The cosmic-ray records of meteorites are used to infer much about their origins and recent histories. The methods used to interpret meteorites' cosmic-ray records, especially identifying simple or complex exposure histories, often are inadequate. Spallogenic radionuclides, stable nuclides, and measurements of products that have location-sensitive production rates, such as the tracks of heavy cosmic-ray nuclei or neutron-capture nuclides, are very useful in accurately determining a meteorite's history. Samples from different, known locations of a meteorite help in studying the cosmic-ray record. Such extensive sets of meteorite measuremetns, plus theoretical modeling of complex histories, improves the ability to predict the production of cosmogenic nuclides in meteorites, to distinguish simple and complex exposure histories, and to better determine exposure ages

Reedy, R. C.

English

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386 SH 7.1-7
COSMIC-RAY EXPOSURE RECORDS AND ORIGINS OF METEORITES
Robert C. Reedy
Nuclear Chemistry Group, Mail Stop J514
Los Alamos National Laboratory
Los Alamos, NM 87545, USA
ABSTRACT
The cosmic-ray records of meteorites can be used to infer much
about their origins and recent histories. Some meteorites had
simple cosmic-ray exposure histories, while others had complex
exposure histories with their cosmogenic products made both
before and after a collision in space. The methods used to
interpret meteorites' cosmic-ray records, especially identi-
fying simple or complex exposure histories, often are inade-
quate. Besides spallogenic radionuclides and stable nuclides,
measurements of products that have location-sensitive produc-
tion rates, such as the tracks of heavy cosmic-ray nuclei or
neutron-capture nuclides, are very useful in accurately deter-
mining a meteorite's history. Samples from different, known
locations of a meteorite help in studying the cosmic-ray
record. Such extensive sets of meteorite measurements, plus
theoretical modeling of complex histories, will improve our
ability to predict the production of cosmogenic nuclides in
meteorites, to distinguish simple and complex exposure
histories, and to better determine exposure ages.
1. Introduction. Meteorites are fragments from a variety of objects in
the solar system and can provide important information about these
bodies. Well-determined cosmic-ray exposure histories are needed to
help to establish the origins of meteorites: where they came from and
how they were produced (1,2). In principle, the exposure history of a
meteorite should be well determined if sufficient measurements are made
of its cosmic-ray record and if the production rates and profiles of the
cosmogenic products used to interpret the measurements are known well
enough. It is unlikely that many meteorites have had their cosmic-ray
exposure histories properly determined. Wetherill (3) noted that complex
histories should be common for most likely meteorite-origin scenarios.
However, complex histories for meteorites are seldom reported, especially
_for stony meteorites. Is this discrepancy between the predicted and
reported frequencies of meteorite complex histories a problem with the
approaches pres@nt]y used to study the exposure records of meteorites?
Inadequacies in the interpretation of the cosmic-ray records of °
meteorites could mean that exposure ages and other results of meteorite
studies, such as cosmic-ray variations, also might not be correct.
Systematic experimental and theoretical studies are needed to study how
well we have been determining the recent histories of meteorites and to
develop better methods for unfolding the cosmic-ray records of
meteorites.
2. Cosmic-Ray Exposure Records. Among the ages of a meteorite, the
youngest are those determined from the activities or concentrations of
cosmogenic products. Exposure ages, the lengths of time that meteorites
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have been exposed to cosmic rays, are typically orders of magnitude
shorter than their radiometric formation ages and, for some meteorites,
their brecciation ages and collisional shock ages (4). The cosmogenic
products used to study exposure records of meteorites include stable and
radioactive nuclides and tracks of heavy cosmic-ray nuclei. If a
meteorite had a simple exposure record, then the concentration of a
cosm_genic product and its production rate can be used to determine an
exposure age. However, the production rates vary considerably with
meteoroid size and shape and with sample location (5), so corrections for
the effects of the sample's shielding geometry must be applied. Most
methods for estimating shielding corrections, such as the Ne-22/Ne-21 or
He-3/Ne-21 ratios, do not yield unique production rates but only narrow
the range of possible values (5).
Many meteorites had simple exposure ages. Detailed investigations
of the cosmic-ray records of several meteorites, such as the tracks,
radionuclides, and spallogenic noble gases in St. Severin (6), show that
the cosmogenic products were made only during one exposure geometry.
However, most meteorites have not had their exposure records shown to
h_ve been simple by such systematic studies. Another clue that a
meteorite probably had a simple history is that its exposure age is the
same as a number of other meteorites of the same type. However, there
are only a few clusters for the exposure ages of meteorites; the exposure
ages for most meteorites form a continuous distribution.
The clusters of exposure ages for a given class of meteorites, such
as the ~675 Ma (675 million years) cluster for IIIAB irons (7) or the _6
Ma peak for H-chondrites (8), suggest that these meteorites were all
produced by single, major collisional events. If these two clusters
really represent individual events, the widths of the peaks, about 15-20%
of the age, indicate that the sums of the uncertainties in determining
cosmic-ray exposure ages are fairly large. As the measurement uncertain-
ties are much smaller than 15-20%, the spread in the exposure ages for
these clusters probably results from interpreting the measurements
(although pre-irradiation or gas losses could account for some of the
spread). One likely source of this width in the exposure-age peaks is
the significant correction for shielding.
Several meteorites clearly had their cosmogenic products made both
before and after a major collisional event in space that changed the
meteoroid's geometry. The Jilin chondrite was irradiated by a signifi-
cant flux of cosmic-ray particles for ~i0 Ma near the surface of a very
