Geology Reference
In-Depth Information
Peridotite
melt zone
Oceanic crust
Oceanic lithosphere
Melt
extraction
zone
Residual zone
Peridotite mantle
Figure 10.12. Sketch of melting under a mid-ocean ridge, assuming a homoge-
neous source. The curves are flow lines.
(passively) under the ridge, the reduction of pressure allows it to begin melting, or
in other words its adiabatic temperature trajectory intersects the peridotite solidus
(Figure 10.11, where pyrolite is similar to peridotite). This is estimated to occur at
about 60 km depth.
The eruption zone at a mid-ocean ridge is very narrow, only about 10 km across.
Yet melting will begin across a much broader zone. It is inferred that the rising
melt is 'focused' into the narrow eruption zone, otherwise it is difficult to explain
the thickness of the oceanic crust. The focusing occurs because the diverging flow
under the ridge generates a pressure deficit that pulls the melt towards the ridge
axis [208].
The rate of melting within the melt zone is proportional to the vertical velocity
of the mantle material. As the off-axis flow lines curve towards the horizontal, the
rate of melting will approach zero. A little of the off-axis melt may not be extracted
to the ridge. Material that has passed through the melt zone will have had some melt
extracted, forming a residue that will move laterally with the overlying plate. The
degree of melting and depletion will be greater for material that reaches shallower
depths. Off-axis melting will be limited at the top by the thermal boundary layer
(the oceanic lithosphere) extending downwards by conduction (Chapters 5 and 6).
10.5.2 Reaction and disequilibrium of eclogite melts
Returning to melting in a heterogeneous source, the reaction of eclogite-derived
melts with peridotite might produce a range of compositions, from lherzolite
through pyroxenite, garnet pyroxenite even to eclogite, depending on the rela-
tive proportions of melt and peridotite. However, Sobolev et al. [209] propose that
the product will be a relatively uniform pyroxenite, with approximately 50% inputs
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