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VERCHOVSKY, SEPHTON: NOBLE GASES IN METEORITES
SOME POSSIBLE SOURCE ENVIRONMENTS/PROCESSES FOR NOBLE GASES IN METEORITES
2 (a): Carbon stars (low-mass AGB stars in which
atmosphere ratio of C/O is >1) such as IRAS
05316+1757. (Atlas image/2MASS)
(b): Supernovae (generated by an explosion of a
massive star) such as that which produced the
remnant SNR0103-72.6. (NASA)
(c): Novae (produced when a star ejects some of its
material to form a luminous cloud) such as is seen
for Nova Cygni 1992. (NASA)
A noble record
ABSTRACT
Meteorites contain unusual mixtures of noble gases that reflect contributions from
several extraterrestrial environments. Processes that predate the formation of the solar
system – such as condensation in stellar atmospheres and violent supernovae explosions
– have introduced noble gases into interstellar grains that include diamonds, silicon
carbide and graphite. The solar wind has implanted solar mixtures of gas into the
surfaces of asteroids from which meteorites originate. An enigmatic carrier of planetary
noble gases remains unidentified. Cosmogenic and radiogenic noble gases reveal the
timing of events during solar system formation and the eventual break-up of the
asteroidal meteorite parent body.
T
he noble gases – helium, neon, argon,
krypton and xenon – have chemical and
physical properties that make them distinct from all other elements. Their most notable
property is a lack of chemical activity in comparison to other elements; they tend not to combine easily with other elements, for example. Yet
these properties ensure that noble gases are an
important source of information for extraterrestrial investigations. That very lack of chemical
activity means that they can preserve features of
the early solar system that are obliterated by
reactions among other elements. The relative
abundances of noble gases are different for many
objects in the cosmos (figure 1) and measurements in meteorites can therefore give an insight
into their origins, as well as into extraterrestrial
processes they may have undergone since. The
rate at which noble gases physically melt, boil,
diffuse, dissolve and adsorb varies systematically
2.12
10 9
10 7
10 5
103
101
10 –1
10 –3
10 –5
10 –7
10 –9
M/10 6 Si
Alexander B Verchovsky and Mark A Sephton review the origins
and significance of noble gases in meteorites, focusing on what
this unique record reveals about the early solar system.
with their mass and can be used to model both
their original abundances and the processes that
changed those abundances over time.
Noble gas distribution is usually expressed in
terms of the relative abundances of the different
elements. Equally or even more important is
their isotope abundance, because this reflects a
wide range of processes, including nuclear reactions in the hearts of stars and the self-destructive process of radioactive decay. The noble gas
content – elemental and isotopic – of ancient
meteorites reveals a cornucopia of contributing
environments. In this account, we will follow
the noble gas evidence in meteorites from the
early solar system and show how it illuminates
different aspect of its history.
Noble gas origins
The first noble gas produced in the cosmos was
helium, created seconds after the Big Bang as
Sun
Venus
meteorites (Q)
Earth
Mars
4He 20Ne 36 Ar
84Kr
132 Xe
1: Abundances of noble gases (per 106 atoms
of Si) in the Sun, atmospheres of terrestrial
planets and meteorites (Q component).
