Geoscience Reference
In-Depth Information
'Traditionl' MORB Range
100
DEPTH < 2.5 km
All near-ridge data
68%
of data
80
60
40
20
0
2
4
6
8
10
12
14
16
18
20
3
He
4
He
R
R
A
Table 16.2
Helium isotope data, as com-
piled over the years. The year of the compi-
lation is also given
Fig. 16.1
Histogram of helium isotopes measured in
samples along the global spreading ridge system, including
new ridges, back-arc basins and near-ridge seamounts. In
most compilations much of the right-hand side of the
distribution is missing since values higher than about 9
R
/
R
a
are filtered out. High values are attributed to plume
contamination. Anomalous sections of the global ridge system
are also avoided. The traditional MORB range, after filtering
the data, is from 7---9
R
/
R
A
. High
R
values are found in
depleted components of basalts that have been identified as
the most common mantle component, possibly peridotite.
The high
R/R
a carriers may be peridotites or olivines with low
U/He ratios. The helium may be acquired from wall-rocks, by
ascending magmas.
R
/
R
A
S.D.
Year
All ridges
9.06
3.26
'Filtered' data
8.4
0.36
1982
8
2
1986
8.2
0.2
1991
8.3
0.3
1996
8.58
1.81
2000
8.67
1.88
2000
8.67
1.88
2000
spreading ridge system, and can only be sam-
pled by smaller scale processes, such as individ-
uallavaflowsonislands.Estimatesof
R
/
R
A
for
OIB and for filtered ridge samples are given in
Table 16.2.
Ratios that are sensitive to degassing of
magma, and atmospheric contamination, such
as He/Ne and He/Ar, show a clear distinction
between MORB and OIB. The
distribution
of the lighter noble gases -- helium,
argon and neon -- in mantle magmas
suggests that degassing of magmas near the
Earth's surface, trapping of volatiles, and possi-
bly more recent atmospheric contamination, may
be involved in the differences [
mantleplumes
].
The heavier noble gases in mantle mag-
mas may entirely be due to atmospheric
and seawater contamination
.
8.75
2.14
2002
Back-arc basin basalts
8.4
3.2
2002
Ocean island basalts
7.67
3.68
1995
9
3.9
2001
Seamounts
6.58
1.7
9.77
1.4
S.D., standard deviation.
Most workers assume a lower-mantle plume
source for high-
R
magmas --- and a very high
[
3
He] --- which must be isolated and preserved
against convective homogenization with the
more radiogenic upper mantle. A 'primordial' or
