Geology Reference
In-Depth Information
preferable to have a method with greater precision and
natural to suppose that combinations of radionuclide
measurements might provide the basis of such a method.
Equation 9.4 is obtained by forming a ratio of activities
measured for two nuclides in one meteorite.
the factor that ultimately limits the accuracy of a terres-
trial age is the activity of that nuclide at the time of fall.
As suggested above in section 2.5,
A
Fall
depends on the
size of the meteoroid, the location of our specimen within
that meteoroid, and the duration of cosmic-ray exposure,
all of which may be unknown. The corresponding term
that appears in equation 9.4 is the
ratio
of activities,
A
1,
Fall
/
A
2,
Fall
. Both modeling calculations and direct exper-
imental measurements confirm that when the two nuclides
are chosen appropriately,
A
1,
Fall
/
A
2,
Fall
is much less sensitive
to size and depth effects than is the value of
A
Fall
for one
nuclide, and that where
A
1,
Fall
/
A
2,
Fall
varies, it does so in
predictable ways [
Neupert et al
., 1997;
Jull et al
. 1994
(Knyahinya);
Kring et al
., 2001 (Gold Basin);
Nishiizumi
and Caffee
, 1998;
Lavielle et al
., 1999 (small iron mete-
orites)]. This ratio is also nearly independent of the
meteorite's elemental composition for
14
C and
10
Be and
certain other pairs of nuclides [
Jull et al
., 1994].
Nishiizumi et al
. [1997] used
36
Cl and
10
Be activities in
equation 9.4 to calculate terrestrial ages for iron meteor-
ites. The
36
Cl/
10
Be ratio has an effective half-life of 382 ka,
and with high-precision measurements of activities,
Δ
T
Terr
can be reduced to about ±40 ka.
Welten et al
. [2000]
calculated terrestrial ages from equation 9.4 using
41
Ca
and
36
Cl activities measured in the metal phase of chon-
drites and small iron meteorites. The
41
Ca/
36
Cl ratio has
an effective half-life of 157 ka, and the
36
Cl/
10
Be ratio has
one of 383 ka. For further discussion, see
Welten et al
. [2006]
and references therein.
Although it is neither practical nor necessary to obtain
a terrestrial age for every Antarctic meteorite, a sizable
database is useful.
Welten et al
. [2011a] have been sepa-
rating metal phases from chondrites and using the
36
Cl/
10
Be method to obtain terrestrial ages. With results in
hand for ~490 chondrites, these authors find that all but a
few Antarctic meteorites arrived within the last 650 ka
and that a majority have terrestrial residence times of
less than 50 ka. A small but significant fraction of the
meteorites have radionuclide activities so low that reliable
terrestrial ages cannot be determined.
More detailed studies on Antarctic meteorites may use
three or more nuclides in order to calculate terrestrial
ages and to correct for undersaturation.
Welten et al
.
[2006] studied 36 meteorites from the Frontier Mountains
blue icefield, which contains a classic “trap” for meteorites.
The terrestrial ages ranged from 6 to 525 ka.
(
)
AT
AT
A
A
(
)
−−
λλ
λλ > 35
T
1
Terr
)
=
1
,
Fall
e
;
>
;
T
./ λ
(9.4)
12
Terr
(
1
2
Exp
2
2
Terr
2
,
Fall
We assume that nuclide 1 has the shorter half-life and
that the CRE lasted long enough for nuclide 2 to reach
>98% of its maximum (or saturation) value. A first
question concerns the range of terrestrial ages for which
this equation is best applied. Comparison of equation 9.4
with equation 9.2
(
)
=
AT Ae
Terr
−λ
T
Terr
(9.2)
Fall
(
)
suggests a formal analogy in which
AT
AT
)
,
A
1
Terr
1
,
Fall
,
(
A
2
Terr
2
,
Fall
and (
λ
1
−
λ
2
) play the roles of
A
(
T
Terr
),
A
Fall
, and
λ
, respec-
tively. Thus, the ratio
AT
AT
( )
( )
behaves like a radionu-
clide that decays with half-life equal to
1
Terr
2
Terr
()
−
ln 2
λλ
. Just as
in the single-nuclide approach, we expect equation 9.4 to
yield the best results for terrestrial ages comparable to
this value. For example, if we take as our two nuclides
14
C
(
λ
= 0.1210 ka
−1
) and
10
Be (λ = 0.0005 ka
−1
), we see that
λ
(
14
C) −
λ
(
10
Be) ≈
λ
(
14
C) and conclude that equation 9.4
gives optimal results over a time scale equal to a few half-
lives of
14
C. With typical H chondrite values for
A
1,
Fall
(
14
C) = 50 dpm/kg and
A
2,
Fall
(
10
Be) = 20 dpm/kg, equation
9.4 becomes
(
)
1
2
14
C
Be
50
20
=
e
−
0 1205
.
T
~.
25
14
e
−
λ
T
,
(9.5)
Terr
Terr
10
where
T
Terr
is in ka. In cases where the cosmic-ray irra-
diation lasted for fewer than five half-lives of
10
Be, a
correction based on the CRE age must be applied (for
details, see
Herzog and Caffee
[2014]).
One might wonder whether this method offers any
great advantage, inasmuch as most of the uncertainties
propagated in equation 9.2 carry over into equation 9.4.
Indeed, the precision of the terrestrial age obtained using
the two-nuclide equation 9.5 is generally poorer than that
obtained using the one-nuclide equation 9.2. For one key
term, however, the relative uncertainty associated with
equation 9.4 is smaller and for this reason the overall
accuracy of
T
Terr
is better. In particular, in equation 9.2,
9.2.13. Terrestrial Age and Terrestrial Alteration
Weathering was documented early in the history of
the Antarctic meteorite collection [
Biswas et al
., 1980;
Gooding
, 1981;
Lipschutz
, 1982], and to this day
weathering remains one of the classification parameters
for Antarctic meteorites. To first order, one would expect