Geoscience Reference
In-Depth Information
and the
40
Ar in the atmosphere (e.g. the first
edition of
Theory of the Earth
, or TOE).
Depletion of the upper mantle alone cannot
explain the continental crust (CC); the MORB
reservoir and the CC are not exactly comple-
mentary. There must be other components and
processes beyond single-stage small-degree melt
removal from part of the primordial mantle to
form CC. There are other enriched components
in the mantle, probably in the shallow man-
tle. Other depleted components or reservoirs, in
addition to the MORB-source, are required by
mass-balance calculations. The upper mantle can-
not be treated as if its composition can be uni-
quely determined from the properties of depleted
MORB -- NMORB or DMORB -- and depleted peri-
dotites, continental crust, and an undifferential
starting condition.
Both the cosmochemical and petrological
approaches utilize terrestrial and meteoritic
data. The common theme is that the Earth
should have an unfractionated chondritic pattern
of the refractory elements. This can be used as a
formal a priori constraint in geochemical model-
ing of the composition of the Earth. This mass
balance approach is consistent with the idea
that most of the radioactive elements, and other
crustal elements, are in the crust and upper man-
tle. Some investigators, however, decouple their
models of the Earth from meteorite composi-
tions. Two extreme positions have been taken:
(1) the Earth is a unique body and is not related
to material currently in the solar system; upper
mantle rocks are representative of the whole
mantle and Earth-forming material is not to be
found in our meteorite collections; (2) only part
of the Earth has been sampled and the upper
mantle is not representative of the whole mantle.
There are several variants of the second option:
(1) the mantle is extensively differentiated and
the deepest layers are the dense residues of this
differentiation, which is irreversible; (2) only the
upper mantle has been processed and differ-
entiated; the deeper mantle is 'primordial'. There
are numerous petrological reasons why the
Earth's upper mantle should be distinct from any
known type of meteorite and why no part of the
mantle should have survived in a homogenous
primitive state. However, some argue that it is
more reasonable that the Earth accreted from
material with major-element compositions that
were distinct from primitive meteorite types than
to accept chemical stratification and petrological
differentiation of the mantle.
'Primitive mantle' or PM is the silicate frac-
tion of the Earth, prior to differentiation and
removal of the crust and any other parts of the
present mantle that are the result of differen-
tiation, or separation, processes. This is called
bulk silicate Earth (BSE). In geochemical models,
which were popular until very recently, it was
assumed that large parts of the Earth escaped
partialmelting,ormeltremoval,andarethere-
fore still 'primitive'. Some petrological models
assumed that melts being delivered to the Earth's
surface are samples from previously unprocessed
material. It is difficult to believe that any part
of the Earth could have escaped processing dur-
ing the high-temperature accretional process.
'Primitive mantle', as used here, is a hypothetical
material that is the sum of the present crust and
mantle. Some petrological models assume that
it is a mixture of the MORB-source and conti-
nental crust. 'Primitive magma' is a hypotheti-
cal magma, the parent of other magmas, which
formed by a single-stage melting process of a par-
ent rock and has not been affected by loss of
material (crystal fractionation) prior to sampling.
It is much more likely that magmas are the result
of a multi-stage process and that they represent
blends of a variety of melts from various depths
and lithologies.
Petrological building blocks
Most mantle magmas can be matched by mix-
tures of depleted MORB (DMORB) and enriched
components (Q). Midocean-ridge basalt (MORB)
represents the most uniform and voluminous
magma type and is often taken as an end-member
for large-ion lithophile (LIL) concentrations and
isotopic ratios in other basalts. The uniformity of
MORB, however, may be the result of sampling,
and the central limit theorem. In this case, MORB
is a good average, but not a good end-member.
The MORB source has been depleted by removal
of a component -- Q -- that must be rich in LIL
