Geology Reference
In-Depth Information
9
Cosmogenic Nuclides in Antarctic Meteorites
Gregory F. Herzog
1
, Marc W. Caffee
2
, and A. J. Timothy Jull
3
9.1. INTRODUCTION
matters for the better. The first Yamato meteorites were
collected by a Japanese team in 1969 [see
Kojima
, 2006].
By 1977, U.S. expeditions had commenced and have
continued ever since. These annual expeditions regularly
retrieved hundreds of meteorites from the Antarctic ice.
As the number of interesting samples rapidly increased,
the limits on laboratory instrument time and staffing
soon rivaled sample availability as a key factor in experi-
mental design.
The new flood of meteorite arrivals sparked interest
in both old and new questions related to cosmogenic
nuclides. The technical task for the Antarctic meteorites
remained the same: to measure the concentrations (atom
g
-1
) of stable cosmogenic nuclides and either the concen-
trations or the activities (usually in dpm/kg = decays
minute
-1
kg
-1
) of radioactive ones. The conversion from
concentration to activity or vice versa is given by radio-
active decay law (equation 9.1),
We discuss applications to Antarctic meteorites of
cosmogenic nuclides. Cosmogenic nuclides are nuclei
produced by cosmic rays (Table 9.1), mostly before the
meteorites collided with Earth. They help us estimate
how long the meteorites were exposed to cosmic rays, a
length of time called the cosmic ray exposure age, and
the duration of a meteorite's stay on Earth, a length of
time called the terrestrial age. Cosmic ray exposure ages
have been reviewed by
Eugster et al
. [2006] and
Herzog
and Caffee
[2014];
Jull
[2006] has summarized data for
terrestrial ages. A comprehensive review of all relevant
material would occupy more space than is available and
so we limit the discussion to articles that demonstrate
special features of the Antarctic collection or that exem-
plify broader trends seen (or not) in the non-Antarctic
collection.
In the 1960s and 1970s, people who studied meteorites
used what was at that time an indispensable tool, the
Catalogue of the Natural History Museum, London
[
Hey
, 1966]. That catalogue included only four Antarctic
finds: the L chondrite Adelie Land, the IAB iron Neptune
Mountains, the pallasite Thiel Mountains, and the
ungrouped iron Lazarev; and so the notion of studying
cosmic-ray exposure histories of Antarctic meteorites
would have seemed constricted. The early harvests from
the Yamato Mountains and Allan Hills instantly changed
)
=−
[]
=
[]
d
A
(
Activity dpm kg
/
λ
A
dt
(
)
(
)
×
=
−1
t
a
1 3179 10
9
.
×
−
,
Concentrationatomg
/
M
12
/
(9.1)
which incorporates the current IUPAC recommendation
that 1 a = 31556925.445 s. In this expression,
t
1/2
is the
half-life of the nuclide of interest.
About the same time that the acquisition of large num-
bers of Antarctic meteorites began, accelerator mass spec-
trometry (AMS) became important for the measurement
of cosmogenic radionuclides. With AMS, samples of
0.01-0.5 g could be analyzed, a mass range hundreds to
thousands of times smaller than accessible with older
methods. The increase in sensitivity revolutionized the
1
Department of Chemistry and Chemical Biology,
Rutgers University, Piscataway, NJ
2
Department of Physics, Purdue University, W. Lafayette, IN
3
NSF-Arizona Accelerator Mass Spectrometry Laboratory,
Departments of Geosciences and Physics, University of
Arizona, Tucson