Geology Reference
In-Depth Information
Table 10.5.
Noble gas abundances. Abundances in atoms per gram. (Plausible
ranges in parentheses.)
4
He
3
He
21
Ne
22
Ne
40
Ar
36
Ar
(10
13
)
8
)
6
)
8
)
(10
13
)
8
)
(
(
(a) Conventional estimates, MORB source
Low gas
7
8
6
0.9
2
7
(5-9)
(6-10)
(4-8)
(0.6-1.1)
(0.5-3)
(2-10)
(b) New estimates, MORB source
Medium gas
14
16
12
1.8
15
∼
30?
High gas
20
24
18
3
22
∼
70?
(c) 'Undegassed lower mantle'
Porcelli and Wasserburg [181]
100
500
570
150
56
600
that these isotopes reflect the amount generated over a residence time of about
1.4 Gyr in their steady-state upper mantle, and use a production ratio of about 3.6,
based on the usual K/U ratio of 1.3
10
4
[184]. The residence times given in
Section 10.7.2 are comparable, so we can use their production ratio. Thus we get
40
Ar
×
10
8
(Table 10.5(a)). Ballentine
et al.
[229] use a production ratio of 1.55, appropriate
for 4.5 Gyr, which probably overestimates the
40
Ar abundance.
Neon follows by similar arguments, from a production ratio for
10
13
and, with the usual
40
Ar/
36
Ar ratio of about 30 000,
36
Ar
=
2
×
=
7
×
21
Ne/
4
He of
10
−
7
[230] and the
21
Ne/
22
Ne MORB ratio of 0.07 (taking account also of
the initial solar ratio of 0.0328).
Abundances in the lower mantle of the old two-layer mantle were built around
the assumption that it retains all of the radiogenic isotopes generated within it over
4.5 Gyr. Values from Porcelli and Wasserburg [181] are reproduced in Table 10.5(c)
as representative of this approach.
0.45
×
10.8.3 New concentration estimates
The abundances of noble gases in the MORB source might not be as well deter-
mined as the above estimates imply. Ballentine
et al.
[229] question whether the
∼
1000 year timescale implicit in ocean sampling of the helium flux from mid-
ocean ridges is long enough to yield a reliable average, given possible fluctuations
in plate tectonic processes on longer timescales. They cite two other estimates that
give higher concentrations of helium. The gas-rich 'popping rock' [231, 232] has
a
3
He concentration of 2.7
10
10
atoms/g. With an assumed 10% partial melting,
this implies a minimum concentration in the MORB source of 27
×
10
8
atoms/g.
×
