Geology Reference
In-Depth Information
40
MORB
Loihi
Kilauea
Reunion
Tahiti
Samoa
30
20
10
ATM
5000 15000 25000
0
40
Ar/
36
Ar
Figure 10.4. Helium and argon isotopes from MORBs and from various ocean
island groups. The helium data are normalised to the atmospheric ratio (
R
A
=
10
−
6
, [180]), denoted by the point ATM (
40
Ar/
36
Ar
1.4
300). After Porcelli
and Wasserburg [181]. Copyright Elsevier Science. Reprinted with permission.
×
∼
Important information comes from observations of helium, neon and argon in
mantle-derived rocks. These gases occur in minute concentrations and the con-
centrations are subject to near-surface processes, so concentrations do not give a
very robust basis for interpretation. Therefore most attention is focused on iso-
topic ratios. Figure 10.4 summarises observations of helium and argon.
4
He is
produced by radioactive decay of U and Th, and
40
Ar is produced by decay of
40
K.
Unfortunately, helium is usually presented with the reference isotope,
3
He, in the
numerator,
R
3
He/
4
He, contrary to the convention for other elements.
Figure 10.4 shows that argon ratios in the mantle are generally much more
radiogenic than those in the atmosphere, whereas the helium ratios are generally
much less radiogenic than those in the atmosphere. For argon, the main reason for
this is that argon accumulates in the atmosphere and comprises about 1% of the
atmosphere, so the atmospheric abundance of
36
Ar is relatively high. On the other
hand helium continuously escapes from the top of the atmosphere so that helium
is a minor part of the atmosphere. The abundance of U and Th in the continental
crust means that the crust contributes substantial
4
He to the atmosphere, making it
more radiogenic.
The helium ratios for MORBs are relatively uniform, around 8
=
1. In contrast
the OIB helium is quite variable, ranging from about 4 to over 30. Thus some
OIB helium is less radiogenic than MORB helium (i.e. above 8) and some is more
radiogenic.
±
