Geology Reference
In-Depth Information
MORB
OIB
(a)
(b)
21
Ne/
22
Ne
21
Ne/
22
Ne
(c)
OIB-
source
MORB-
source
Nucleogenic
addition
Solar
14
12
10
mfl
Air
Planetary
8
0.02
0.03
0.04
0.05
0.06
0.07
0.08
21
Ne/
22
Ne
Figure 10.5. Neon isotope ratios for MORBs and OIBs. SW: solar wind. Ne-B:
'neon-B' component in meteorites. (a) MORB data, (b) OIB data, (c) interpreta-
tion. ATM: atmosphere. mfl: mass-fractionation line. (a) and (b) Modified from
Porcelli and Wasserburg [181]. Copyright Elsevier Science. Reprinted with per-
mission. (c) After McDougall and Honda [180].
The argon data show that both MORB and OIB ratios are quite variable, but the
MORB data range to higher values than OIB data. The highly radiogenic values
of both types are inferred to reflect the fact that argon is very incompatible in the
mantle and most of it has escaped, leaving very low concentrations of
36
Ar. On
the other hand, potassium is relatively compatible, so its abundance has probably
not changed by more than a factor of 2 or so and
40
Ar keeps accumulating in the
mantle at a significant rate.
Neon observations are summarised in Figure 10.5. Three isotopes are observed.
21
Ne is produced in the Earth by the reactions of neutrons, produced mainly by the
decay of U and Th, with other elements. The other two isotopes are not nucleogenic.