soon as temperatures dropped low enough to
allow any neutrons and protons to combine. At
that point helium made up around 25% of matter with hydrogen accounting for almost all of
the rest. Around a billion years later matter
aggregated into clumps to form galaxies where
the first-generation stars formed, using primordial hydrogen and helium as fuel. The original
Big Bang helium was joined by helium generated
in stars from the process of hydrogen burning –
the fusion of four hydrogen nuclei (single protons) into a single helium nucleus (two protons
and two neutrons). Hydrogen burning is the
most common nuclear process in stars, and the
ultimate source of their luminosity. Over time,
hydrogen burning is making the cosmos poorer
in hydrogen and richer in helium. Many stars
end their lives by expelling their matter into
space and this material is then involved in the
formation of a new generation of stars. ConseA&G • April 2005 • Vol. 46
VERCHOVSKY, SEPHTON: NOBLE GASES IN METEORITES
(d): The solar wind (a flow of gas and energetic
charged particles) originating from our Sun. (NASA)
(e): Products from all of (a) to (d), and those from
other environments/processes, are represented in
primitive asteroids. (NASA)
quently, younger generations of stars are
increasingly rich in helium and the heavier elements. The relative amount of elements heavier
than helium in a star is described as its “metallicity” and is an expression of both star age and
the number of stellar cycles the constituent matter has experienced. Further processing in stars
produced increasing amounts of heavier noble
gases. The noble gas mixtures in meteorites are a
combination of products from the Big Bang and
subsequent star chemistry.
When stars end their lives in supernova explosions, they eject small micrometre sized (or less)
particles that join the interstellar medium and can
become part of a dense molecular cloud. Such
grains in one particular interstellar cloud were
caught up in the developing solar nebula. Some
of the grains survived formation of the Sun and
avoided destruction by geological activity on the
planets by being accreted into relatively small
asteroid-size bodies. Fragments of asteroids that
fall to Earth as meteorites provide us with tangible
samples of this star dust. There have been several
presolar grains discovered so far (Zinner 1998,
Nittler 2003, Anders and Zinner 1993), and some
contain noble gases whose isotopic compositions
are significantly different from what is normally
observed in the solar system. These anomalous
isotopic compositions were the key evidence for
their extrasolar origin and the main feature that
allowed identification of the grains in painstaking laboratory isolation procedures.
Nanometre-sized diamonds (average grain size
~3 nm) were the first presolar gains identified
(Lewis et al. 1987) and they are the most abundant presolar material in meteorites, at up to
1500 ppm. But their origin is the least well understood among the known types of presolar grains.
Xenon isotopes suggest that they originate from
mixing of different layers of an explosive supernova (Ott 1993). Yet correlation between isotopic compositions of noble gases, carbon and
nitrogen with grain size of the diamonds indicates that they consist of several populations
formed in different stellar environments (VerA&G • April 2005 • Vol. 46
chovsky et al. 1998). Diamonds, it seems, may
be ubiquitous components of interstellar space.
Krypton and xenon in presolar silicon carbide
grains (typical grain size from submicrometre to
a few micrometres) point directly to a provenance in the atmospheres of carbon stars (lowmass Asymptotic Giant Branch stars in which
atmosphere ratio of C/O is >1) in which carbonaceous grains condense. The carbon star origin of silicon carbide grains has been supported
by their detection in these environments by spectroscopic observations.
In contrast to the relatively straightforward
provenance of presolar silicon carbide, noble
gases in presolar graphite reveals a more complicated history (Zinner 1998). As well as a carbon
star component similar to that for silicon carbide,
presolar graphite contains neon that has decayed
from a short-lived isotope of sodium with a half
life of 2.6 years. Its origin requires a quick explosive supernovae. Isotopic compositions of other
elements – such as krypton – in the graphite
grains also point to diverse stellar sources,
including carbon stars, novae and supernovae
(Zinner 1998). Like diamonds, graphite appears
to be widespread in cosmic environments.
Solar noble gases
The most abundant noble gases in the solar system are those in the Sun. In meteorites the distinctive patterns of solar noble gases are rare,
depleted by many orders of magnitude compared to their abundance in the Sun. The small
amounts that are present were implanted into
the surfaces of asteroids by the solar wind. The
process that separated solids – the meteorites –
from the noble gases of solar composition must
have occurred very early in the history of the
solar system and appears to be associated with
the violent solar winds common during the tempestuous T-Tauri stage of the early Sun (Kallenbach et al. 2003). Yet the element abundance of
noble gases in meteorites is usually different
from that observed in the solar wind because
lighter noble gases are preferentially lost relative
(f): Meteorites such as Murchison are fragments of
primitive asteroids that have fallen naturally to
Earth.
to heavier ones during metamorphism (mineral
changes resulting from heat and pressure) on the
parent asteroid.
