Geoscience Reference
In-Depth Information
The isotopic homogeneity of NMORB has
strongly influenced thinking about the presumed
homogeneity of the upper mantle and the inter-
pretation of 'anomalous' sections of midocean
ridges. It is common practice to avoid 'anoma-
lous' sections of the ridge when compiling MORB
properties, and to attribute anomalies to 'plume--
ridge interactions.' In general, anomalies along
the ridge system -- elevation, chemistry, physi-
cal properties -- are part of a continuum and
the distinction between 'normal' and 'anoma-
lous'
Earth, composed of materials with different
intrinsic densities, will tend to stratify itself by
density. Plate-tectonic processes introduce het-
erogeneities into the mantle, some of which
can be mapped by geophysical techniques. On
the other hand, diffusion, chaotic advection
and vigorous unidirectional stirring, are homog-
enizers. Convection is thought by many geo-
chemists and modelers to homogenize the man-
tle. Free convection driven by buoyancy is not
the same as stirring by an outside agent. Melt-
ing of large volumes of the mantle, as at ridges,
however, can homogenize the basalts that are
erupted, even if they come from a heterogenous
mantle.
There are numerous opportunities for gener-
ating (and removing) heterogeneities associated
with plate tectonics (Figure 24.1). The tempera-
tures and melting temperatures of the mantle
depend on plate-tectonic history and processes
such as insulation and subduction cooling. Ther-
mal convection requires horizontal temperature
gradients; cooling from above and subduction of
plates can be the cause of these temperature gra-
dients. The mantle would convect even if it were
not heated from below. Radioactive heating from
within the mantle, secular cooling, density inho-
mogeneities and the surface thermal-boundary
layer can drive mantle convection. An additional
important element is the requirement that ridges
and trenches migrate with respect to the under-
lying mantle. Thus, mantle is fertilized, contam-
inated and extracted by migrating boundaries --
a more energy-efficient process than moving the
mantle to and away from stationary plate bound-
aries, or porous flow of magma over large dis-
tances. Lateral return flow of the asthenosphere,
and entrained mantle flow, are important ele-
ments in plate tectonics. Embedded in these flows
can be fertile patches. Even if they are confined to
the asthenosphere these patches will move more
slowly than plates and plate boundaries, giving
the illusion of fixed hotspots.
ridge
segments
is
arbitrary
and
model
dependent.
Mantle heterogeneity: toward
a new paradigm
It is increasingly clear that the upper mantle is
heterogenous in all parameters at all scales. The
evidence includes seismic scattering, anisotropy,
mineralogy, major- and trace-element chemistry,
isotopes, melting point and temperature. An
isothermal homogenous upper mantle, however,
has been the underlying assumption in much of
mantle geochemistry for the past 35 years.
One must distinguish
fertility
from (trace ele-
ment)
enrichment
, although these properties may
be related.
Fertility
implies a high basalt--eclogite
or plagioclase--garnet content.
Enrichment
implies
high contents of incompatible elements and long-
term high Rb/Sr, U/Pb, Nd/Sm etc. ratios. Because
of buoyancy considerations, the most refractory
products of mantle differentiation -- harzburgite
and lherzolite -- may collect at the top of the man-
tle and bias our estimates of mantle composition.
The volume fractions and the dimensions of the
fertile components -- basalt, eclogite, pyroxenite,
piclogite -- of the mantle are unknown. There
is no reason to suppose that the upper mantle
is equally fertile everywhere or that the fertile
patches or veins in hand specimens and outcrops
are representative of the scale of heterogeneity
in the mantle.
There are three kinds of heterogeneity of
interest to petrologists and seismologists, radial,
lateral and random, or statistical. Melting and
gravitational differentiation stratify the man-
tle. Given enough time, a petrologically diverse
Creation of mantle heterogeneity
Recycling contributes to chemical and iso-
topic heterogeneity of the source regions of
basalts but it also contributes to the fertility
and
productivity
of
the
mantle.
Temperature
