Geoscience Reference
In-Depth Information
Figure 8.16.
A computer
model of convection in
the upper mantle. Half the
heat is supplied from
below, and half is
supplied internally. The
Rayleigh number is
1.4
10
6
. The model has
several adjacent cells,
each with separate
circulation, although over
time the cell boundaries
move and material is
exchanged between
adjacent cells.
(a) Isotherms
(temperature contours),
(b) fluid flow lines and
(c) locations of marked
fluid; (a), (b) and (c) are all
at the starting time.
Deformation of the
marked fluid at
subsequent times:
(d) 33 Ma, (e) 97 Ma and
(f) 155 Ma. (From Hoffman
and McKenzie (1985).)
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'slab graveyard'. Another idea, which is in agreement with the convection and
geochemical models, is that the sub-continental lithosphere provides a source
for OIB. Isotopic anomalies can easily form in the deep lithosphere beneath the
continents. Deep continental material could become denser and delaminate or
fall into the upper mantle, this process perhaps being triggered by a continent-
continent collision. Such a cold body would descend at least to the base of the
upper mantle, where it would warm before rising to the surface as part of the con-
vection system. It would remain a viable magma source for about 100-300 Ma.
After 150 Ma a body that was originally 100 km thick would be mixed into
5-km-thick sheets. Another proposal for the origin of OIB is that they are the
result of partial melting of material that has risen from a separately convecting
primitive lower mantle (Fig. 8.17). Even though this model cannot explain why
these basalts do not have the same isotopic composition as the bulk Earth, it is
