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Fig. 16.5 Plot showing relation
between helium and argon isotopes
in MORB and OIB and the effects of
degassing and 'contamination' with
air and older CO 2 -rich vesicles. The
relative rates of production of 4 He
and 40 Ar are shown by the
horizontal dashed line (after
Anderson, 2000a,b).
1000
MORB
Degassed
Magmas
10
OIB
Production
Popping
Rock
Gas
XENOLITHS
Undegassed
MORB
0.1
OIB
Air
10 8
10 6
10 4
4 He (cc
/
g)
4 He
these popping rocks, and even some of these gas-
rich rocks appear to have lost some gas.
Histogram of 3 He/ 22 Ne and 4 He/ 21 Ne in MORB
and OIB show that MORB and OIB are frac-
tionated in opposite directions relative to the
solar ratio and the production ratios, respectively
( mantleplumes noble gases ).
Aplotof 4 He/ 40 Ar vs. [ 4 He] and related inter-
element plots (e.g. 3 He/ 21 Ne vs. [He]) shows a con-
tinuous transition from MORB to OIB to air (e.g.
Figure 16.5). The MORB to OIB trends could be
interpreted as air-contamination of noble gases
degassed from MORB and trapped in vesicles or
cumulates.
So-called primitive ratios of noble gas isotopes
may be preserved by ancient separation of He and
Ne from U
(
7.7)
and
the
radiogenic
to
nucleogenic
21 Ne
10 7 ). The linear cor-
relation implies that the elemental fractiona-
tion event (perhaps degassing of magma), which
enriched MORB glasses in [He] with respect to
[Ne], is recent, otherwise ingrowth of radiogenic
4 He and nucleogenic 21 Newouldhavesystem-
atically shifted the data points from the cor-
relation line. Basalts exhibit positive correla-
tions between 3 He/ 22 Ne and [ 3 He], and between
4 He/ 21 Ne and [ 4 He]. The
production ratio (2
.
2
×
3 He/ 22 Ne,
4 He/ 21 Ne and
4 He/ 40 Ar
ratios
in
MORB
glasses
are
system-
3 He/ 22 Ne
atically
higher
than
the
primordial
4 He /nucleogenic
21 Ne
ratio or the radiogenic
4 He /radiogenic
40 Ar production
and radiogenic
ratios.
The production rate of 21 Ne parallels that
of 4 He since they both result from the decay
of U
+
Th and storage of the gas in low-
U
Th environments such as depleted lithosphere
or olivine cumulates. In this way, the radiogenic
isotopes 4 He and 21 Ne are not added to the gas
and old isotopic ratios can be 'frozen in.' This con-
trasts with the situation in more fertile materials
such as MORBs or undepleted peridotite, where
U
+
Th (Table 16.2). The 3-isotope plot show-
ing OIB (Loihi) as forming one trend from atmo-
sphere to solar and MORB as forming another
(Figure 16.3) can be interpreted as a rotation of
the OIB trend by nucleogenic ingrowth, i.e. MORB
source is aged OIB source. In this simple model
MORB
+
Th contents are relatively high and contin-
ued radiogenic and nucleogenic ingrowth occurs.
Usually, high R are interpreted as high- 3 He but
they could, with equal validity, be interpreted as
low 4 He or from a low-U source.
The He/Ne ratios observed in mantle xeno-
liths and basaltic glasses vary by orders of mag-
nitude
+
time. OIB and MORB could have
evolved from a common parent at some time in
the past. Old MORB gases (from ancient degassed
MORB) stored in a depleted refractory host (e.g.
olivine crystals, U
=
OIB
+
Th-poor lithosphere) will have
high 3 He/ 4 He and low He/Ne compared with cur-
rent MORB magmas. One of the mantle 'reser-
voirs' for noble gases may be isolated gas-filled
inclusions or vugs in a peridotite. This 'reservoir'
+
and
define
a
linear
correlation
with
a
slope
of
unity
which
passes
through
the
3 He/ 22 Ne ratio
point defined by the primordial
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