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seamount
chains
ophiolites
pluton
plateau
sediments
volcanic arc
batholiths
CRUST
imbricated
slabs
CRATON
basalt
eclogite
delamination
Asthenosphere
flat subduction
more than 10% of the seafloor area is capped
by seamounts and plateaus. The delamination
of over-thickened continental crust also intro-
duces fertile material into the asthenosphere;
this is warmer and perhaps thicker than sub-
ducted oceanic crust, and will equilibrate and
melt sooner. These warm delaminates are poten-
tial fertile spots and can create melting anoma-
lies. They may account for 5% of all recycled
material.
The rate at which young oceanic crust and
delaminated lower continental crust enters the
mantle
Fig. 24.1 Illustration of one end game of the plate-tectonic
cycle - the closure of ocean basins. The other end of the
cycle is continental breakup, when this diverse material can
be involved in breakup magmatism. The many oceanic
plateaus in the newly opened Atlantic and Indian ocean basins
may be, in part, this reactivated material.
variations are long wavelength while chemical
heterogeneity can be of the scale of slabs and
the source regions of volcanoes. Melting anoma-
lies may be primarily due to high homologous,
not high absolute, temperature.
Basalts, mafic and ultramafic cumulates and
depleted harzburgitic rock are constantly formed
along the 60 000-km-long mid-ocean spreading-
ridge system. The mantle underlying diverging
and converging plate boundaries undergoes par-
tial melting down to depths of order 50--200 km
in regions up to several hundred kilometers
wide, the processing zone for the formation of
magmas -- MORB, backarc-basin basalts (BABB)
and island-arc basalts (IAB). Midplate volcanoes
and off-axis seamounts process a much smaller
volume of mantle, and the resulting basalts are
therefore -- as a consequence of the central limit
theorem -- much more heterogenous.
Before the oceanic plate is returned to the
upper mantle in a subduction zone, it accu-
mulates sediments and the harzburgites become
serpentinized; this material enters the man-
tle (Figure 24.1). Plateaus, aseismic ridges and
seamount chains also head toward trenches.
About 15% of the current surface area of oceans
is composed of young (
is
comparable
to
the
global
rate
of
2km 3 /yr. Melting anomalies
may therefore be due to fertile blobs. The fates
of older plates with thin crust, and deeper slabs
are different.
'hotspot' volcanism,
Fate of recycled material
Large-scale chemical heterogeneity of basalts
sampled along midocean-ridge systems occur on
length scales of 150 to 1400 km. This hetero-
geneity exists in the mantle whether a migrating
ridge is sampling it or not. Fertile patches, how-
ever, are most easily sampled at ridges and may
explain the enigmatic relations between physi-
cal and chemical properties along ridges. Nor-
mal oceanic crust may start to melt after being
in the mantle for
60 million years (Myr), while
delaminated crust may melt after only 20--40 Myr
because it starts out warmer (Figure 5.2).
Tomographic correlations suggest that
middle-aged plates reside mainly in the bottom
<
20 My) lithosphere and
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