Geoscience Reference
In-Depth Information
Isotopic work has indicated that the subducting slab provides only a small
component of erupted lavas. Data from Chile have shown that the overriding
mantle provides a significant proportion of all elements: only volatile species
such as H
2
O and the
large-ion lithophile
elements such as Rb, K, Ba, Th and Sr
were supplied in any great quantity by the subducting slab. These elements are
transported upwards to the overlying mantle in the volatiles ascending from the
subducted slab to become incorporated into the rising melt.
The overriding mantle wedge
It is assumed in many thermal models of subduction zones that heat is transferred
by conduction alone, which is an extreme simplification; more realistic models
incorporate a viscous, convecting mantle. The inflow of overriding mantle ensures
a supply of fertile asthenosphere, with a potential temperature (Eq. (7.94a)) of
about 1280
◦
C, to the region above the descending slab.
Two thermal models are shown (Figs. 9.44 and 10.7). Temperatures close to
the wet solidus (Fig. 10.6(b)) are reached in the vicinity of the descending slab
at depths in excess of about 100 km. This is important because it means that the
mantle temperatures there are in the range within which addition of water results
in copious partial melting. Most melt is generated in the upper mantle beneath
the volcanic arc (the mantle wedge) as a result of the addition of water and other
volatiles from the subducting slab. As discussed earlier, water is lost by the slab
at all depths down to about 100 km, initially due to compaction (the closing of
the pores) and then to the dehydration reaction. However, it is only at depths at
which the overlying mantle temperature is more than 1000
◦
C that partial melting
can take place in the mantle wedge; this condition seems to be satisfied at about
100 km depth in most subduction zones.
The effects of subduction on the overriding mantle wedge can be summarized
as follows: (1) an influx from the descending slab of upward-moving volatiles;
(2) some melt rising from deeper parts of the slab; and (3) the driving of convective
flow in the wedge, the flow-lines of which show movement of mantle material
from the distant part of the wedge (on the right in Fig. 10.8) and pulled downwards
(counterclockwise in Fig. 10.8)bythe slab.
The upward-moving volatiles from the descending slab consist of H
2
O and
CO
2
, probably accompanied by a substantial flux of mobile elements such as Rb,
K, Ba, Th and Sr. Melting of any subducted sediment may be important. The
contribution of melt (as opposed to volatiles, etc.) from the slab itself is probably
relatively small. The deeper parts of the slab may leak some melt upwards, leaving
residual quartz-eclogite behind. Compositionally, any melt rising from the slab
is probably hydrous, siliceous magma, which is roughly similar to calc-alkaline
magmas erupted in island arcs but probably with a higher CaO/(FeO
+
MgO)
ratio.
The addition of streams of volatiles, plus perhaps some melt at deeper lev-
els, causes partial melting in the warm peridotite overlying the subducted slab