Planetary noble gases
The term “planetary” was first used to describe
mixtures of noble gases in an attempt to distinguish between the solar noble gases and those
observed in the Earth’s atmosphere. The latter
has a significant (several orders of magnitude)
depletion of light noble gases compared to the
solar component. What was formerly called the
planetary component in meteorites initially bore
a superficial resemblance to the mixtures seen in
the Earth’s atmosphere, but later turned out to
be a mixture of several components including
those from presolar grains. It was during the
separation of presolar grains in the laboratory
that a phase was identified that appeared to host
a very high concentration of planetary noble
gases. This was named phase Q, for quintessence (Lewis et al. 1975), and was found to be a
separate carbonaceous phase comprising less
than 0.03% of the total meteoritic mass. Q
accounts for all of the noble gases in meteorites
that do not belong to presolar grains, implanted
solar wind, or matter of cosmogenic or radiogenic origin (the latter two discussed below).
To this day, the identification of Q remains elusive and only its eventual isolation in pure form
and its subsequent chemical characterization
will provide an answer to the questions about
both its provenance and associated planetary
noble gases (Verchovsky et al. 2002).
Cosmogenic noble gases
Cosmogenic isotopes are formed in meteorites
as a result of the interaction between galactic
cosmic rays – high-energy protons – and meteoritic matter (Wieler 2002). The protons break
atomic nuclei in spallation reactions to create
new lighter ones, as well as inducing nuclear
reactions with secondary neutrons that also
form new nuclides. Among the many isotopes
formed in this way only noble gases show sig2.13
VERCHOVSKY, SEPHTON: NOBLE GASES IN METEORITES
Table 1: Radiogenic
noble gas isotopes and
their parents
Parent
isotope
Half-life
(years)
Daughter
isotope(s)
40
1.277×109
4.468×109
7.038×108
1.405×1010
1.57×107
8.08×107
40
K
U
235
U
232
Th
129
I
244
Pu
238
Ar
He
4
He
4
He
129
Xe
4
131,132,134,136
Xe
(a)
(b)
3: An example of a presolar grain and its stellar source. (a) SiC grain from Murchison meteorite.
(b) Planetary nebula NGC6751. (From L R Nittler 2003 Earth and Planet. Sci. Lett. 209 259)
nificant shifts in the ratios of different isotopes,
because their background concentrations in
meteorites are extremely low. Cosmic rays penetrate to about one metre below the surface of
an object and, therefore, cosmogenic noble
gases are found in meteorites representing either
the very surface of their parent bodies or small
fragments that resulted from asteroid collisions.
The main implication of the existence of cosmogenic isotopes is that it makes it possible to calculate exposure ages of meteorites, i.e. the time
between separation of a small, metre-size, fragment from its parent body and its fall to the
Earth’s surface as a meteorite (Wieler 2002).
The exposure age is calculated from the production rate of noble gas isotopes by cosmic rays,
based on the chemical composition of meteorites and the assumption of constant cosmic
ray intensity throughout time.
The production rate depends also on the
geometry of the meteorite body before it entered
Earth’s atmosphere and the position of the sample inside the meteorite.
Exposure ages of meteorites of the same chemical group can reveal complex histories. For
example, plotting the exposure ages on agedistribution diagrams can often reveal several
peaks, indicating that many fragments were
formed at these times, suggesting the ages of
major collision events in the asteroid. For
instance, one of the largest meteorite groups, the
H-chondrites (meteorites with relatively high
metallic iron content), show signs of three large
collision events that happened 7, 24 and 33 million years ago.
