Geology Reference
In-Depth Information
“medium-grained, feldspathic pyroxenite with abundant
megacrysts of olivine and some megacrysts of pyroxene.”
Lithology B is a “coarse-grained equivalent of Lithology
B, but without the megacrysts.” And Lithology C is a
complicated mixture of glass and microcrystalline mate-
rials. A white druse consists of salts that apparently
formed in place, a portion of them on Earth.
Using revised production rates for noble gases,
Eugster et al
. [1997] recalculated CRE ages for EET
A79001 based on earlier analyses of
3
He,
21
Ne, and
38
Ar.
The
21
Ne CRE ages,
T
21
, which are probably the most
reliable, were approximately 0.7 Ma.
Jull and Donahue
[1988] measured the
14
C concentration and obtained a
terrestrial age of 12 ± 2 ka based on
A
Fal
l
= 65 ± 11 dpm/
kg for a body with the average composition of shergot-
tites. CRE ages can also be calculated from the activities
at the time of fall of
26
Al,
10
Be, and
53
Mn, all of which
have half-lives comparable to or longer than
T
21
.
Pal
et al
. [1986] reported
10
Be ages of 0.7 ± 0.2 Ma and
0.9 ± 0.2 Ma for EET A79001A and B, respectively based
on composition-corrected production rates of 22 and
26 dpm/kg.
Sarafin et al
. [1985] measured both the
26
Al
and the
10
Be activities of EET A79001. For each radio-
nuclide we have
AP e
Table 9.4.
Terrestrial and exposure ages (
T
Terr
and
T
Exp
) of
martian meteorite ALH A77005.
Nuclide
T
Terr
(ka)
T
Exp
(Ma)
Source
81
Kr,
38
Ar
0.19 ± 0.07
3.6 ± 0.4
a
81
Kr,
36
Cl,
10
Be
0.21 ± 0.07
2.5 ± 0.3
b
,
c
10
Be
2.8 ± 0.6
d
3
He,
21
Ne,
38
Ar
3.3 ± 0.6
e
3
He,
21
Ne,
38
Ar
2.9 ± 0.7
f
a
Schultz and Freundel
, [1984]
b
Nishiizumi et al
., [1994]
c
Nishiizumi et al
., [1986]
d
Pal et al
., [1986]
e
Bogard et al
., [1984] recalculated by
Eugster et al
., [1997]
f
Miura et al
., [1995]
satisfy constraints from short-lived cosmogenic radionu-
clides, which imply a prelaunch burial depth of at least
150 cm. The authors conclude, therefore, that EET 79001
assimilated martian regolithic material. The assimilated
material evidently carries a Sm but not a
21
Ne signature
altered by martian preirradiation.
ALH A77005, 482 g, was the first martian meteorite
discovered in in the Antarctic. The Martian Meteorite
Compendium [
Meyer
, 2012;
http://curator.jsc.nasa.
gov/antmet/mmc/ALH77005.pdf
] summarizes the main
experimental results. ALH A77005 consists of olivine
(~55%) and pyroxene (~30%), with a balance mostly of
maskelynite and melt. All the major phases show signs of
heavy shock, which is also suggested by the presence of
melt pockets and glassy (pseudotachylite) veins.
Table 9.4 presents published terrestrial and CRE ages
for ALH A77005. The weighted mean CRE age is
2.93 ± 0.20, very close to the value of 2.87 ± 0.20 recom-
mended by
Nyquist et al
. [2001]. We base the ages given in
Table 9.4 on older half-lives:
10
Be (1.5 Ma) and
81
Kr
(0.213 Ma) rather than the currently accepted values, in
order to facilitate comparisons with ages in the literature,
which assume the older half-lives. The data hint that the
ALH A77005 lherzolite is somewhat older than most
basaltic shergottites.
In Table 9.5 we have compiled ejection and terrestrial
ages of martian meteorites collected in the Antarctic. The
CRE age distributions for Antarctic and for non-Antarc-
tic martian meteorites (Figure 9.7) are indistinguishable.
Most martian meteorites fall into one of a small number,
five to seven, of groups. This clustering of ages implies a
correspondingly small number of launches from a few
locations on Mars. Radiometric dating (mostly Sm/Nd
with some Rb/Sr) yields a clumpy distribution of much
older ages clustering about 180 Ma or 600 Ma for sher-
gottites and about 1.2 Ga for nakhlites. The outlier
ALH 84001 and two recent martian finds from North
Africa have a much older age of 4.1 Ga [see, e.g.,
Nyquist
(
)
−
λ
T
−
λ
Terr
.
T
=
1
−
e
Aexp
A
A
Adopting the values
P
(
26
Al) = 84 dpm/kg and
P
(
10
Be) = 21.2 dpm/kg, Sarafin and coworkers solved
these two equations iteratively and found
T
Exp
= 0.78 ± 0.14 Ma and
T
Terr
, = 0.32 ± 0.17 Ma. Both
Pal
et al
. [1986] and
Sarafin et al
. [1985] assumed a
10
Be
half-life of 1.6 Ma, rather than the currently accepted
value of 1.387 Ma. This lower half-life decreases the CRE
ages estimated from
10
Be by about 10%. More important
than the details, however, and as noted by several early
workers, EET A79001 has a CRE age on the order of 1 Ma.
By comparison, the CRE ages of basaltic and other sher-
gottites are older. Recent work by
Berezhnoy et al
. [2010]
and especially by
Nishiizumi et al
. [2011] show the
existence of a prominent 1-Ma peak in the CRE age
distribution of phyric shergottites, strongly suggesting a
single launching event and neighboring locations of the
stones on Mars.
One of the minor mysteries concerning martian mete-
orites has been the apparent absence of material that lay
close enough to the martian surface to record cosmic-ray
irradiation prior to launch.
Hidaka et al
. [2009] searched
for evidence of such pre-exposure by measuring Sm
isotope abundances in a group of 12 martian meteorites
that included EET A79001. These authors concluded that
EET A79001, among others, shows shifts in Sm isotope
abundances that require irradiation on Mars. Ironically,
however, neither a simple nor even a multistage exposure
history of the intact meteoroid and/or its precursor in the
parent body can simultaneously explain the Sm data and
