Geology Reference
In-Depth Information
Table 9.6.
Pre-ejection lunar depths (
D
2π
) and durations (
T
2π
), and ejection, CRE exposure, and terrestrial ages (
T
Ej
,
T
Exp
,
and
T
Terr
) of selected lunar meteorites found in the Antarctic.
a
Meteorite
D
2π
(g/cm
2
)
T
2π
(Ma)
T
Ej
(Ma)
T
Exp
(Ma)
T
Terr
(ka)
ALH A81005
150-180
520
0.04 ± 0.02
<0.05
18 ± 1
Asuka 881757
>1000
0.9 ± 0.1
0.9 ± 0.1
<50
EET 87521, EET 96008
540-600
26 > 10 ± 1
c
0.08 ± 0.04
<0.01
80 ± 30
LAP 02205
b
>43
c
0.055 ± 0.005
<0.01
20 ± 5
MAC 88104, MAC 88105
360-400
630 ± 200
0.28 ± 0.02
0.04-0.05
230 ± 20
MET 01210
0.95
<0.01
~20
MIL 05035
>1-3
c
PCA 02007
—
0.95
<0.01
~30
QUE 93069, QUE 94269
65-80
1000 ± 400
0.16 ± 0.02
0.15 ± 0.02
10 ± 2
QUE 94281
270-320
400 ± 60
0.05 ± 0.03
<0.05
23.5 ± 1.8
Y 791197
4-8
450
<0.1
<0.1
30-90
Y 793169
500
50 ± 10
1.1 ± 0.2
1.1 ± 0.2
<50
Y 793274, Y 981031
140-180
600 ± 150
0.032 ± 0.003
<0.01
20-35
Y 793885
~90
—
—
>36
Y 82192, 3, 86032
d
>1000
<10 Ma
11
80±8
80±8
a
References to the original data for these and other lunar meteorites may be found in
Lorenzetti et al
. [2005],
Jull
[2006],
Righter
and Gruener
[2013], and
Herzog and Caffee
[2014].
b
Paired with LAP 02224/02226/02436/03632.
c
Fernandes et al.
[2009]. These are lower limits because they assume maximum
38
Ar production rates that apply only at the very
surface of the Moon.
d
Lorenzetti et al., 2005.
an anorthositic breccia and appears to be a very rare bird.”
Although lunar meteorites from “hot” deserts now out-
number Antarctic ones, the latter are more “pristine,” that
is, they show less weathering, which may simplify the inter-
pretation of experimental results. The first measurements
of
26
Al and
10
Be in ALH A81005 gave low activities, which
limited the transit time to Earth to 1 Ma [
Tuniz et al
., 1983];
measurements of NTL suggested an even shorter one,
2500 y, a value also compatible with extremely low den-
sities of cosmic-ray tracks [
Sutton and Crozaz
, 1983]. Later
reports of
14
C,
41
Ca,
36
Cl,
26
Al,
10
Be, and
53
Mn activities
enabled the construction of a much more detailed exposure
history [
Nishiizumi et al
., 1991;
Jull and Donahue
, 1992].
With five activities available for modeling, these authors
were able to assign to ALH A81005 a terrestrial age of 9 ka
and a transit time of only 2 ka.
Consistent with these observations, theoretical studies
carried out in the 1960s give short transit times from the
Moon [e.g.,
Arnold
, 1965;
Wetherill
, 1968], as does recent
work [e.g.,
Gladman et al
., 1995]. More specifically,
Gladman et al
. [1995] concluded that much lunar orbital
debris ejected with low velocities would reach the Earth
in under 10 ka and that the spatial distribution of the
meteorites, even from single launch events, would cover
the globe. Thus, the relatively short CRE age of ALH
A81005 confirms expectations and foreshadowed similar
experimental results in years to come.
By comparing values of
T
Ej
for meteorites such as ALH
A81005 we can identify meteorites that are launch paired.
With supplementary information from remote sensing
and the mineralogy and petrology of the objects, it even
may be possible to identify the source location on the
Moon [
Gnos et al
., 2004]. Table 9.6, adapted from
Jull
[2006] with additional material from
Righter and Gruener
[2013], summarizes terrestrial and ejection ages of
selected lunar meteorites recovered in the Antarctic.
So far omitted from this discussion is irradiation on the
Moon itself. To push the time horizon to the period pre-
dating exposure as a meteoroid, one needs to measure
cosmogenic nuclides with half-lives longer than the
ejection age. In this respect, the stable rare gas isotope
21
Ne and to a lesser degree the isotopes
38
Ar,
83
Kr, and
126,128
Xe have proved useful.
Eugster
[1989] and
Lorenzetti
et al
. [2005] have reviewed the exposure ages of lunar
meteorites.
Herzog and Caffee
[2014] discuss the general
process of modeling a detailed cosmic-ray exposure his-
tory and refer to many papers in the primary literature. In
short, in the simplest case, one assumes a single period of
cosmic-ray exposure on the Moon characterized by a
duration
T
2π
at a depth
d
followed by irradiation in space
in a body of radius
R
at a depth
d
for a time
T
4π
, followed
by near quiescence on Earth for a time
T
Terr
. The modeler
seeks values of these six parameters that lead to a calcu-
lated match to the observed activities or concentrations
of cosmogenic nuclides.
The fitting of experimental results indicates that
Antarctic lunar meteorites often record 10 Ma or more of
cosmic-ray exposure in the topmost two or three meters