Radiogenic noble gases
Radiogenic noble gases are formed as a result of
the radioactive decay of unstable isotopes. The
most common radiogenic noble gases and their
parent isotopes are listed in table 1. Some of
them, produced from long-lived parents, are
generated in meteorites throughout the life of
the solar system while others, originating from
short-lived precursors, are produced only dur2.14
ing the very early stages of the formation of the
solar system. An important factor is the abundance of the parent isotope in meteorites, which
in turn determines how widespread is the corresponding radiogenic product.
The radioactive isotope of potassium (40K) is
by far the most abundant parent isotope in
meteorites, and its decay product, the radiogenic
noble gas argon (40Ar), is also relatively abundant. Because almost no 40Ar was incorporated
into asteroids when they formed, nearly all of
the 40Ar present in meteorites is assumed to be
radiogenic. Because the decay rate of 40K is well
known, comparing the ratio of 40K/ 40Ar in a
meteorite can be used to determine when the
object formed. The radiogenic noble gases,
therefore, are valuable for establishing a solar
system chronology (Swindle 2002). Events that
can be dated by potassium–argon (K–Ar) methods include mineral formation or recrystallization as well as various metamorphic alteration
processes on the meteorite parent bodies. One
of the examples of a successful use of the
method is related to dating of lunar rocks that
established that a major catastrophic impact
crater formation event occurred early in the
solar system around 3.8–4 Ga, the so-called late
heavy bombardment (Swindle 2002).
Radiogenic products of short-lived and therefore now extinct isotopes have important implications for chronology of the very early solar
system (Swindle 2002). For example, the presence of 129Xe implies the existence of its parent,
radioactive iodine (129I), in the early solar system. 129I is produced by nucleosynthesis in stars,
but it is so short-lived (half life of 15.7 Ma) that
the time interval between inputs of nucleosynthetic products, including this, into the solar
nebular and formation of the first solids must be
short, estimated at ~100 Ma. As 129I decayed, its
decrease relative to the background nonradiogenic 127I was locked up in solids as they
formed and the ratio can be used for relative
dating of the first 50 Ma of solar system history
with a resolution of <1 Ma. The method has
been used for establishing formation sequences
of the major types of meteoritic parent bodies
and characteristic inclusions found within them
such as enigmatic spherical silicate blobs (chondrules) and refractory objects enriched with Ca
and Al (calcium aluminium inclusions; CAIs)
the origins of which are hotly debated.
Conclusions
Noble gases found in meteorites are valuable
probes of extraterrestrial environments. Noble
gas abundances and isotopic compositions
reflect various processes including nucleosynthesis in stars that existed before our solar
system was born, radioactive decay, and interaction with cosmic rays and solar winds. Their
ability to act as fingerprints for specific sources
and indicators of the means by which solid
objects formed, make noble gases vital to constraining the environments that contributed raw
materials to our solar system and deducing the
sequence of events that followed. ●
Alexander B Verchovsky, PSSRI, Open University,
Milton Keynes. Mark A Sephton, Dept of Earth
Sciences and Engineering, Imperial College London.
References
Anders E and Zinner E 1993 Meteoritics 28 490.
Kallenbach R, Robert F, Geiss J, Herbst E, Lammer H,
Marty B, Millar T J, Ott U, Pepin R O 2003 Space
Science Reviews 106 58.
Lewis R S, Srinivasan B and Anders E 1975 Science
190 1251.
Lewis R S, Ming T, Wacker J F, Anders E, Steel E 1987
Nature 326 160.
Nittler L R 2003 Earth and Planetary Science Letters 209
16.
Ott U 1993 Nature 364 25.
Swindle T D 2002 Review in Mineralogy and
Geochemistry 47 101.
Verchovsky A B, Fisenko A V, Semjonova L F, Wright I
P, Lee M R, Pillinger C T 1998 Science 281 1165.
Verchovsky A B, Sephton M A, Wright I P, Pillinger C T
2002 Earth and Planetary Science Letters 199 243.
Wieler R 2002 Review in Mineralogy and Geochemistry
47 21.
Zinner E 1998 Annual Review of Earth and Planetary
Sciences 26 42.
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