The upper mantle - New Theory of the Earth - page 310

Attempts are made to avoid accessory phases
and hydrothermal alteration. The net result is
that estimates of DMM, DM or DUM, and the
whole upper mantle in some models, are biased
toward very depleted end members. Remaining
questions are, what is the fraction of EM and recy-
cled components in the source regions of MORB
and OIB, what is the origin of EM, and what is the
average composition of the upper mantle? The
standard model is that MORB derive from a vig-
orously convecting well-stirred reservoir and that
enriched materials, and hidden heat-producing
reservoirs are in the deep mantle.
Although MORB and their presumed residues,
abyssal peridotites, have some degree of heteroge-
neity in radiogenic isotope ratios (Sr--Nd--Pb--Hf),
they have a small range of values relative to
ocean island basalts and are, with some excep-
tions, depleted, relative to bulk Earth values, thus
requiring a long-term history of low Rb/Sr, Hf/Lu
and Nd/Sm (i.e. incompatible element depletion).
The small range in composition is partly a result
of the data selection and filtering operations just
discussed and partly due to the averaging accom-
plished by natural processes at ridges (Meibom
and Anderson, 2003).
Estimates of Donnelly et al. (2004), Salters and
Stracke (2004) and Workman and Hart (2005)
for the NMORB source can be made consistent
with chondritic and accretional differentiation
constraints, and the compositions in Chapter 13
simply by using different proportions of the com-
ponents, and by adding enriched components
representing small degrees of melt, or recycled
material. Mass-balance calculations provide no
constraints on the sizes and locations of the var-
ious components, reservoirs or blobs. Chondritic
constraints, however, can be satisfied even if all
the components are in a volume less than the
size of the upper mantle.
Table 23.1 contains estimates of BSE, CC,
NMORB, EMORB, OIB, KIMB and various ultra-
mafic rocks (UMR). The fertile-mantle values tab-
ulated in Table 23.1 are derived by mixing MORB,
EMORB, OIB, KIMB, CC and various estimates
of the compositions of spinel and abyssal peri-
dotites in order to retain the LIL and refrac-
tory element ratios in BSE (Chapter 13). Peri-
dotites account for 92--94% of the mix and MORB

accounts for 4.5--6%. EMORB and OIB can be up
to 0.7% of the mix and KIMB can be up to 0.15%.
CC is constrained to be 0.55%. Since the upper
mantle is 30--40% of the mantle, there is no mass
balance reason to suppose that the enriched and
fertile components -- EMORB, OIB, KIMB -- must
come from the lower mantle.
Primitive upper mantle (PUM) in the standard
model of mantle geochemistry is

PUM = CC + DUM

In the current model, fertile mantle (FM),
prior to removal and stabilization of the conti-
nental crust, was

FM = CC + MORB + EMORB + OIB + KIMB

+ residual refractory peridotites (UMR)

The currently accessible outer reaches of the
mantle probably contain the MORB-source, the
EMORB-source, the OIB-source and the KIMB-
source. The non-fertile mantle (BSE minus FM),
based on cosmic abundances of the major ele-
ments, has more silica and less magnesia than
FM and is therefore less olivine-rich.

Enriched blobs
The NMORB-source region, DMM, is just part
of the mantle, probably just part of the upper
mantle. A representative mass-balance calcula-
tion using published estimates of the composi-
tions of DMM (DMORB abyssal peridotite), con-

tinental crust, recycled oceanic crust (now in the
mantle as eclogite) and an OIB-source yields, in
round numbers, 74% DMM, 5% recyling MORB
(eclogite), 21% OIB-source and 0% primitive man-
tle. The fraction of mafic material in this and sim-
ilar calculations is less than the mass of oceanic
crust generated throughout Earth history, assum-
ing current rates for 4.55 Ga (6--7% the mantle
mass).
One point of this exercise is to show that
there is no mass-balance contradiction with the
hypothesis that most of the fertile and heat-producing
elements are in the upper mantle .Itisonlyby
assumption, in simple box-model calculations,
that the entire upper mantle is required to be
depleted and low in radioactive elements. This

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