Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 208

The amount of ambient water present in the mantle as a
whole is thought to be c .0.03 weight percent, so the aver-
age basaltic melt produced at the midocean ridges is
largely anhydrous. Most interstitial water taken with ocean
crust and sediments into subduction “factories” is rather
efficiently processed back into the atmosphere and terres-
trial environment via arc volcanoes and subsurface magma
bodies. It has been calculated that of about 10 12 kg of
water taken into the subduction zones of the world every
year,

Magma
chamber

MOR

Crust
Lithosphere

Magma ascent

Partial melting

Asthenosphere

92 percent is recycled in arc volcanism. This is just
as well, because without recycling, the water-rich oceanic
crust would effectively drain the oceans in only 10 9 years.
How exactly the majority of this water is recycled by
Cybertectonica (Section 1.6.7) shall be briefly explored
below.

Upwelling
convection
loop

Fig. 5.10 Decompression melting under a midocean ridge magma
chamber. Volcanism at the midocean ridges is by far the most
voluminous on Earth.

5.1.4 Why and where does melting occur in
the Earth's crust and mantle?

The mobility of the Earth's convecting mantle (Sections
4.20 and 5.2) means that there are ample opportunities for
large-scale circulation to cause hotter material to rise up
from below. The process may be part of the large-scale
flow to the midocean ridges (Section 5.2; Fig. 5.10), with
melt volumes produced at rates of c .25 km 3 a 1 . Or it may
be on a more regional scale, around thermal plumes
(Fig. 5.9b) whose head area may be up to of order
10 4 km 2 . In either case, the melting associated with the
slow upward motion by plastic flow, of order 10 2 mm a 1 ,
is coincidental. It occurs because of what has been termed
decompression melting under conditions approximating the
adiabatic thermal transformation discussed in Section 3.4.
Remember that a volume undergoing adiabatic transfor-
mation is treated as being thermally isolated from its
surrounding environment. In the adiabatic rise and thus
decompression of deep mantle rock, despite some energy
loss due to work done in expansion, the rising and expand-
ing hot rock loses so little heat that it eventually intersects
the mantle solidus (Fig. 5.9a) thereby causing melting.
In this case, the adiabatic transformation is possible
because of the very low thermal diffusivity (Section 4.18)
of mantle material. The work done in expansion, as the
pressure decreases upward, requires a certain amount
of internal heat energy to be expended but this has very lit-
tle effect on the temperature of a rock volume. The tem-
perature path illustrated in Fig. 5.9a slopes gently negative
to illustrate the point, with the actual solid adiabatic
gradient, d T /d z , given by the expression g

capacity. Computed values of d T /d z for mantle peridotite
are about 0.4 Ckm 1 . In the rising decompressing

mantle, numerical calculations indicate that substantial
partial melt fractions (25 percent) can be produced over
20 km or so near the surface. The partial melt fraction pro-
duced at the solidus is of anhydrous basalt composition
and its intrusion and extrusion at the Earth's midocean
ridges leads to the formation of new lithospheric plate
(Section 5.2).
The second cause of melting (Figs 5.9c and 5.11)
explains the large-scale distribution of volcanoes and melt
zones associated with volcanic arcs, such as those around
the Pacific “ring-of-fire” (Fig. 5.1) and returns to the sub-
theme of water in melts outlined above. Melting in arcs is
associated with the sinking of lithosphere plates back into
the mantle via subduction zones (Section 5.2); but it is not
the sliding process and frictional heat generation (see
below) that causes the melting, for the descending plate is
actually quite cool, and remains so for considerable
depths. Rather, it is the transformation of the oceanic
mantle of the descending slab that causes melting in the
overriding plate. The transformation involves a mineral
group called serpentinite , which forms in the suboceanic
mantle as olivine is altered by deep penetration of water
along fracture zones and by subsea convection.
Serpentinite contains up to 12 percent by weight of water
in its mineral lattice. As the descending slab heats up, but
still well below the limits of the mantle solidus, it loses its
structural water at 400-800 T/c p , where C under pressures of

3-6 GPa. The water percolates upward, perhaps aided by
pressure changes in fractures opened during deep

T is the initial solid temperature, volume coefficient

of thermal expansion and c p is the isobaric specific heat