Geoscience Reference
In-Depth Information
reviews of the rhenium--osmium and oxygen iso-
tope systems, see Shirey and Walker (1998), Eiler
(2001).
A large number of other components have
been suggested: DUPAL, FOZO, C (Common),
PHEM, LONU and so on. Most of these have
been attributed to plumes, or the lower man-
tle, or the core--mantle boundary, but with little
justification. Isotopes cannot constrain the loca-
tions, depths, protoliths or lithologies of the
sources of these isotopic signatures. The compo-
nents have been attributed to separate reservoirs,
such as DMM (depleted MORB mantle), DUM
(depleted upper mantle), EM (enriched mantle),
PM (primitive mantle), PREMA (prevalent mantle)
and so on. These hypothetical reservoirs have
been equated with mantle subdivisions adopted
by seismologists, e.g. crust, upper mantle, tran-
sition region, lower mantle and D
. The compo-
nents, however, may be distributed throughout
the upper mantle.
In addition to MORB, there are also a vari-
ety of depleted magmas including picrites and
komatiites at
hotspot islands
(Hawaii, Iceland,
Gorgona) and at the base of continental flood
basalts. Many so-called hotspot basalts have very
low
3
He concentrations and low
3
He/
4
He isotope
ratios. The crust and shallow mantle account for
most of the incompatible elements in the Earth,
such as U, Th, K, Ba, Rb, Sr and isotopes that
are used to finger-print so-called plume influence.
The origins and locations of these components
are actively debated. It is conventional, in
iso-
tope geochemistry treatises
, to assume
that the upper mantle is the source of depleted
MORB, and only of depleted MORB. Thus, any-
thing other than depleted MORB must come from
the deep mantle. The reasoning is as follows:
MORB is the most abundant magma type; it
erupts passively at ridges; the MORB-source must
therefore be shallow; since MORB is derived from
the upper mantle, nothing else can be. MORB,
at one time, was thought to be a common com-
ponent in magmas from the mantle; the most
likely location for such a common component is
the shallowest mantle. Early mass balance calcu-
lations suggested that about 30% of the mantle
was depleted by melt extraction. If this was due
to removal of the continental crust, then a vol-
ume equivalent to the mantle above about 650-
km depth is depleted.
These arguments are not valid. Based on more
complete mass-balance calculations in
Theory
of the Earth
, I showed that most of the mantle
must be depleted and much of it must be infertile
[www.resolver.caltech.edu/CaltechBOOK:1989.001].
Most of the mantle is not basaltic or fer-
tile (there is not enough Ca, Al, Na and so on).
About 70--90% of the mantle must be depleted
to explain the concentrations of some elements
in the continental crust alone and the amount
of
40
Ar in the atmosphere. Enriched basalts are
the first to emerge at a new ridge, or upon con-
tinental break-up, and enriched MORB (EMORB)
occurs along spreading ridges. Enriched islands
and seamounts occur on and near ridges. A
chemically and isotopically heterogenous shallow
mantle is indicated. In the early isotope mod-
els of mantle geochemistry and crustal growth,
there was no recycling; crustal growth and upper
mantle depletion were one-way and continuous
processes. The upper mantle was assumed to be
vigorously convecting and chemically homoge-
nous. The plume hypothesis emerged naturally
from these assumptions.
Plate tectonics continuously subducts sedi-
ments, oceanic crust and lithosphere and recy-
cles them back into the mantle. The basalts and
peridotites are generally altered; subduction zone
processes modify this material. Upper continen-
tal crust enters subduction zones at trenches,
and lower continental crust enters the mantle
by delamination and subcrustal erosion. Some of
the melts and gases in the mantle are trapped
and never make it to the surface. These are some
of the components that one might expect to
find in the mantle. They have distinctive trace
element and isotope signatures.
The following are some of the isotopically dis-
tinctive components that have been defined in
oceanic and continental magmas and which may
correspond to some of the above materials man-
tle mixing [see
HIMU EM1 EM2 DMM
for examples
of mixing trends].
HIMU may represent old recycled hydrother-
mally altered oceanic crust, dehydrated and
converted to eclogite during subduction. It is
depleted in Pb and K and enriched in U, Nb and
