Page 48 - Fundamentals of Physical Volcanology

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MAGMA GENERATION AND SEGREGATION 25

not a setting in which melting by decompression or
(a)
by heating is likely to occur. Instead it is thought
that melting occurs due to the release of volatiles,
especially water, from the descending lithospheric
slab. The slab is topped by water-saturated sedi-
Accretionary New arc
wedge crust Oceanic crust ments, some of which will descend with the slab.
More importantly, the rocks of the slab itself con-
tain many hydrous mineral phases as the result of
chemical reactions between the rocks and the
Melting occurs in
the mantle wedge hydrothermal water that was circulating through
the rocks while they formed the ocean floor.
Water is released
from the As the slab descends, reactions occurring within
descending plate
the rocks lead to the dehydration of the rocks and
the release of this bound water. It appears that the
Oceanic lithosphere
water rises into the mantle wedge which overlies
the descending slab (Fig. 2.8), reducing the solidus
of the mantle material sufficiently to induce melt-
(b) ing (Fig. 2.2). As in the MOR setting, melting of
the mantle is likely to produce basaltic magmas
but, although present, they are not the dominant
magma type seen in subduction settings. In island-
Magma undergoes
fractionation and arc settings it is thought that ponding of the basaltic
assimilation within
the crust melts at the base of the lithosphere and within
Crustal underplating the crust itself allows fractional crystallization to
occur, with the residual liquids giving rise to the
Melting occurs in more common basaltic andesites and andesites found
the mantle wedge
there (Fig. 2.8).
Water is released In continental arcs, where any magma reaching
from the
descending plate the surface must travel through a considerable
thickness of continental crust, there is the potential
for a wide range of processes to operate. Here,
Oceanic lithosphere
then, although the primary magma produced is
basalt from the mantle wedge, interaction of the
Fig. 2.8 The two types of subduction zone that can occur magma with the continental crust creates the diver-
on Earth. In (a), one oceanic plate subducts beneath another sity of magma erupted at the surface (Fig. 2.8).
oceanic plate producing an island arc. In (b) an oceanic There is great potential here for melting of crustal
plate subducts beneath a continental plate generating a
rocks, for assimilation of crustal material during
continental arc or active continental margin. In both
magma ascent, for fractional crystallization
cases melting occurs in the mantle as water released from
(where crystals form and are left behind by the
the descending plate infiltrates the mantle and changes the
solidus and liquidus temperatures (see Fig. 2.2). However, remaining liquid magma), and for mixing of mag-
differences in the surface volcanism occur because the mas at different depths beneath the surface. The
mantle melts interact with rocks of differing composition relative importance of each of these processes is
as they rise, oceanic crust in case (a) and continental crust still a source of considerable debate amongst
in case (b).
igneous petrologists and geochemists, and is well
beyond the scope of our discussion here. We are
more concerned with the physical properties of
the resulting magmas, and these, together with the
properties of less evolved magmas such as basalts,