Geology Reference
In-Depth Information
to self-regulate their bending radius so the bending resistance does not become
dominant. There would in any case be questions about the consistency of this kind
of thermal history with the geological record, as it would seem to imply rather
extreme levels of magmatism and tectonics in the Archaean. This, however, is less
definitive than the implausibility of the parameters required.
At present it thus seems that there is no plausible proposal for how a long-term
imbalance of heat loss and heat generation might be maintained. We are left with
the other two possibilities, that the geochemical estimates are wrong or that we are
in a period in which the heat loss is unusually high. Either of these options seems
possible. A little more will be said as we go along, but the resolution of the heat
source puzzle will remain for the future.
9.4 Compositional buoyancy
The convection theory of Chapters 5 and 6 and the thermal evolution calculations
of this chapter so far assume that thermal buoyancy is the dominant driver of mantle
convection. Yet we know that plate tectonics generates density differences due to
different compositions. Melting at mid-ocean ridges forms basaltic oceanic crust,
and it also leaves a zone of mantle depleted of its basaltic component to a depth of
around 60 km. The depleted mantle tends to be buoyant because the erupted basalt
is more enriched in iron than its source, according to conventional views. This
means that subducted lithosphere is not uniform - it is stratified in composition
and in density.
The oceanic crust is also buoyant relative to the mantle, having a density of
about 2900 kg/m
3
, compared with a mantle density of 3300 kg/m
3
.However,by
a depth of about 60 km it is expected to have transformed to a different mineral
assemblage, called eclogite, with a density around 3500 kg/m
3
. Mantle minerals
undergo further phase transformations as they go further down, because of the
increasing pressure. There is a significant transformation around 410 km depth and
a larger one around 660 km depth. Subducted oceanic crust undergoes comparable
transformations with depth, and generally it is a little denser than average mantle at
the same depth, by 100-200 kg/m
3
[152, 153]. There is, however, a depth interval
within which it is less dense than adjacent mantle, because its transformation to
lower-mantle phases is delayed until around 750 km depth. Thus between 660 km
and 750 km depth it is still in its transition zone phases and is less dense by about
150 kg/m
3
.
The density difference between oceanic crust and the mantle, and its several
changes of sign, has some important effects on mantle dynamics. This topic is still
being explored, but there are some important conclusions or indications that will
be outlined here.
