Geology Reference
In-Depth Information
(a)
(b)
100
80
subduction
80
free plates
60
60
40
40
buoyancy-limited
20
20
neutral buoyancy
0
0
1320
1340
1360
1380
1400
1350
1400
1450
1500
1550
1600
Mantle temperature (
°
C)
Temperature (
°
C)
Figure 9.7. (a) Age at which plates become neutrally buoyant, and average age of
plates as they reach a subduction zone. (b) Heat removed by free plates and by
plates limited by compositional buoyancy versus mantle temperature. Actual heat
loss would be given by the lower (solid) parts of the curves. After Davies [154].
Copyright by the Geological Society of America.
demonstrate another phenomenon that seems to overcome the argument. The argu-
ment and its antidote will be summarised.
Davies [154] estimated the positive buoyancy contributed by both the oceanic
crust and the depleted melt zone from which the crust is drawn and calculated
the age at which lithosphere becomes neutrally buoyant. Using estimates of how
melting increases as mantle temperature increases, the age of neutral buoyancy was
calculated as a function of mantle temperature, and is shown in Figure 9.7(a). The
theory of convection from Chapter 5 can be used to calculate plate velocity as a
function of mantle temperature. The plate velocity determines how old a plate is
when it reaches a subduction zone, and Figure 9.7(a) also shows the average age
at subduction versus mantle temperature.
The curves show that, when the mantle was only about 50
◦
C hotter than now,
plates would be neutrally buoyant as they reached subduction zones. At higher
temperatures, plates would not be able to subduct if they moved at speeds calculated
from the convection theory. According to Figures 9.1 and 9.2 the neutral buoyancy
condition would have applied only about 1.5-2 Gyr ago.
Before that time, plates would have had to move more slowly, so heat removal by
plates would have been less than predicted by free-plate theory. This is illustrated
in Figure 9.7(b). At a little higher temperature again, heat loss would be less than
radioactive heating, so the mantle would have been warming up rather than cooling
down. If that were true, then it would never have been able to cool to the neutral
point anyway - a paradox. The only way out of the paradox is to assume that some
other heat removal mechanism could operate. It is possible to envisage different
behaviour of the cool thermal boundary layer, and hence a different tectonic regime.
Discussion of such possibilities will be deferred to Section 9.6.
