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of rocks or magmas thought to come from the
mantle is greater than the atmospheric ratio is
another paradox for standard models of man-
tle degassing and atmospheric evolution because
20
Ne and
22
Ne are essentially primordial in the
sense that
20
Ne is stable and only very small
amounts of
22
Ne are produced in the Earth. Their
ratio is thus expected to be uniform throughout
the Earth, if the solar system reservoir is uniform
and invariant with time.
Non-atmospheric neon found in MORB, OIB,
volcanic gases and diamonds is enriched in
20
Ne
and nucleogenic
21
Ne, relative to
22
Ne. This have
been attributed to primordial components, possi-
bly solar neon. Basalt glasses from Hawaii have
a range of Ne isotope ratios, stretching from
atmospheric to
20
Ne-enriched compositions that
approaches the solar wind composition. These
signatures are similar to those in cosmic dust
particles accumulated in deep-sea sediments.
These dust particles become impregnated with
Ne from the solar wind during their exposure in
space. If these particles can survive the
noble
gas subduction barrier
and deliver cos-
mic (solar) Ne to the mantle this could explain
the ratios of submarine glasses without having
to invoke ancient primordial Ne in the Earth, or
an undegassed lower mantle reservoir.
12
IDP
SOLAR
11
10
Air
9
0.030
0.032
0.034
0.036
21
Ne
22
Ne
/
Fig. 16.4
'Three-isotope plot.' Some ocean islands appear
to be mixtures of air and solar or interplanetary dust
particles (IDP). MORB has relatively more
21
Ne suggesting
that its source is older or has higher U than the OIB sources.
21
Ne/
22
Ne (Figure 16.4). Neon isotope ratios are
more susceptible to atmospheric contamination
than helium isotope ratios because Ne is more
abundant in the atmosphere. Most basalts plot
on what appear to be mixing lines between atmo-
spheric values (
10), and solar wind --- SW or
interplanetary dust particle --- IDP values (13.7)
of
20
Ne/
22
Ne. The corresponding values for the
21
Ne/
22
Ne ratios are 0.029 (air),
∼
>
0.04 (OIB) and
>
0.06 (MORB).
The mixing line for MORB extends to greater
values of
21
Ne/
22
Ne than the mixing line for OIB,
indicating more
21
Ne in-growth in MORB than
in OIB. This is analogous to the higher
4
He/
3
He
ratios found in some MORB samples compared
with the extremes found in some OIB samples.
The
20
Ne/
22
Ne ratios in mantle-derived rocks,
both MORB and OIB, extend from atmospheric
to the solar or IDP ratio. The identification of
solar-like ('primordial' or IDP) Ne isotopic ratios
in some OIB and MORB samples implies that solar
neon trapped within the Earth has remained vir-
tually unchanged over the past 4.5 Gyr (the stan-
dard 'primordial mantle' model) or, alternatively,
that noble gases have been added to the man-
tle since
Inter-element ratios
Both melting and degassing fractionate the noble
gases, and separate the parent from the daugh-
ter isotopes. Helium and the light noble gases are
more readily retained in magma than the heavy
noble gases during degassing. Thus, the He/Ne
and He/Ar ratios of different basalt types con-
tain information about their degree of degassing.
The He/Ne and He/Ar ratios of MORB are gener-
ally higher than OIB suggesting that MORB is
more degassed than OIB. Nevertheless, [He] is
higher in MORB than in OIB, which suggests
that OIB initially contained less [He] and possibly
less [Ne] than MORB, prior to MORB degassing.
Some of the more noble-gas-rich igneous rocks
are the 'popping rocks' found along the mid-
Atlantic ridge (e.g. Sarda and Graham, 1990; Javoy
and Pineau, 1991; Staudacher
et al.
, 1989). Ordi-
nary MORB appear to be degassed versions of
2 Ga, perhaps from noble-gas-rich sed-
iments or cosmic rays. Some basalts and some
diamonds essentially have pure 'solar' (SW or IDP)
20
Ne/
22
Ne ratios. The fact that the
20
Ne/
22
Ne ratio
∼
