Geology Reference
In-Depth Information
excluded for unexamined meteorites,” in agreement with
the results of
Evans et al
. [1982].
of Antarctic glacier movement and assuming that the
ice surrounding the meteorite did not thaw and refreeze,
this result implies that Allan Hills 82102 was transported
for only a short distance. By extension, the young terres-
trial ages of Yamato meteorites, which
Jull et al
. [1993]
confirmed for many additional stones, and the older ter-
restrial ages of Allan Hills Main Icefield meteorites may
imply transport over shorter and longer distances, respec-
tively, if the incoming ice moves at a constant rate.
Using NTL data,
Benoit et al
. [1993] reached a quali-
tatively similar conclusion about the relative terrestrial
ages of Yamato and Allan Hills meteorites. They also
suggested that meteorites from the Far Western Icefield
of Allan Hills tended to be younger than those of other
Allan Hills areas. In a more detailed study based mostly
on
14
C,
Jull et al
. [1998a] found that many meteorites
from the Allan Hills Near, Middle, and Far Western
icefields had terrestrial ages of <50 ka, values well
within the range of
14
C dating. In contrast, a much
lower percentage of meteorites from the main icefield
had low terrestrial ages (Table 9.3, adapted from
Jull
[2000]). The large span of the results (Figure 9.5)
emphasizes the need to choose the radionuclide with a
task-appropriate half-life.
9.2.11. Terrestrial Ages of Yamato and Other Icefields
From the 1980s onward, evidence accumulated that
meteorites collected from different icefields and within
different parts of the same icefield have different terrestrial
ages.
Kigoshi and Matsuda
[1986] presented
14
C terrestrial
ages from Yamato meteorites;
Nishiizumi et al
. [1989a]
presented 135 terrestrial ages based on
36
Cl analyses made
for meteorites collected in the Allan Hills, the Yamato
Mountains, and other locations (Figure 9.5).
Nishiizumi
and coworkers
concluded that “the terrestrial ages of Allan
Hills meteorites vary from 0.2 to 1 Ma and are clearly
longer than those of Yamato meteorites and other
Antarctic meteorites” [1989a]. The apparent reason for
this difference in terrestrial ages is that meteorites fall in
various locations and then travel (in the ice) to the recovery
location over considerable distances, along different paths,
and at varying velocities [
Whillans and Cassidy
, 1983]. A
similar dispersion of terrestrial ages is reported for the
Frontier Mountains site [
Welten et al
., 2006].
Folco et al
.
[2006] noted that the terrestrial ages of meteorites could
also be used to constrain the age of the ice itself.
Nishiizumi et al
. [1989b] singled out for special attention
the H5 chondrite Allan Hills 82102, which was found
in
rather than
on
the surface of the ice at the Far Western
Icefield. Its terrestrial age is ~11 ka. Given the slow rates
9.2.12. Terrestrial Ages from Two or More
Radionuclide Activities
The determination of terrestrial ages using cosmogenic
nuclides is limited in part by our confidence in the
radionuclide activity at the time of fall,
A
Fall
. While this
parameter often can be constrained well enough to allow
the determination meaningful terrestrial ages, it would be
50
40
Table 9.3.
Percentage of meteorites recovered from different
Antarctic locations and with terrestrial ages falling in different
time periods (adapted from
Jull
[2000]).
30
% Meteorites per Radionuclide and
T
Terr
14
C
20
14
C or
36
Cl
36
Cl or
81
Kr
Recovery Location
<25 ka
25-70 ka
>70 ka
>480
10
Far Western Ice
field, Allan Hills
65%
35%
~0%
0
Yamato Mountains
27%
54%
19%
0
60 20
180 240 300
Terrestrial age (ka)
360
420
480
Middle Western Ice
field, Allan Hills
30%
70%
~0%
Elephant Moraine
14%
37%
49%
Figure 9.5.
Terrestrial age distributions of meteorites. Yamato
site (slanted bars) taken from
Jull et al
. [1993, 1998a, unpub-
lished data] and
Nishiizumi et al
. [1989a, 1999]. The Allan
Hills Main Icefield (black bars) and Elephant Moraine (white
bars) taken from
Nishiizumi et al
. [1999],
Michlovich et al
.
[1995],
Jull et al
., [1998b], and
Jull
[unpublished data, 2003].
Figure reprinted from
Jull
[2006] by permission of the University
of Arizona Press.
Lewis Cliff
10%
25%
65%
Frontier Mountains
8%
31%
61%
a
Allan Hills Main
Ice field
7%
c
39%
54%
b
a
Welten et al
. [2000].
b
Estimate of
Nishiizumi et al
. [1999] on 102 samples.
c
Jull et al
. [1998a] and unpublished data.
