Meteorites 273
FIGURE 15 Three-isotope plot for stable Ne isotopes in ALH
85151 chondrite. Data for separated mineral grains lie on lines
connecting average Solar Ne (composed of keV/nucleon solar
wind and MeV solar energetic flare particles) with cosmogenic
Ne produced by GeV galactic cosmic rays. Data points in the
upper left of the line represent fine Ne in dust grains exposed on
a regolith surface with constituent Ne being almost entirely solar.
The point at the lower right is from grain interiors (with low
surface-to-volume ratios) whose Ne is nearly all cosmogenic. The
box in the upper left is expanded in the inset to show isotopic
compositions of individual Ne components in meteorites with
low^21 Ne/^22 Ne ratios. Pure^22 Ne (so-called Ne-E) is formed by
radioactive decay of very short-lived (2.60 years)^22 Na in the
protoplanetary nebula or presolar grains. If ALH 85151
contained substantial Ne-E, data points would lie in the
triangular region defined by E, solar and cosmogenic Ne. The
inset depicts isotopic compositions of Ne from solar wind, solar
energetic particles, the terrestrial atmosphere, and an absorbed
presolar, planetary component (Q) that is released when mineral
grains are etched with nitric acid.
meteorites,^40 Ar is mainly radiogenic, deriving from decay
of^40 K(t 1 / 2 = 1 .28 Ga). This monoisotopic^40 Ar compo-
nent limits the use of three-isotope plots for interpreting
the trapped Ar component.
Krypton and Xe systematics are complicated for several
reasons. The Kr and Xe isotopes derive from several nucle-
osynthetic sources, two of which are especially important.
One is now-extinct^129 I, which decayed to produce^129 Xe and
gives chronometric information (cf. Section 6.5). The sec-
ond involved fission of now-extinct^244 Pu which produced
a Xe component with a characteristic fission–yield curve.
In addition to induced and spontaneous U fission products,
different trapped components exist. Kr and Xe in presolar
grains provide almost pure gas from individual nucleosyn-
thetic events.
Each solar system body has its particular formation his-
tory and, thus, its own noble gas isotopic “fingerprint.” Gases
on the Earth, Moon, Venus, and Mars can be distinguished
from each other and from those in chondrites. As discussed
in Section 4.2, glass in the EET A79001 shergottite contains
martian atmospheric gas indicating that it (and the other 31
martian meteorites) formed there.5.2 Noble Gas Components and Mineral Sites
Our brief Ne discussion outlined, in principle, how to dis-
entangle several Ne components from an average mete-
oritic datum. Actually, the situation is more complicated
because each “component” may, in fact, be resolvable into
constituents from specific sources, each with reproducible
isotopic patterns involving more than one noble gas. Inge-
nious laboratory treatments can yield a phase enriched in
one true gaseous constituent from others. These include
investigation of individual grains, selective acid dissolution
of specific minerals, enrichment by mineral density using
heavy liquids, stepwise heating and mass-analysis of gases
evolved in some temperature interval, or some combination
of these steps (and others).5.2.1 INTERSTELLAR GRAINS IN METEORITES
Until about 1970, the solar system was considered “isotopi-
cally homogeneous,” objects in it having formed from a
well-mixed and chemically and isotopically homogenized
primordial nebula. (The later discovery of oxygen isotopic
variations, e.g. Fig. 11, disproved this.) However, even then,
rare samples extracted from meteorites exhibited anoma-
lous contents of, for example, Ne or Xe isotopes. These
anomalies cannot be explained by well-established pro-
cesses like decay of naturally occurring radionuclides, cos-
mic ray interaction with matter, or mass-dependent physical
or chemical fractionation.
These isotopic anomalies, usually orders of magnitude
larger than in other solar system materials, are associated
with very minor mineral phases of primitive chondrites
distributed irregularly in unequilibrated meteorites. These
minerals include diamond, graphite, silicon carbide, and
aluminum oxide, with typical grain-sizes being 1–10μm,
with diamond being much smaller (∼0.002μm). Presolar
SiC grains, at least, follow a power-law mass distribution
dominated by submicron particles, with rare large ones.
These minerals are rare in meteorites (e.g., SiC in the CM
chondrite, Murchison, is about 5 ppm by mass). Figure 16
depicts such an anomaly, the Ba isotopic composition in
presolar SiC separated from Murchison. The data are nor-
malized to terrestrial values of^130 Ba and^132 Ba, the anoma-
lous s- and r-process isotopes (see below) lying far above
the horizontal line.
Since the isotopic composition of these grains differs
wildly from those of ordinary solar system matter, they must