Geology Reference
In-Depth Information
Porous melt flow
zone
Eclogite body
Melt channel
+ reaction zone
Streamlines
Figure 10.13. Sketch of the migration of melt emerging from eclogite bodies. The
melt will react with surrounding peridotite, and will tend to refreeze. Some might
react completely and become trapped in the mantle. However, melt from larger
bodies might form a reaction zone that insulates later melt from reaction, and
allows it to rise into the zone where pervasive peridotite melting will allow porous
melt flow. After Davies [118]. Copyright American Geophysical Union.
reflect, in its trace element composition, the presence of eclogite in the source.
This has important implications for the interpretation of mantle geochemistry.
10.5.3 Melt trapping and recirculation
An implication of Kogiso
et al.
's study [204] is that some melt may react with the
peridotite matrix, refreeze and not be remelted. Material close to the spreading axis
is likely to ascend close to the Earth's surface and therefore to remelt and to be
extracted. Any melt from heterogeneities that ascends into the zone where the peri-
dotite matrix begins to melt is also likely to be extracted, as the peridotite melt will
form a connected network through which the melt from heterogeneities can migrate.
However, eclogites and hybrid pyroxenites begin to melt much deeper than
peridotite, about 110 km (Figure 10.11 and [80]), so their melting zone will be
much wider. Heterogeneities further from the spreading axis may not rise shallow
enough to remelt, nor may they rise into the peridotite melting zone. This material
may be carried away laterally without remelting. A sketch of how melt may be
trapped is shown in Figure 10.14. The eclogite melting that occurs outside the
peridotite melting zone will tend to be in disconnected pockets. Melt from larger
eclogite bodies may migrate, but much of it may not reach the surface.
Because the eclogite melting zone is deeper and wider than the peridotite melting
zone, much of the eclogite melt, or its hybrid pyroxenite product, will pass outside
the peridotite melting zone and may remain in the mantle. This effect will be
enhanced if the peridotite matrix is more refractory than the 'fertile peridotite'
usually assumed in models of the melting of a homogeneous source. This is because
the refractory peridotite melting would begin shallower than 60 km and so be even
less likely to capture the pyroxenite products.
