suggests mantle
material of a basaltic composition derived from the wide convection and hence mixing (Section 12.9).
upper mantle suggests that the upper mantle is com- Helffrich & Wood (2001) consider that the various lines
posed of either peridotite or eclogite (Harrison & of geochemical evidence can be reconciled with whole
Bonatti, 1981). The main difference between these two mantle convection if various small- and large-scale het-
rock types is that peridotite contains abundant olivine erogeneities in the lower mantle revealed by seismo-
and less than 15% garnet, whereas eclogite contains logical studies are remnants of subducted oceanic and
little or no olivine and at least 30% garnet. Both possess continental crust. They estimate that these remnants
a seismic velocity that corresponds to the observed make up about 16% and 0.3% respectively of the mantle
−1
upper mantle value of about 8 km s . volume.
Several lines of evidence now suggest very strongly Although estimates of bulk mantle composition vary
that the upper mantle is peridotitic. Beneath the ocean in detail, it is generally agreed that at least 90% of the
basins the P n velocity is frequently anisotropic, with mantle by mass can be represented in terms of the
velocities over 15% higher perpendicular to ocean oxides FeO, MgO, and SiO 2 , and a further 5–10% is
ridges. This can be explained by the preferred orienta- made up of CaO, Al 2 O 3 , and Na 2 O.
tion of olivine crystals, whose long [100] axes are
believed to lie in this direction. None of the common
minerals of eclogite exhibit the necessary crystal elon- 2.8.4 The mantle
gation. A peridotitic composition is also indicated by
estimates of Poisson’s ratio from P and S velocities, and low velocity zone
the presence of peridotites in the basal sections of
ophiolite sequences and as nodules in alkali basalts. The low velocity zone (Fig. 2.16) is characterized by
The density of eclogites is also too high to explain the low seismic velocities, high seismic attenuation, and a
Moho topography of isostatically compensated crustal high electrical conductivity. The seismic effects are
structures. more pronounced for S waves than for P waves. The
The bulk composition of the mantle can be esti- low seismic velocities could arise from a number of
mated in several ways: by using the compositions of different mechanisms, including an anomalously high
various ultramafic rock types, from geochemical com- temperature, a phase change, a compositional change,

putations, from various meteorite mixtures, and by the presence of open cracks or fissures, and partial

using data from experimental studies. It is necessary to melting. All but the latter appear to be unlikely, and it
distinguish between undepleted mantle and depleted is generally accepted that the lower seismic velocities
mantle which has undergone partial melting so that arise because of the presence of molten material. That
many of the elements which do not easily substitute melting is likely to occur in this region is supported
within mantle minerals have been removed and com- by the fact that it is at this level that mantle material