The upper mantle - New Theory of the Earth - page 306

generally viewed as well-mixed with the refrac-
tory infertile components -- depleted peridotites,
lherzolites, ultramafic residues (UMR). They are
more likely to reside in large blobs.
Various combinations of materials give chon-
dritic or BSE ratios of the refractory LIL elements.
The fertile components can account for all the LIL
and they occupy about 10% of the mantle; the
average enrichment -- of the fertile part of the
mantle -- is therefore a factor of 10 times the con-
centration levels of primitive mantle. About
0.5--2% melting is implied to obtain material
as enriched as CC and KIMB from BSE, and to
deplete DM to the extent observed; up to approx-
imately 25% melting of this still fertile residue,
at a later time, is implied in order to generate the
more abundant depleted basalts. The time sepa-
ration of these events can be estimated from iso-
topes to be of the order of 1.5 to 2.5 Gyr. This has
traditionally been taken as the convective over-
turntimeofthemantle.Fromaplate-tectonic
or top-down point of view it is the revisitation
time of a migrating spreading ridge. There is no
contradiction between small-degree melting in
the past, when the mantle was hotter, and large-
degree melting at the present, after the mantle
has cooled down. Since the mantle is close to
the melting point, the formation and removal
of very small-degree melts may have occurred in
the thermal boundary layer at the surface of the
Earth, where the current temperature rises from
near 0 ◦ C to about 1400 ◦ C. Low-degree melting
can occur in a surface or internal TBL or in recy-
cled mafic material. High-degree melting requires
special circumstances.
Kimberlites, OIB and EMORB are probably all
produced from the upper mantle; there is no rea-
son to suppose that they are not. Mass-balance
constraints on the composition of the mantle
can be achieved by adding EMORB, EM or KIMB
to the very depleted components involved in
DMORB genesis. For example, having 5% EMORB
or 0.5% kimberlite plus MORB plus peridotite in
the upper mantle gives approximately chondritic
ratios of the refractory LIL.
Petrological and cosmochemical constraints
can be satisfied if the basaltic or fertile com-
ponents of the mantle are mainly DMORB but
also include about 10% OIB, 10% EMORB and

0.001 -- 0.002 kimberlite, or some combination.
All of this material fits easily into the upper
mantle, and is potentially available for incor-
poration into ocean-ridge basalts, ocean-island
basalts, seamounts and continental basalts. Esti-
mates of the composition of fertile mantle are
given in Table 23.1.
Original unprocessed mantle may have
included the equivalent of 7.5--9.5% NMORB, 1%
OIB, 2% EMORB plus the present continental
crust (Chapter 13). This particular combination
gives chondritic ratios of the refractory LIL and
accounts for almost all the LIL and Na of BSE.
The crust itself accounts for about 50--80% of the
original LIL material in PM. Almost all the rest
is contained in the various MORB components,
in kimberlites and in OIB. Other considerations
suggest that the mantle may contain from 6--15%
eclogite.

Melt generation

Current upper-mantle conditions allow a wide
range of melt fractions, with the largest extents
of melting permitted at midocean ridges and
other thin-lithosphere spots where mantle near
the melting point can increase its melt content
by adiabatic ascent to shallow levels, a mech-
anism not available under thick plates. Large
degrees of melting can also occur in the more
fertile -- or eclogite-rich -- regions of the man-
tle. Small-degree melts occur on the wings of
midocean-ridge melting zones, and at greater
depths, and under thick plates, and in the man-
tle wedge above subducted plates. As slabs warm
up to ambient mantle temperature they can also
experience small-degree melting. Thus, there are
many opportunities for generating and removing
enriched small-melt fractions, and for enriching
and depleting various regions of the upper man-
tle. On the present Earth, and probably through-
out Earth history, small-degree and large-degree
melting conditions can both operate.
The simplest melt-generation scenerio is one
in which the source rock is melted by a cer-
tain amount and the melt is then completely
extracted. This single-stage process can be mod-
eled, and the melt and residue compositions