Geology Reference
In-Depth Information
9.2.2. Choice of Radionuclide
information about the fall, this value cannot be measured
for a find and therefore must be estimated. In the simplest
approximation for
A
Fall
, we use average or selected activ-
ities measured for fresh meteorite falls comparable to the
meteorite whose terrestrial age,
T
Terr
, we want to know.
These data define
ranges
of possible values for
A
Fall
, and
the resulting imprecision contributes appreciably to the
overall uncertainty of the terrestrial age (equation 9.3).
For example, the minimum activity for
14
C in ordinary
chondrite falls is about 20 dpm/kg and the maximum
about 60 dpm/kg, depending on the size of the precursor
meteoroid, its composition, and the position of the ana-
lyzed sample within it. Lower values are unusual and are
most often found in objects that were atypically small or
large in space. In the likely worst case then,
A
Fall
for
14
C
may differ by 50% from the midpoint of the total range.
Inspection of equation 9.2 shows that a 50% uncertainty
14
C
Fall
for a meteorite with
T
Terr
= 1/
λ
translates to a
minimum Δ
T
terr
/
T
terr
of 50%.
Most fresh falls, however, contain between 40 and
60 dpm/kg
14
C. It follows that for
T
Terr
= 1/
λ,
the typical
(relative) uncertainty attributable to Δ
A
Fall
would be approx-
imately 15%-20%. Relative uncertainties of the measured
14
C activities after corrections for blank normally range
from <1% to 5% but may exceed 50% for samples that give
signals close to blank levels. Blanks typically correspond to
a terrestrial age of about 30,000 years; a 20% error in the
14
C
determination corresponds to an absolute error in the
terrestrial age of about 1650 years, which in many contexts
is negligible (see discussion below).
As a rule of thumb, the radionuclide chosen for calcu-
lating a terrestrial age should have a half-life,
t
1/2
, comparable
to the terrestrial age of the sample; this rule follows from
standard propagation of error considerations. Specifically,
we have for the relative uncertainty of
T
Terr
2
2
2
(
)
∆
T
T
A
TA
∆
∆
AT
TAT
∆
λ
λλ
Terr
Terr
=+
Fall
+
+
, (9.3)
(
)
λ
Terr
Terr
Fall
Terr
Terr
where Δ denotes the uncertainty of an experimental
quantity. All three terms on the right may contribute signif-
icantly to the overall uncertainty. At large values of
λT
Terr
,
measurement difficulties usually lead to increased values of
∆
AT
AT
(
)
Terr
. At small values of the terrestrial age, the terms
(
)
Terr
λT
Terr
in the denominator approach zero and increase the
relative uncertainty. Consequently, in the determination of
terrestrial age it is usually best to choose a radionuclide
with a half-life comparable to the terrestrial age.
For a meteorite that fell within the last 15 ka or so,
14
C would be optimal, while for one that fell 100 ka ago,
41
Ca would be the better choice. With AMS, the relative
uncertainty of the measured
14
C activity,
∆
AT
AT
(
)
Terr
, is
(
)
Terr
usually 3%-10%, mainly because of blank corrections.
9.2.3. Activity Measurements
9.2.5. First Terrestrial Ages for Antarctic Meteorites
from
14
C and
26
Al
Historically, the radionuclide activities
A
(
T
Terr
) were mea-
sured by low-level decay counting. Today, decay counting is
mostly limited to short-lived (
t
1/2
< 1000 a) species such as
7
Be,
60
Co, and
54
Mn. Some γ counting of longer-lived
26
Al
continues and
81
Kr concentrations are measured by conven-
tional (low-energy) mass spectrometry. For the radionu-
clides
14
C,
41
Ca,
36
Cl,
41
Ca,
53
Mn,
60
Fe, and
129
I, however,
AMS has become the method of choice, although even
with the greater sensitivity of AMS relative to decay
counting, activities may be below detection limits. When
the activities are too low to measure, terrestrial ages are pre-
sented as lower bounds. The practical upper limits for
quantitative determinations of terrestrial age are about
30-40 ka and 400-1500 ka for
14
C and
36
Cl, respectively.
Fireman et al
. [1979] and
Fireman and Norris
[1981]
used
14
C measurements to calculate the terrestrial ages
of 12 Allan Hills (ALH) meteorites, which included a
eucrite, a diogenite, and several L and H chondrites. For
A
Fall
, they adopted the value of 57 ± 3 dpm/kg, a value
Fireman
[1978] determined for the L6 chondrite
Bruderheim; the stated uncertainty made no allowance
for the influence of shielding. Their results established a
range of terrestrial ages beginning at 10 ka and extending
upward (Table 9.2). Later observations agreed with this
early important result, verifying Fireman's techniques.
At about the same time, the late 1970s, John Evans and
John Wacker at Battelle Pacific Northwest Laboratories
began a program to measure cosmic-ray effects, and
particularly
26
Al (t
1/2
= 0.7 Ma) activities in Antarctic
meteorites. Measured activities in ordinary chondrites at
the time of fall typically lie between 30 dpm/kg and
120 dpm/kg. The substitution of reasonable values for the
uncertainty of a measured
26
Al activity in an Antarctic
find (5%-10% in most instances) into equation 9.3 shows
9.2.4. Activity at the Time of Fall (A
Fall
) and the
Uncertainty of the Terrestrial Age
The use of equation 9.2 to calculate a terrestrial
age requires knowledge of the radionuclide activity at
the time of fall,
A
Fall
. Absent independent historical
