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and the density of the mantle under the spread-
ing centers is the same. The marginal basins
of Southeast Asia, however, tend to be deeper
than similar age oceanic crust elsewhere, by one-
half to one kilometer, and to deepen faster with
age. The presence of cold subducting material
underneath the basins may explain why they are
deeper than average and why they sink faster. The
low-density material under the ridge axis may be
more confined under the major midocean ridges.
On average the upper 200--300 km of the mantle
in the vicinity of island arcs and marginal basins
has slower than average seismic velocities and the
deeper mantle is faster than average, probably
reflecting the presence of cold subducted mate-
rial. The depth of back-arc basins is an integrated
effect of the thickness of the crust and litho-
sphere, the low-density shallow mantle and the
presumed denser underlying subducted material.
It is perhaps surprising then that, on average,
the depths of marginal basins are so similar to
equivalent age oceans elsewhere. The main dif-
ference is the presence of the underlying deep
subducting material, which would be expected
to depress the seafloor in back-arc basins. Basalts
in marginal basins with a long history of spread-
ing are essentially similar to MORB, while basalts
generated in the early stages of back-arc spread-
ing have more LIL-enriched characteristics. A sim-
ilar sequence is found in continental rifts (Red
Sea, Afar) and some oceanic islands, which sug-
gests a vertical zonation in the mantle, with the
LIL-rich zone being shallower than the depleted
zone. This is relevant to the plume hypothesis,
which assumes that enriched magmas rise from
deep in the mantle. The early stages of back-
arc magmatism are the most LIL-enriched, and
this is the stage at which the effect of hypo-
thetical deep mantle plumes would be cut off
by the presence of the subducting slab. Conti-
nental basalts, such as the Columbia River basalt
province, are also most enriched when the pres-
ence of a slab in the shallow mantle under west-
ern North America is indicated. The similarity of
the isotopic and trace-element geochemistry of
island-arc basalts, continental flood basalts and
ocean-island basalts and the slightly enriched
nature of the back-arc-basin basalts all suggest
that the enrichment occurs at shallow depths,
Xenoliths
Kimberlite
0.5132
MORB
Marianas
Sc
Or
S. Sandwich Is.
0.5130
A
Ic
Ecuador
G
B
0.5128
Sb
Sb
Sunda
Arc
P
0.5126
E
T
K
Br
Chile
0.5124
0.5122
0.703
0.704
0.705
0.706
0.707
87
Sr/
86
Sr
Fig. 14.6
Neodymium and strontium isotopic ratios for
oceanic, ocean island, continental and island-arc (hatched)
basalts and diopside inclusions from kimberlites. The numbers
are the percentage of the depleted end-member if this array
is taken as a mixing line between depleted and enriched
end members. Mixing lines are flat hyperbolas that are
approximately straight lines for reasonable choices of
parameters. The fields of MORB and CFB correspond
to
>
97% and 70--95%, respectively, of the depleted
end-member. The enriched end-member has been arbitrarily
taken as near the enriched end of Kerguelen (K) basalts. The
most enriched magmas are from Kerguelen, Tristan da Cunha
(T) and Brazil (Br). Other abbreviations are Sc (Scotia Sea), A
(Ascension), Ic (Iceland), G (Gouch), Or (Oregon), Sb
(Siberia), P (Patagonia) and E (Eifel). Along top are strontium
isotopic data for xenoliths and kimberlites.
the lower mantle by a slab. Yellowstone is not in
a classic back-arc basin but it has high
3
He/
4
He
in spite of the underlying thick U-rich crust and
no deep low-velocity zone. BABB are similar to,
or gradational toward, ocean-island basalts and
other hotspot magmas.
In most respects active back-arc basins appear
to be miniature versions of the major oceans,
and the spreading process and crustal structure
and composition are similar to those occurring
at midocean ridges. The age--depth relationships
of marginal seas, taken as a group, are indistin-
guishable from those of the major oceans. To
first order, then, the water depth in marginal
sea basins is controlled by the cooling, contrac-
tion and subsidence of the oceanic lithosphere,
