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pyrolite
eclogite
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T (°C)
Figure 10.11. Solidi (lower) and liquidi (upper) for an average mantle composi-
tion (pyrolite) and for eclogite, the upper-mantle form of subducted ocean crust.
The solidus is the temperature at which melting begins and the liquidus is the tem-
perature at which melting is complete. An approximate solid adiabat is included.
After Yasuda et al. [80]. Copyright American Geophysical Union.
therefore react with the peridotite to form an assemblage of intermediate composi-
tion, and it may solidify as it does so, yielding 'refertilised', hybrid lherzolites or
pyroxenites [205-207]. If such material continues to rise under a mid-ocean ridge,
it may remelt and mix with melt derived from melting peridotite. It is possible
that the end-product of this process is a melt that is similar to the melt that would
have been produced by a homogeneous source of the same average composition.
However, we must consider that not all of the intermediate material may remelt, or
that some of the eclogite-derived melt may reach the surface without fully mixing
with peridotite melt.
On the one hand, the studies so far have yielded evidence that melt from het-
erogeneities does indeed contribute discernibly to MORBs and OIBs. On the other
hand, they have produced arguments that melt from the heterogeneities often may
not equilibrate with the surrounding peridotite. The same arguments suggest that
significant amounts of melt may not be erupted at mid-ocean ridges, and thus may
be trapped in the mantle and recirculate internally.
10.5.1 Melting a homogeneous source
Before discussing heterogeneous melting, it is helpful to depict the picture of
melting under mid-ocean ridges that has developed from the assumption of melt-
ing of a homogeneous source, as sketched in Figure 10.12. As material rises
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