Geoscience Reference
In-Depth Information
dry and wet extremes: voluminous melting can start anywhere between the two
curves, depending on the amount of water present. However, except in limited
regions where heating is especially rapid, melting of the subducted oceanic crust
is unlikely to occur during the early stages of descent.
The subducted oceanic crust carries wet oceanic sediment. At or near the
trench, much of this sediment is scraped off and becomes part of the accre-
tionary wedge (see Section 10.2.2). However, a small part of the sediment may
be subducted. This subducted sediment melts at comparatively low temperatures
(although these temperatures might not be attained on the surface of a subducting
slab until depths
150 km) and provides components (such as CO 2 ,Kand Rb)
to the stream of volatiles rising upwards. 238 U- 230 Th disequilibrium evidence
shows that, beneath the Mariana arc, at least 350 000 yr elapses between melting
of sediment and its incorporation into mantle melt.
The lower, plutonic (gabbro) portion of the subducted crust and the upper-
most subducted mantle (peridotite) may also have been partially hydrated by
sub-seafloor hydrothermal processes. Figure 10.6(b) shows the controls on dehy-
dration of hydrated ultramafic rock in the lowest crust and topmost mantle as
water is driven off any serpentine in the rock. The subducted oceanic mantle,
which is depleted and refractory, does not usually melt. Being cooler than nor-
mal upper mantle at these depths, the subducted mantle simply heats up slowly
towards the temperature of the surrounding mantle.
>
The descending slab: heating
The descending slab heats up by conduction from the overlying hotter mantle,
but other factors that combine to speed up the heating process are also operating.
Some contribution to the heating of the slab may come from friction on its
upper surface. This frictional or shear-stress heating is, however, not well quanti-
fied. Estimates of shear stress (0-100 MPa) are used in thermal modelling. Results
of detailed studies of the heat flow measured in the fore-arc region, where there
is no thermal contribution from the volcanic arc, indicate that the shear stress
is low and probably lies between 10 and 30 MPa. Figure 10.7 shows a series of
conductive models of the thermal structure of subduction zones dipping at 26.6 ,
the average dip of subduction zones down to 100 km depth (see Fig. 9.59) and
with shear-stress heating increasing from zero at the surface and then decreasing
to zero when the top of the slab reaches 100 km depth. The temperatures of the
subducted oceanic crust are not high enough for it to melt in this region; the
wet solidus for basalt (shown in Fig. 10.6(a)) is not attained in the upper part of
the slab. Thermal models suggest that normally subducted oceanic crust reaches
temperatures no greater than 500-700 Catdepths of
125 km (Fig. 10.6(a)).
The subducting oceanic crust is progressively subjected to temperatures and pres-
sures appropriate to blueschist to eclogite facies. Greatly increased values of the
shear stress and/or much higher initial temperature gradients for the subduct-
ing and overriding plates would be necessary for melting of the crust itself to
take place in this region. This is a good check on the validity of models and the
Search WWH ::




Custom Search