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et al. 1992 ). This sequence is: olivine ( ˙ Mg Cr
Spinel), olivine C plagioclase ( ˙ Mg Cr Spinel),
olivine C plagioclase C clinopyroxene. The av-
erage composition of MORBs includes 50.5 %
SiO 2 , 15.3 % Al 2 O 3 , 10.4 % FeO T , 11.3 %
CaO, 7.6 % MgO, 2.7 % Na 2 O, and 1.6 % TiO 2
(Hofmann 1988 ).
The oceanic crust has a characteristic layered
structure, which can be investigated both seis-
mically and by on-land observation of ophio-
lite sequences . These rock assemblages represent
remnants of oceanic crust that has been obducted
onto a continental margin by plate tectonic pro-
cesses. For example, if an oceanic plate is sub-
ducting beneath another oceanic plate, and if it
also carries continental crust in the rear, then
a collision between the continent and the intra-
oceanic subduction zone (comprising the asso-
ciated island arc) is unavoidable, as soon as the
oceanic crust has been entirely destroyed. In this
instance, subduction will be rapidly stopped, be-
cause the low-density continental crust cannot be
subducted. However, a small sliver of the overrid-
ing oceanic plate will be eventually transported
(obducted) onto the continental margin, where it
will form an ophiolite sequence. Figure 1.6 shows
a typical cross-section of oceanic crust, based
on refraction and reflection seismology experi-
ments (e.g., White et al. 1992 ; Christeson et al.
2012 ), and direct field observation of ophiolite
sequences.
The sequence starts (in top-down direction)
with a thin layer of deep-sea sediments, usually
less than 0.5 km, which can be formed by car-
bonate oozes, radiolarites, or argillites depending
from sea floor depth, distance from the ridge,
and latitude. Then, we find the effusive high-
porosity MORB layer 2, 0.75 km thick, formed
by ovoidal masses resembling pillows (pillow
lavas). This stratum can be further divided into
two layers, 2A and 2B, with a boundary that
is observed at 400-600 m below the sea floor
away from the ridge axis (Christeson et al. 2012 ).
The 2A layer has P -wave velocities of 2-3 km s 1
in the upper 250-300 mt, followed by a high-
gradient region, with velocity increasing linearly
up to 4.7 km s 1 at 500 m depth. The transition
from such high-gradient region to velocities of
having a minimum width w D H=F .For
example, for H D 7kmandF D 20 %, a 110 km
wide melting regime is necessary, granted that
the entire amount of melt is transported to the
ridge axis. In general, the degree of melt focusing
to the spreading center is elevated in the higher-
degree melting regime zone. The pattern of melt
migration depends from the anisotropy of perme-
ability of the peridotite rocks, which determines
the preferred directions of channeling of the melt
between mineral grains. Such anisotropy results
from the process of deformation of these rocks
within the melting regime, where the upwelling
asthenosphere flow is deviated to the horizontal.
Figure 1.5 shows the pattern of melt focusing in
the model of Phipps Morgan ( 1987 ). It is clear
that a small amount of melt is always retained,
even in the higher-degree melting zone, because
melts that cross the lateral boundaries of the
melting regime will be incorporated within the
horizontal mantle flow and will never reach the
ridge. Therefore, the asthenosphere outside the
melting regime always contains a small amount
of melt, which contributes to decrease the viscos-
ity of this layer.
The average de gre e of melting beneath nor-
mal ocean ridges, F , increases with increasing
spreading rate. The compositions of the oceanic
crust and the residual asthenosphere depend from
the process of mixing and equilibration of the
melts that are produced during the mantle up-
we lling. An important feature of the quantity
F is that it reflects the degree of enrichment
of the oceanic crust in highly incompatible el-
ements . These are elements, such as K, Rb, Sr,
U, and rare-earth elements (REE), that do not
easily fit into the crystal lattice structures of
minerals such as olivine, pyroxene, spinel, and
garnet, and thereby are the first to be parti-
tioned into the melt. Conversely, the residual as-
thenospheric column leaving the melting regime
will be depleted in these elements. The final
product of sea floor spreading is represented by
a 6-10 km thick oceanic crust layer made by
extrusive MORBs and intrusive gabbros. Mid-
ocean ridge basalts are fine-grained rocks, glassy
to porphyritic, whose sequence of crystallization
starts at depths shallower than 18 km (Grove
 
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