Geoscience Reference
In-Depth Information
that the shallow mantle, and the source region
for various basalts, including OIB and NMORB,
are the same and is a peridotite. Since the LIL-
depleted components in the mantle have already
lost a melt fraction, they should be depleted in
garnet -- and therefore infertile -- unless they are
eclogite cumulates, or delaminated lower conti-
nental crust. Peridotites depleted in basalt have
less Al
2
O
3
and garnet than nondepleted or fer-
tile peridotites. But the depletion event may have
involved a very small melt fraction, in which case
the incompatible trace elements will be affected
more than the major elements.
The
regions of the upper mantle. This averaging effect
of geophysical and geochemical data can distort
views regarding homogeneity of the mantle. But,
there is no doubt that peridotites can and do
come from the shallow mantle; some regions
of the upper mantle and lithosphere are proba-
bly mostly peridotite. Even if basalts derive from
eclogitic regions of the mantle they traverse and
evolve in peridotitic surroundings.
The average composition of the Earth is proba-
bly close to ordinary chondrites or enstatite mete-
orites in major-element chemistry, the mantle
therefore contains abundant, although not neces-
sarily predominant, olivine. By the same reason-
ing the mantle contains even more pyroxene plus
garnet. The above arguments do not prove that
the source region of the most abundant basalt
types, is garnet peridotite or that the regions of
the mantle that appear to be peridotitic, on the
basis of seismic velocities, are the regions where
midocean basalts are generated. Although some
of the older ideas about source regions, such as
a glassy or basaltic shallow source, can be ruled
out, the possibility that basalts involve eclogite,
pyroxenite, recycled crust or cumulates, cannot be
ruled out. Eclogite, garnet pyroxenite, peridotite--
eclogite mixtures, or piclogite, are also candi-
date source 'rocks'; the 'grains' in such 'rocks'
can be tens of km in extent. Hand-specimen-sized
rocks are a different scale from what volcanoes
and seismic waves see.
The trace-element inhomogeneity of the man-
tle plus the long-term isolation of the vari-
ous components suggests that differentiation has
been more effective in the long run than mixing.
Mixing can be avoided in a chemically inhomoge-
nous -- or chemically stratified -- mantle if the
components are large chunks and/or have large
intrinsic density and viscosity contrasts. Garnet
has the highest density of any abundant upper-
mantle mineral and therefore plays a role in
determining the density of various components,
and regions, of the mantle. However, the chemi-
cal heterogeneity may also be dispersed through-
out the upper mantle. Eclogites come in a variety
of compositions and densities; they all have low
melting points compared to peridotites. Some
eclogites have densities similar to some upper
mantle peridotites; the density of eclogite is very
traditional
emphasis
on
homogenous
olivine-rich
and
peridotite
source
regions
for
mantle
magmas
is
based
on
the
following
arguments.
(1) Peridotite is consistent with seismic velocities
for the shallow mantle; basalts come from the
mantle; the upper mantle is therefore mainly
peridotite.
(2) Garnet peridotite is stable in the upper man-
tle.
(3) Garnet peridotites have close compositional
relationships to meteorites.
(4) Partial melts of natural samples of garnet
peridotite at high pressure have basaltic com-
positions.
(5) Eclogites are the high-pressure chemical
equivalent of basalt and partial melting of
eclogite does not recreate the composition of
basalt.
(6) Melting of eclogites would have to be very
extensive, and melt--crystal segregation would
occur before such extensive melting can be
achieved.
These arguments are all suggestive rather than
definitive. They do not rule out other lithologies
for the upper mantle or for the source regions
of at least some basalts. There is increasing evi-
dence that large parts of the upper mantle are
eclogitic or composed of garnet pyroxenite and
are therefore more fertile than most peridotites.
Most of the seismic information about the upper
mantle is derived from seismic waves with wave-
lengths from 20 to 300 km. Much of the petrologi-
cal information comes from midocean ridges and
large volcanoes, which sample comparable size
