Geoscience Reference
In-Depth Information
process that blends the various enriched and
depleted products.
During accretion, melting is extensive and
there should be good separation between the
elements -- LIL -- that enter the magmas and
those that are retained by the residual solids.
The LIL include U, Th and K -- the heat-producing
elements -- the concentrations of which were
much higher during the first Gyr after Earth for-
mation. The relative buoyancy of magma, and
the self-heating tendency of potentially fertile
parts of the mantle combine to concentrate the
radioactive elements in the outer shells of the
Earth, including the crust and upper mantle. To
a good approximation the large-ion and large-
charge elements -- LIL and HFS elements -- can
be considered to reside in the fertile parts of
the mantle and the crust. This is not necessar-
ily true of non-LIL elements such as He and Os,
although these also are probably in the shallow
mantle.
components are those that have relatively low
melting points and which provide the main com-
ponents for basalts. The incompatible LIL ele-
ments enter the melt. The refractory components
have higher melting temperatures and are left
behind when partial melts are extracted. There
is no requirement that the fertile and refractory
components are intimately mixed or remixed in
the mantle or that the fertile components are
veins in a refractory matrix rather than km-
sized blobs. When small amounts of melting are
involved the separation tendencies are quantified
by partition coefficients, which give the parti-
tioning of a given element between the magma
and the residual solid. When melting is exten-
sive, most of the incompatible and basalt form-
ing elements in the source end up in the magma
or the fertile components.
DMORB
and
abyssal peridotites
are, respectively,
the 'blank' slates -- depleted basalt and refrac-
tory residue -- upon which various enriched sig-
natures are written. These are among the most
depleted of mantle materials and form the basis
for recent estimates of upper-mantle composition
(e.g. Donnelly
et al.
, 2004, Salters and Stracke,
2004, Workman and Hart, 2005). These are esti-
mates of the most depleted parts of the upper
mantle, not the upper mantle as a whole.
Kimberlites and continental crust are the ma-
in enriched complements to the above. EMORB
and OIB are intermediate in trace element prop-
erties -- but extreme in some isotopes. Eclogite
reservoirs/components are required to balance
such trace elements as Re, Zr, Ti and Na but
are also implied by recycling models and by
mass balance using chondritic constraints. Mass-
balance calculations (e.g. Part IV and
Chap-
ter 8 of Theory of the Earth
) imply that
the mantle contains less than about 10% of the
fertile -- basalt, gabbro, eclogite, garnet pyroxen-
ite -- component; this can be regarded as poten-
tial basalt for ridges, islands, CFB and LIPs; some
of this material may be recycled or delami-
nated crust/eclogite. This is about the amount
of oceanic crust recycled into the mantle over
billions of years at current subduction rates.
Delaminated lower continental crust also intro-
duces fertile material -- eclogite cumulates -- into
the upper mantle. The fertile components are
Composition of the upper mantle
An estimate of the original composition of the
mantle (see Part IV) can be derived simply by
mixing together all the products of mantle dif-
ferentiation. For example, the mantle produces
MORB, EMORB, OIB, kimberlite (KIMB), island
arc basalts and so on, and has also produced
the continental crust (CC). There is direct evi-
dence that the mantle contains a variety of
peridotites, pyroxenites and eclogites. One can
alternatively or in addition focus on the recy-
cled materials that are known to be entering the
mantle -- sediments, oceanic plates and delami-
nated lower continental crust. All of these can
be put into the mix. If one assumes that the mix-
ture must have chondritic ratios of the refrac-
tory elements and that the present mass of
thecrustisalowerboundontheCCcompo-
nent then one can estimate the mixing ratios
of the components and the original composition
of BSE, or at least that part that has provided
material to the surface. This can be posed as
a least-squares or geophysical inverse
problem
.
The components in the mantle can be alter-
natively subdivided into
fertile components
and
refractory
or
residual-solid
components. The fertile
