Geology Reference
In-Depth Information
1700.
1700.
400.
400.
400.
400.
(a)
(b)
Heat loss
Heat
generation
Mean mantle
No basalt
(F9.2)
Upper mantle
Plate velocity (cm/yr)
1200.
1200.
0.
0.
0.
0.
0
0
4500
4500
0
0
0
0
4500
4500
4500
4500
Time (Myr)
Time (Myr)
Figure 9.6. Thermal evolution of the model depicted in Figure 9.5. The mean
mantle temperature from a model with no basaltic tracers (Figure 9.2) is included
for comparison.
using tracers that each carry a small mass corresponding to the negative buoyancy
of a piece of oceanic crust. Details of the model, including how the basalt tracers
are implemented, are given in Section B.4.
As tracers rise into a defined melting zone, they are removed and placed in
a thin oceanic crust. This simulates melting under mid-ocean ridges and results
in the formation of compositionally heterogeneous lithosphere. This subducted
lithosphere can be seen in the right panels as the white (empty) zones bordered by
a thin sheet of concentrated tracers.
The figure shows that some of the tracers settle to the bottom of the model and
accumulate there. Tracers are also removed from accumulations and mixed back
into the mantle interior by warm upwellings. Over time, a relatively steady number
of tracers are located in the bottom accumulations, though the number declines
slowly as the model evolves. The zone where the tracers accumulate has a higher
density that tends to resist being mixed up into the mantle interior. Because of
its limited circulation, heat accumulates within it, and it becomes several hundred
degrees hotter than the mantle above (left panels). This zone of dense but hot mantle
raises the average temperature of the mantle, as can be seen in Figure 9.6, which
shows the thermal evolution of this model. The mean temperature of this model is
significantly higher than for the model in Figure 9.2 that lacked basaltic tracers.
The tendency of subducted oceanic crust to accumulate at the base of the mantle
was first demonstrated by Christensen and Hofmann in 1994 [122]. The improve-
ment in computer power since then permits models that have greater realism,
including running at the full mantle Rayleigh number and evolving for the full age
of the Earth.
The basaltic accumulations have significant structure within them (Figure 9.5,
right panels). There tends to be a thin zone, 50-100 km thick, with concentrations
two or more times the average. Above that zone are regions of lesser and vari-
able concentration that extend upwards for hundreds of kilometres. The shapes of
