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five times higher than measured in depleted peri-
dotites.
The shallow mantle is a sink of subducted sed-
iments, slab-derived fluids, altered ocean crust
and serpentinized peridotites and may con-
tain trapped small-degree melts, kimberlites and
metasomatic fluids. If so, it is a non-negligible
heat source and, because of its large volume,
may have an integrated productivity compara-
ble to the continental crust. Even if the conti-
nental crust has achieved a steady state between
construction and destruction, there is a certain
amount of radioactive-rich crustal material circu-
lating in the mantle.
materials as representative of the upper mantle;
this is
ad hoc
. An alternative approach is to mix
terrestrial materials together in proportions that
satisfy chondritic or cosmic ratios of the refrac-
tory elements.
Upper mantle
If the total heat productivity of the Earth is 19--
31 TW then the upper mantle share, if uniformly
distributed, is 5--10 TW. If the crustal inventory
of 8--9 TW was derived only from the upper man-
tle then the UM would be barren indeed. There
is little room for recycled sediments and crust
or metasomatic fluids. However, this is only a
model. Detailed mass-balance calculations and
the amount of
40
Ar in the atmosphere suggests
that most or all of the mantle must have con-
tributed to the LIL inventory of the crust and UM.
This indicates an efficient differentiation and
melt extraction process. However, inefficient melt
extraction and recycling keep the upper mantle
from being completely barren. If the LIL were
concentrated into the outer layers of Earth dur-
ing accretion, then extraction of the crust leaves
10 to 23 TW in the UM. These numbers can be
matched by making the UM out of Pacific MORB
and peridotitic xenoliths or a mix of enriched
and depleted MORB and peridotites and a small
fraction of enriched components such as recy-
cling crust or kimberlitic material.
Table 26.2 shows how much heat can be gen-
erated in the upper mantle if it has the com-
position of various mantle samples. The upper
410 km can generate 9 TW if it is entirely made of
continental peridotites and 13 TW if made from
a representative MORB. The upper 650 km can
generate 28 TW if it is made up of Pacific MORB
or an average eclogite. A plausible range, using
basaltic and peridotitic mantle samples, is from
10--22 TW. This is about an order of magnitude
higher than estimates based on depleted MORB.
If most of the U and Th is in the upper man-
tle then most of the
4
He will be generated there.
The missing
4
He in the integrated volcanic flux
implies that He is not readily outgassed. This is
consistent with measured U/He ratios and abso-
lute He concentrations in midocean ridge basalts.
Mantle components
Mantle melts are buoyant compared to resid-
ual solids, at least in the shallow mantle, and
frozen melts (basalts) are buoyant until they con-
vert to eclogite at about 50 km. These materi-
als strip radioactivity out of the source man-
tle and deliver it to shallow depths where they
pond, underplate, intrude or erupt. The erosion
of continents puts radioactive-rich material into
seawater and onto the ocean floor where some
gets recycled into the mantle. Some of this gets
returned quickly to island arc and back-arc vol-
canoes but some remains in the shallow mantle
as an enriched component. Delaminated conti-
nental crust also recycles into the mantle. Never-
theless, most estimates of the composition of the
shallow mantle adopt the most depleted basalts
erupted along the midocean-ridge system and
assume that these, along with residual peridotite
having little or no radioactivity, are character-
istic of the entire mantle above the 650 km
phase change. Many mantle peridotites have U
and Th contents comparable to depleted basalts
and this alone can raise estimates of upper man-
tle radioactivity by a factor of ten.
Basalts found along the global ridge system
have radioactivities that vary by more than an
order of magnitude. Kimberlites, carbonatites
and alkalic basalts extend the range of upper
mantle materials even further. Peridotites also
have a large range of compositions. It is con-
ventional to adopt the most depleted of these
