Geology Reference
In-Depth Information
21
Ne is obtained by assuming
in Jupiter's atmosphere [234]. The initial value of
the solar ratio
21
Ne/
22
Ne
0.0328 [180].
If all of the radiogenic
4
He were retained in the mantle, this would imply a
present
4
He/
3
He ratio of about 280 000, triple the observed MORB value. Even
with degassing accounted for, the calculated present ratio is still about 150 000,
because of the slow degassing rates inferred above.
However, it is plausible that the degassing rate was higher during the first 0.5 Gyr
or so: during this time the mantle temperature drops rapidly from a high post-
accretion value to a value sustained by radiogenic heating, according to thermal
evolution calculations (Chapter 9), and the rate of mantle overturn would be much
higher than is implicit in Eq. (10.9). Because
4
He is generated at a much higher
rate early on, proportionately more of it is removed by the early degassing and the
effect is to reduce later values of
4
He/
3
He. Argon values behave similarly.
This early degassing phase was crudely approximated in these calculations
by increasing the degassing rate for the first 0.5 Gyr by a factor of 3 over that
inferred from Eq. (10.9). This is sufficient to bring
4
He to reasonable present
values. A little trial and error then established that multiplying the preliminary
initial concentrations of primordial species (
3
He, initial
4
He,
22
Ne, solar
21
Ne,
40
Ar) by a factor of 4 over those inferred from Eq. (10.9) compensated for the extra
early loss of these species. The resulting initial values are included in Table 10.6(a).
These values are still plausible relative to estimates of initial abundances based on
solar ratios [180, 234].
The consequent evolution of helium, neon and argon isotopes in the MORB
source is shown in Figure 10.25 (solid curves). The resulting present MORB-
source values of all species are given in Table 10.6(a), and the corresponding
ratios are given in Table 10.6(b). Good agreement with observed ratios has been
achieved, and present concentrations are near the ranges given in Table 10.5(b),
except for argon, which is low, and will be discussed later. The early phase of more
rapid degassing is equivalent to the inference previously made that much of the
mantle's argon must have degassed within the first 1 Gyr of Earth history [214,
233]. These results demonstrate that a relatively simple and plausible model based
on the hybrid pyroxenite hypothesis is capable of reproducing the MORB-source
noble gas observations reasonably well.
A stronger test of the hypothesis comes from calculating the OIB concentrations.
Following the discussion in Section 10.8.1, in which it was noted that the OIB
material has a longer residence time and consequently degasses more slowly, an
OIB evolution has been calculated simply by assuming that the degassing rate is
60% of the MORB-source degassing rate, with all other parameters the same. This
degassing rate would correspond to that component of OIB material with a final age
of about 2.5 Ga, compared with a mean MORB age of about 1.5 Ga. (Recall that
=
