Geology Reference
In-Depth Information
26 TW
27 TW
28 TW
(c)
(a)
(b)
3.5 TW
12 TW
25 TW
+ 3 TW cooling
25 TW
+ 6 TW cooling
+ 2 TW radiogenic
+ 10 TW radiogenic
+ 9 TW cooling
19 TW
+ 13 TW radiogenic
+ 6 TW cooling
19 TW
+ 10 TW radiogenic
+ 3 TW cooling
7 TW
7 TW
+ 1 TW radiogenic
6 TW core
6 TW core
6 TW core
Figure 8.7. Heat budgets of three mantle models [118]. (a) The whole-mantle
model, with a thin D
layer at the bottom. (b) The deep layer model, containing
about half the heat-generating elements. (c) The old primitive lower-mantle model,
containing about two-thirds of the heat-generating elements. After Davies [118].
Copyright by the American Geophysical Union.
On the other hand, the main reason for proposing the thick layer depicted in
Figure 8.6(b) was to accommodate extra trace elements [108]. Meteorite studies
provide estimates of the Earth's complement of many refractory trace elements,
such as uranium and thorium. Many of these elements tend to be concentrated in
the continental crust, but the continental crust accounts for only about half of the
total. A major rationale for the earlier primitive lower-mantle model was that it
accounted for the balance of these trace elements [107]. When that model became
untenable, there seemed to be nowhere these elements could be accommodated, so
the deeper layer was proposed as a place that could accommodate them without
obviously contradicting geophysical evidence. However, this model does contradict
important evidence.
If the deep layer contains about half of the Earth's U, Th and K, the main
heat sources, then a large fraction, one-third to one-half, of the Earth's radiogenic
heat budget would be generated within it. A reasonable estimate is that about
13 TW would be added to the core heat flow (6 TW; Figure 8.7(b)). This heat
would conduct through the top interface of the deep layer, forming a thermal
boundary layer that would generate plumes. By the time the plumes rose the
2000 km to the shallow mantle, their heat flow would have attenuated to about
12 TW, much greater than the 3.5 TW inferred from the hotspot swells. In other
words, this model implies that hotspot swells should be 3-4 times more prominent
than they are (Figure 8.6(b)). They should be quite obvious in Figure 2.4, instead
of being rather subtle. The hotspot topography should be nearly half the size of
the mid-ocean ridge topography, which is associated with the transport of about
30 TW.
This argument applies to any deep layer that contains a significant proportion
of the Earth's radiogenic heat sources. It applies to the old primitive lower-mantle
