Geology Reference
In-Depth Information
In space, the concentration of the trapped electrons,
n
,
increases with the duration of the irradiation up to a lim-
iting or saturation value. As soon as the electrons are
trapped, they can decay to lower energy states through a
process of thermal de-excitation that emits light termed
natural thermoluminescence (NTL). Thus, NTL can be
loosely regarded as a decay product of the (unstable)
population of trapped electrons. The rate of decay
increases with temperature and other factors.
As with production of cosmogenic radionuclides,
production of the trapped electrons mostly ends with a
meteorite's fall because the Earth's atmosphere attenuates
the cosmic-ray flux by a factor of ~1000. Decays, however,
continue and so the concentration of trapped electrons
decreases as time passes. In the laboratory, heating of a
meteorite sample to high temperature (from ~100 °C to
450 °C) greatly hastens this decay or draining (in the
jargon of the trade) or annealing of the trapped electrons,
which is accompanied by the emission of NTL. Measured
as a glow curve and integrated over suitable temperature
ranges, the NTL data give signals related to the trapped
electron concentration at the time of measurement. To
the extent that prior annealing in the Antarctic occurs at
a constant or known rate, NTL may serve as a basis for
calculating terrestrial ages [see, e.g.,
Sears and Hasan
, 1986].
0.1
1
10
8
Observed falls
6
4
2
Prairie state finds
6
4
2
6
4
2
0
Antarctic finds
0.1
1
Natural TL normalized to high-TTL
10
Figure 9.2.
Thermoluminescence (TL) measurements for
Antarctic and non-Antarctic meteorites. Adapted from
Sears
and Hasan
[1986].
age, an assumption not universally accepted, a comparison
of the NTL and
26
Al results implies an effective half-life of
~0.2 Ma for the decay of NTL, about the same as that of
81
Kr or
36
Cl.
9.2.7. Quantitative Relation Between NTL Data
and Terrestrial Age
9.2.9. The
26
Al and NTL Surveys of the U.S. ANSMET
Collection of Antarctic Meteorites
The calculation of terrestrial ages from NTL measure-
ments is model dependent [
McKeever et al
., 1982], and we
have never seen an equation in closed form from which
we could make a calculation ourselves. Nevertheless
and although over the years Sears and coworkers have
emphasized different methods for calculating terrestrial
ages, the general pattern of results has changed little.
In 1987, the Meteorite Working Group, which oversees
the U.S. collection, curation, and distribution of Antarctic
meteorites, announced that it had approved “two types
of surveys of Antarctic meteorites which will identify
meteorites with particularly short or long terrestrial
ages or unusual thermal or radiation histories” (
Antarctic
Meteorite Newsletter
, vol. 10, of February 1987), as John
Annexstad had suggested eight years earlier. About one
year after the announcement, the
Antarctic Meteorite
Newsletter
of February 1988, Table 4, presented NTL
measurements from Derek Sears' laboratory for 199
meteorites from Lewis Cliffs. And in the following issue
of August, 1988, 110 more measurements appeared
along with 94 measurements of
26
Al activities from John
Evans, John Wacker, and James Reeves of Battelle
Northwest Laboratories. The determination of NTL in
Antarctic meteorites was to continue for fourteen years
[
Sears et al
., 2011].
In the active days of the NTL survey of Antarctic
meteorites, the curators at NASA's Johnson Space Center
took considerable pains to sample material at least 1 cm
from fusion crusts. They did so in order to avoid the
possibility that atmospheric heating had bleached the
9.2.8. Terrestrial Ages Based on NTL Measurements
Sears and Hasan
[1986] did not calculate terrestrial ages
explicitly but examined the distribution of a quantity,
NTL
NTL
, for 20 Antarctic meteorites (Figure 9.2).
This quantity is expected to decrease with increasing ter-
restrial age. Indeeed, as observed for
26
Al, the distributions
for both Antarctic and non-Antarctic finds show shifts to
lower values by a factor of ~2 relative to the one for falls.
The respective averages for falls and for Antarctic finds
are 2.8 and 1.3. (The data set of
Hasan et al
. [1987]
includes three additional meteorites but is otherwise sim-
ilar.) Assuming that the selections of meteorites for
26
Al
and NTL are similar and that the shift toward lower NTL
values in Antarctic stones reflects primarily terrestrial
log
lowT
−
high T
−
