Geology Reference
In-Depth Information
samples, that is, had erased some or the entire NTL
signal. Ironically, the proposal of
Miono and Nakanishi
[1994] stands this practice on its head. These authors
examined material from just under meteorite fusion
crusts, where atmospheric heating reset the NTL clock to
count up from Earth arrival, rather than down from
irradiation in space. As it turns out, the temperature
dependence of NTL decay limits the accuracy of this
method [
Akridge et al
., 2000].
To summarize, from the basic physics, we know that
some portion of the variability of NTL measurements
reflects terrestrial age. The precise portion may be diffi-
cult to establish in general because of the possibilities of
varying rates of decay, anomalous fading, and solar
heating. Given the existing state of the science, we believe
cosmogenic nuclides are the most robust means by which
to determine terrestrial age. If one is interested, however,
in a statistical profile of a large group of meteorites or in
investigating pairing relations [e.g.,
Ninagawa et al
., 2011],
then a cautious interpretation of NTL measurements
may provide useful guidance. NTL has a role to play in
constructing exposure histories of individual meteorites.
Figure 11 of the recent review by
Sears et al
. [2013]
presents a compilation of terrestrial ages for Antarctic
and other meteorites inferred from NTL measurements.
Mokos et al
. [2000] present a cautionary tale of Antarctic
H chondrites meteorites for which cosmogenic radionuclide
measurements forced a reassessment of groupings based
on NTL.
25
R10
R20
R25
R30
R35
R40
R50
R65
R85
R100
R120
20
15
10
5
0
25
50
75
Depth (cm)
100
125
Figure 9.3.
Model calculations [
Ammon et al
., 2009] for
36
Cl
activities in meteoritic metal. The numbers following the letter
R
denote the meteoroid radius in cm.
0.8
0.6
y
=
x
0.4
9.2.10. Terrestrial Ages Based on
36
Cl for Meteorites
from the Yamato Mountains
0.2
Best fit:
y
= (0.9 ± 0.3)
x
+(0.1
5
± 0.1)
To improve the estimation of terrestrial ages obtained
from
14
C and
26
Al, whose half-lives were at the respective
short and long ends of the terrestrial age distribution, a
radionuclide with an intermediate half-life was needed.
In the late 1970s, a group at the University of Rochester
led by David Elmore developed the AMS techniques
needed to measure
36
Cl (half-life, 0.3 Ma) with exquisite
sensitivity. And while the early measurements of
14
C and
26
Al were taking place in the United States, in Tokyo, a
team that included Kuni Nishiizumi and Masatake
Honda had begun measuring several radionuclide activ-
ities in the Antarctic meteorites collected by Japan's
National Institute of Polar Research [
Nishiizumi et al
.,
1978;
Nishiizumi et al
., 1979a, b]. Shortly thereafter,
Nishiizumi et al
. [1981] presented
36
Cl activities (in mete-
orite metal) along with
26
Al activities (in bulk samples)
obtained by counting. Figure 9.3 shows the results of cal-
culations that model the production of
36
Cl in space and
hence the activity expected at the time of fall. For a large
majority of meteorites (those with exposure ages >1 Ma
and derived from meteoroids with radii less than 65 cm),
0.0
0.0
0.2
0.4
0.6
0.8
T
Terr
(
36
Cl) (Ma)
Figure 9.4.
Comparison of terrestrial ages calculated from the
activities of
26
Al and of
36
Cl [in Antarctic meteorites
Nishiizumi
et al
., 1981].
the range of variation of
A
Fall
of
36
Cl in the metal of stony
meteorites is relatively small, from 19 to 24.5 dpm/(kg Fe).
The narrowness of the range helps improve the accuracy
of terrestrial ages based on
36
Cl.
Figure 9.4 compares the
36
Cl with the
26
Al terrestrial
ages of seven ALH meteorites [
Nishiizumi et al
., 1981].
Overall, the two sets of results agree reasonably well. The
range is from 50 ka to 800 ka; typical uncertainties are
70 ka and 160 ka for the terrestrial ages based on
36
Cl
and
26
Al, respectively.
Nishiizumi et al
. [1981] concluded
that “the terrestrial ages of Antarctic meteorites are less
than ~3 × 10
5
y, although longer terrestrial ages cannot be