" large object prior to its ejection _0.4 Ma ago as an 85-cm-radius sphere
(9). Several other meteorites have had complex histories, such as
several inferred from the disagreements among exposure ages determined
with different radionuclides or stable cosmogenic products. In very
large meteorites, such as the Canyon Diablo iron meteorite (10), one part
could have been removed by a minor collision without seriously affecting
the formation of cosmogenic products in other parts of the meteoroid.
The distinction between simple and complex exposure histories can be
difficult in some cases. Some meteorites probably have small amounts of
cosmogenic products made prior to the event that produced the object that
eventually hit the Earth. In most cases, the amounts of cosmogenic
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products made during the meteorite's final exposure geometry overwhelm
any previous production. However, if this last exposure stage was short,
then possibly the products made during the previous exposure could be
detected. As suggested by (9), the high production rates inferred for
Ne-21 from meteorites that have undersaturated AI-26 activities (that is,
exposure ages of N1Ma) could be a consequence of pre-irradlatlon. A
very special case of a complex history is when a foreign clast in a
brecciated meteorite has had a previous exposure to the solar wlnd or
cosmic rays prlor to incorporation in the meteorite (11).
3_ Systematic Experimental Studies. To properly calibrate the
predlcted production rates and profiles of cosmogenic products in
meteorites, extensive sets of measurements are needed. The measurements
should be made for samples from different, known locations and also of
products with a wide range of production-rate-versus-depth proflles
(tracks, high- and low-energy spallation, and neutron capture) and a
variety of half-lives (including stable isotopes). For example, the
Na-22 and Co-60 actlvities measured in Jilin (9) serve as a calibration
points for cosmogenic nuclides in a meteoroid with a radius of 85 cm.
The measurements of cosmogenic products in meteorite cores, such as those
from St. Severin (6) and Keyes, also are valuable reference data.
However, few such well-characterized data sets for cosmogenic
nuclides in meteorites exist (5). Most meteorites that have been
investigated in detail have radii of ~10-40 cm, which are typical of most
meteorites. There have not been many extensive studies of smaller
meteorites. Laboratory irradiations of small spherical targets are
helping to understand the production of cosmogenic nuclides in small
meteoroids (12). While Jllin had a radlus of 85 cm for its last 0.4 Ma,
its initial exposure limits studles of cosmogenic nuclldes in large
spherical meteoroids mainly to short-lived radionuclides, such as Na-22
and Co-60. More detalled measurements are needed to calibrate the models
used to predict the production rates and profiles of cosmogenic nuclides,
especially for meteorites that had unusual preatmospheric sizes or
shapes.
4. Theoretical Modellng. Calculated size- and depth-dependent produc-
tion rates can be used to illustrate the use of a meteorite's cosmic-ray
record to accurately unfold its exposure history. Most models do a
fairly good job of reproduclng the profiles of cosmogenic products in
typical-sized meteorites, such as St. Severin (6). However, some models
have trouble with very large meteorolds, like Jilin (9). Very small
meteoroids are hard to model because the cascade of secondary particles,
especially neutrons, is not fully developed (5,12) and solar-proton
production is important. Improved models are needed for all sizes,
shapes, and types (irons and stones) of meteorites.
Some of these model results have been used to study criteria for
identifying complex histories. For example, the calculated track produc-
tion rates and the Ne-22/Ne-21 ratios for meteorites with simple histor-
ies cover a small region of all possible values (13). Meteoritic samples
with tracks and neon ratios that are outslde this reglon have complex
histories. The activities of cosmogenic radlonuclides are often used in
looking for complex histories, and often these activities versus track
38 9 SH 7.1-7
production rates or Ne-22/Ne-21 ratios are used to search for unusual
exposure histories. Artificial exposure records derived from calculated
production rates and profiles can be used to test the methods used to
infer the exposure histories of meteorites. These theoretically con-
structed records for complex histories will especially help to show which
features of a meteorite's cosmic-ray record are best suited for distin-
guishing complex from simple histories and which types of complex
histories are most easily detected. Such theoretical studies would help
to find the areas where meteorite measurements or more laboratory simula-
tions or cross sections would be most useful.
5. Conclusions. Many experimental and theoretical studies are needed
to improve our ability to unfold the cosmic-ray exposure records of
meteorites. More extensive measurements of cosmogenic products in
meteorites, especially shielding-sensitive ones like tracks and neutron-
capture-produced nuclides, will help to identify meteorites that had
complex histories and to yield better exposure ages. Such improved
recent histories could aid in identifying the parent bodies for certain
meteorites, such as primitive carbonaceous chondrites (possibly from
cometary surfaces) or the SNC meteorites (the Shergottites, Nakhlites,
and Chassigny, which were possibly ejected from the martian surface).
Detailed measurements of cosmogenic products in meteorites could be used
to improve the models for predicting the production rates of these
products. Theoretical studies of the production systematics of cosmogenic
products in meteorites will help to identify areas where additional
laboratory or meteorite measurements are needed.
6. Acknowledgements. This research was supported by NASA work order
W-14,084 and done under the auspices of the U.S. Department of Energy.
K. Marti and S. Regnier contributed to the ideas presented here.
References
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558-578.
5. Reedy, R. C., (1985), J. Geophys, Res. 90, C722-C728.
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7. Voshage, H., and H. Feldmann, (1979), Earth Planet. Sci. Lett. 45,
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8. Crabb, J., and L. Schultz, (1981), Geochim. Cosmochim. Acta 45,
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Cosmic-ray exposure records and origins of meteorites

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