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a large conflict between models that have been
constructed by specialists in these separate fields.
The mantle is heterogenous on all scales and
there are vast regions of the Earth that are
unsampled today because they were isolated by
gravitational
provides information about physical properties
and boundaries in the mantle.
Recycling of crust into the upper mantle is
an important process. It is possible to estimate
the composition of the
fertile upper man-
tle
by combining known components of the
upper mantle -- basalts, peridotites, recycled crust
and so on -- in such a way as to satisfy cosmic
ratios of the lithophile refractory elements. Other
elements are not necessarily concentrated into
the crust and upper mantle. The MORB source is
just part of the upper mantle and it is not the
onlyLILdepletedpartofthemantle.Itisnot
necessarily convectively homogenized.
Attempts to establish an average composi-
tion for the upper mantle have focused on
MORB because of the assumption the whole
upper mantle is the MORB-reservoir. This pro-
cedure involves major assumptions about melt
generation, melt transport and differentiation
processes that have affected these melts, and
the sources of non-MORB melts. The
depleted
upper mantle
, that part of the mantle that
is assumed to provide MORB by partial melt-
ing is variously called DUM, DM, DMM and the
convecting upper mantle
. Simplified mass
balance calculations suggested to early workers
that this depleted mantle constituted
stratification
during
accretion
of
the planet.
The composition of the crust and the upper
mantle are the results of a series of melting
and fractionation events, including the high-
temperature accretion of the planet. Attempts
to estimate upper-mantle chemistry usually start
from the assumption that it initially was the
same as
bulk silicate Earth
(BSE) and dif-
fers from it only by the extraction of the
crust, or that the most depleted midocean-ridge
basalts (MORB) plus their refractory residues
constitute the entire upper mantle. Tradition-
ally, geochemists have assumed that the lower
mantle is still undifferentiated BSE. On the other
hand, large-scale melting and differentiation
upon accretion probably pre-enriched the upper
mantle with incompatible elements, including
the radioactive elements; the crust and the vari-
ous enriched and depleted components sampled
by current melting events were probably already
in the upper mantle shortly after accretion and
solidification.
Midocean-ridge basalts represent large
degrees of melting of a large source volume, and
blending of magmas having different melting
histories. The central limit theorem explains
many of the differences between MORB and other
kinds of melts that sample smaller volumes of
the heterogenous mantle. Observed isotopic
arrays and mixing curves of basalts, including
ocean-island basalts (OIB), can be generated by
various stages of melting, mixing, melt extrac-
tion, depletion and enrichment and do not
require the involvement of unfractionated, prim-
itive or lower, mantle components. However, the
first stage in building an Earth -- the accretional
stage -- does involve large degrees of melting that
essentially imparted an unfractionated -- but
enriched -- chondritic REE pattern to the upper
mantle. Small-degree melts from this then serve
to fractionate LIL. Mass-balance and box-model
calculations
30% of
the mantle; the 650--670-km discontinuity was
adopted as the boundary between DUM and '
the
primitive undepleted undegassed lower
mantle
'. The starting condition for the upper
mantle (UM) was taken as identical to
primi-
tive mantle
(PM) and the present lower man-
tle (LM). There are many estimates of PM and
BSE based on various cosmological and petrologi-
cal considerations. The primitive upper mantle --
crust plus DUM -- is labeled PUM. It was further
assumed that the upper mantle was vigorously
convecting, well-stirred and chemically homoge-
nous, and extended from the base of the plate to
650-km depth. Thus, this part of the mantle was
also called the
convecting mantle
. The non-MORB
basalts that occur at the initiation of spreading
and at various locations along the global spread-
ing system were attributed to plumes from the
lower mantle.
Most or all of the mantle needs to be depleted
and degassed to form the crust and upper mantle
∼
can
go
just
so
far
in
constrain-
ing
the
chemistry
of
the
mantle.
Geophysics
