Geoscience Reference
In-Depth Information
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