Geology Reference
In-Depth Information
Huang [217] developed a further extension of the sampling theory to account
for numerical models in which the melting depth decreases with time, according to
the decrease in mantle temperature [203, 217]. The processing rate then decreases
with model time, and decreases faster with Earth time. Results show that the degree
of processing (as in Figure 10.21(b)) depends mainly on the early processing rate,
whereas the final tracer residence times (as in Figure 10.21(a)) depend mainly on
the late processing rate.
10.7.4 OIB ages
The 'OIB' ages shown in Figure 10.21(a) are significantly larger than the observed
OIB apparent ages of about 1.8 Gyr. They are in quotes because the sample is,
in retrospect, not really a good simulation of OIB sampling. In the numerical
models the 'OIB' samples were simply taken from a thin zone at the bottom of
the model [203, 215-217], assuming plumes would sample that region. However,
when there are denser accumulations of subducted oceanic crust, plumes will come
from the top or sides of the accumulations. There is considerable structure within
the accumulations (Figures 9.5 and 9.10), with a thinner dense layer overlain by a
more gradational zone generally with much lower concentrations of tracers. It is
plausible that plumes would sample those outer zones rather than the denser zone
at the bottom, and it is also plausible that the dense bottom zone would have larger
residence times.
An indication of this was obtained by Davies [156] from several evolving models
including those of Figures 9.5 and 9.10 (Cases 1 and 2 below). Table 10.4 shows
horizontally averaged ages (residence times) from various depth ranges near the
bottom of the model, compared with the nominal MORB and 'OIB' ages. The
MORB age is from those tracers present in the oceanic crust at the end of the run,
and the 'OIB' age is from the lowest 20 km of the model. Table 10.4 shows that
the mean ages are greatest in the lowest 200 km, and decrease at higher levels. The
ages in the 'piles' extending up to 1000 km are still significantly greater than the
MORB ages.
Thus, for these models to be consistent with observed OIB ages, which do not
seem to be notably larger than MORB ages, plumes would have to incorporate only
a modest fraction of the basal tracers. We should bear in mind, however, that Chase
[222], who was the first to highlight the age significance of the lead isotope plot
(Figure 10.3) in 1981, separated the various OIBs and found apparent ages ranging
from 1 Ga to 2.7 Ga. This suggestion ought to be revisited with more modern data.
Huang [216] further extended the sampling theory to address this in a differ-
ent way, by assuming a population of tracers in the bottom accumulation that
never leave once they have arrived. The proportion of such tracers was estimated
