Geology Reference
In-Depth Information
relation relating the activity of a cosmogenic radionu-
clide to the duration of exposure
(
)
−λ
AExp
APe
Fall
=
1−
(9.6)
A
or a variant derived from it such as
PT
Pe
S
A
S xp
=
.
(9.7)
(
)
−λ
Aexp
1−
Fall
A
In equations 9.5 and 9.6,
P
denotes the production rate
of a cosmogenic nuclide production rate (in space) and
S
the concentration of a stable cosmogenic radionuclide
such as
21
Ne. When the product
λ
A
T
Exp
is large, equation
9.6 simplifies to
Figure 9.6a.
Robbie Score, finder of EET A79001, cutting a slice
from the meteorite.
PT
P
S
A
SExp
=
(9.8)
Fall
A
1cm
The nuclides
A
and
S
are chosen to minimize the
variability of
P
S
/
P
A
, whose value must be known inde-
pendently. A full discussion is beyond our scope, but two
constraints on this approach are directly relevant to the
discussion below. As noted in connection with terrestrial
ages, the value of
A
Fall
is not known for finds. In practice,
to solve equation 9.5 or 9.6, we need a value for the
terrestrial age, which in turn means that we usually
need to measure not one but two or more cosmogenic
radionuclides (see equation 9.4) to obtain
T
Exp
. Second,
equation 9.7 presupposes that the meteorite retains no
memory of any earlier irradiation. This assumption
seems to hold for many meteorites from Mars, but not
from the Moon. For simplicity, therefore, we begin our
brief discussion of the CRE ages of Antarctic meteorites
with the simpler martian meteorites.
Figure 9.6b.
Schematic drawing of a slice of EET A79001 showing
different lithologies.
Lithology A
Lithology C
Lithology B
White druse
measured for the Victoria Land meteorites established
the broad outlines of the measurements to follow, so too
the CRE histories of the first recovered Antarctic mar-
tian meteorites were consistent with and representative of
later findings for other martian meteorites. We discuss
as representative examples two of those meteorites, EET
A79001 (olivine-phyric) and ALH A77005 (lherzolitic),
both of which were acquired by ANSMET (Plates 70 and
72, respectively).
EET A79001 (7.9 kg) was discovered in 1979 and was
only recently displaced by Tissint (recovered in Morocco)
as the second most massive meteorite from Mars. EET
A79001 has three different lithologies (Figure 9.6), whose
descriptions we take from
Martinez and Gooding
[1986].
Lithology A, the most abundant, consists mainly of
9.3.1. Martian Meteorites
In 1962, the unique “SNC” group of meteorites com-
prised six stones distinguished by their mineralogy and
composition. By 1999 six more like them had been dis-
covered, five in the Antarctic and one in Los Angeles,
California, USA. During the same period, circumstantial
evidence for the idea that these objects came from Mars
gained acceptance, which stimulated intense interest and
helped maintain funding for the Antarctic collection pro-
grams. As of October 2013, the count of martian meteor-
ites stood at 125, 28 of them from the Antarctic and 65
from Northwest Africa. Just as the first terrestrial ages
