Geoscience Reference
In-Depth Information
calculated, by use of partition coefficients for
each element. This results in LIL-fractionated and
enriched melts and strongly depleted residues.
The more likely situation is that melt is contin-
uously extracted as melting proceeds, and some
melt is left behind. The extracted melt interacts
with the matrix it is percolating through. Incom-
plete melt extraction on the one hand, and trap-
ping of small-degree melts on the other, result
in source rocks that are enriched compared to
the theoretical residues of complete melt extrac-
tion calculations. The small-degree melts act as
enriching or metasomatizing agents for both
magmas and mantle xenoliths and are essential
components when attempting to do mass-balance
calculations. Theoretical attempts to reconstruct
the composition of the primordial upper mantle
from the compositions of MORB, peridotite and
continental crust (CC) rely heavily on data selec-
tion, melting models and partition coefficients.
Attempts are made to determine the average com-
positions of the MORB and peridotite endmem-
bers, and to make corrections for
contamination
.
An alternative method (Chapter 13) just mixes
together materials thought to represent a spec-
trum of products of mantle melting and metaso-
matism.
enrichment include fractionation during melt-
ing or metasomatic events.
Enriched MORB may
occur without any clear relationship
to plumes
.
Many estimates of the composition of the
upper mantle equate the NMORB source with
the whole upper mantle. The use of depleted
MORB and ultra-depleted residues in the esti-
mates of DUM or DMM gives a lower bound on
the LIL content of the upper mantle. Use of BSE
as the starting composition for the upper man-
tle and then removing CC from it leaves a very
depleted composition for the subsequent gener-
ation of magmas. Calculations in
Theory of
the Earth
, abbreviated in Chapter 13 of the
present volume, suggest that accretional differen-
tiation pre-enriched the upper mantle by a factor
of about 3 compared with primitive abundances,
and the CC, and the depleted and enriched com-
ponents evolved from that starting condition.
The whole mantle was processed (mined) dur-
ing accretion and the continental crust repre-
sents only part of the enriched material that was
in the starting UM. The NMORB 'reservoir' was
depleted by removal of small-degree melts, most
of which remained in the shallow mantle and
some entered the crust. EMORB and OIB are prod-
ucts of those parts of the mantle that have been
enriched by these small-degree melts, as well as
those parts of the mantle affected by recycling
and delamination of the lower continental crust.
Mantle peridotites, in general, have a wide
range of compositions and can be viewed as
mixtures of the most depleted ones and an
enriched fluid such as kimberlite. Continental
peridotites tend to have 1--2 orders of magnitude
higher LIL concentrations than
theoretical
abyssal peridotites
from melt extraction
calculations. Infertile refractory peridotites can
be enriched in trace elements by a low-degree
melt, or other metasomatic fluids, without nec-
essarily being very fertile or a suitable source
rock for basalts. Other enriched materials, such
as recycled eclogite, may also reside at depth, e.g.
in the transition region.
The outer 1000 km of the mantle -- 40% of
the mantle -- appears not to be well stirred or
homogenous (Meibom and Anderson, 2003). Since
the observed compositions of NMORB, EMORB,
Enriched upper mantle?
The earliest studies of ocean-ridge basalts recog-
nized that some ridge segments display enrich-
ments in highly incompatible elements. The ori-
gin of enriched midocean-ridge basalts (EMORB)
is controversial. EMORB have been sampled both
on- and off-axis at various ridges and fracture
zones, and at places not normally considered
to be hotspots. EMORB have been attributed to
interaction of plumes from the deep mantle
with the depleted upper-mantle source of nor-
mal midocean-ridge basalts (NMORB). Enriched
basalts have also been attributed to preferen-
tial melting of veins in the mantle, to small
plumes dispersed as small-scale heterogeneities
and to melting of enriched eclogitic veins from
subducted and stretched oceanic crust recycled
into the upper mantle. EMORB along the EPR
has been attributed to plume material that
was transported 5000 km from Hawaii, through
the asthenosphere. Other proposed sources of
