Geology Reference
In-Depth Information
Table 10.4.
Ages (in Ga) from numerical models in specified
depth ranges [156].
Case
1
2
3
4
Mean age, MORB (model crust)
1.63
1.47
1.61
1.50
Mean age, 'OIB' (0-20 km)
2.97
2.54
2.59
2.45
Mean basaltic ages of deep layers:
0-50 km
3.01
2.54
2.59
2.48
50-100 km
3.03
2.54
2.58
2.53
100-200 km
2.99
2.36
2.55
2.52
200-500 km
2.68
2.21
2.25
2.38
500-1000 km
2.27
2.13
2.23
2.09
from the rate at which the tracer concentration increases in the bottom zone, and
their age distributions and mean ages were calculated, the latter being included in
Figure 10.21(a).
To account for the lower observed ages of OIBs compared with model 'OIB'
ages, Huang assumed that entrainment into a plume is inversely proportional to
tracer concentration, as a proxy for density and following the theory of Sleep
[223]. He estimated that in the conditions of the model a plume would entrain
about 14% of tracers from the accumulation, so the OIB age would be 1.98 Ga,
relative to model MORB and 'OIB' ages of 1.90 and 3.02, respectively. This value
is sufficiently close to the model MORB age that the difference would not be very
noticeable.
These results indicate that the models are reasonably consistent with observed
OIB ages, though more accurate tests are desirable.
10.7.5 Heterogeneities remelted, not homogenised
There is a considerable literature devoted to modelling the rate at which chemical
heterogeneities are stirred by convective flows (e.g. [199, 201]) as briefly discussed
in Section 10.4.2. The motivation for this work was the idea that the apparent
lead age reflects the time it takes for heterogeneities to be stirred down to a
thickness at which it is effectively homogenised. This could be either the scale at
which solid-state diffusion chemically equilibrates the material, which may be only
centimetre scales [197], or, more likely, a scale at which melting and melt extraction
will homogenise the chemical signals, which could be kilometres or larger. The
challenge was to show how heterogeneities could survive mantle mixing for billions
of years, which at first seemed intuitively implausible to many, as discussed earlier.
