Geology Reference
In-Depth Information
Table 9.1.
Useful
a
cosmogenic nuclides.
teorite in relation to the preatmospheric surface of the
meteoroid. The meteoroid is usually approximated as a
sphere of radius
R
, and the position of the protometeor-
ite by a depth
d
from the surface.
More complicated irradiation histories occur. Some mete-
orites, lunar ones especially, retain a record of cosmic-ray
irradiation that preceded the launch from the parent body.
Occasionally, meteoroids may undergo collisions while in
transit, and the resulting changes in size and shape can affect
the concentrations of the radionuclides to varying degrees.
The Antarctic meteorites also opened some new lines
of investigation. By virtue of their large numbers, they
were expected to include unusual or previously unknown
types of objects. Moreover, the Antarctic, a long-term
cold-storage locker, seemed likely to preserve meteorites
with very old terrestrial ages. If so, then a study of the
meteorite census as a function of terrestrial age might
reveal changes in the fluxes of meteorites to Earth over
time [
Zolensky et al
., 2006]. In this connection, pairing
information is important in sorting out the rates and
incoming mass distributions of different meteorite
types. We distinguish two kinds of pairing. Two meteor-
ites are said to be launch or source paired if a single
event ejected them from a parent body. Source-paired
meteorites may follow different paths to Earth and
hence have different CRE ages, but as noted above, they
should share an ejection age. Two meteorites are said to
be fall paired if they derive from a meteoroid that broke
up in the Earth's atmosphere. Fall-paired meteorites
should have the same terrestrial age and a common CRE
age as well.
Nuclide
Half-life (Ma)
Radionuclides
14
C
0.005730
59
Ni
0.076
41
Ca
0.1034
81
Kr
0.229
b
36
Cl
0.301
26
Al
0.717
10
Be
1.387
c
53
Mn
3.74
129
I
15.7
Stable nuclides
3
He
6,7
Li
21
Ne
38
Ar
83
Kr
126
Xe
Gd
Sm
W
Pt
a
Many shorter-lived radionuclides such as
22
Na and
54
Mn
are also useful, but in most Antarctic meteorites they
have decayed to levels below current detection limits and
are not considered here. For a listing see, e.g.,
Herzog
and Caffee
[2014].
b
Baglin
, 2008.
c
Chmeleff et al.
, 2010;
Korschinek et al
., 2010.
field. Some early examples of AMS studies of Antarctic
meteorites are
Nishiizumi et al
. [1979a, b; 1981; 1983].
Many, but not all, of the scientific goals remained
unchanged. The most important one is to devise a history
of exposure to cosmic rays for each specimen, a history
that explains the measured concentrations of cosmogenic
nuclides. In the simplest but nonetheless fairly common
cases these cosmic-ray exposure (CRE) histories are
defined by a small number of parameters.
•
A terrestrial age,
T
Terr
:
how long after the time of fall
(
T
Fall
) the meteorite spent in the Antarctic after it landed.
•
A CRE age,
T
Exp
:
the time the meteorite took to travel
to Earth as a relatively small body (a meteoroid). In some
cases the meteoroid may undergo collisions and in
others the meteoroid may retain some memory of earlier
cosmic-ray irradiation on the parent body.
•
An ejection age,
T
Ej
=
T
Terr
+
T
Exp
:
the total time
interval from launch from the parent body to the present,
which is important for establishing source or launch pair-
ing, discussed further below.
•
Geometric conditions of irradiation or “shielding
conditions”:
(1) how big the body (meteoroid) hosting the
protometeorite was just before it passed through the
Earth's atmosphere; and (2) the position of the protome-
9.2. TERRESTRIAL AGES
9.2.1. Simplest Calculation of Terrestrial Ages
The terrestrial age,
T
Terr
, of a meteorite refers to the time
elapsed between a meteorite's fall (
T
Fall
) and its recovery (or
more rigorously, the present). We can calculate terrestrial
ages in several ways. The most common, generally appli-
cable, and dependable methods rely on measurements of
cosmogenic radionuclides. The simplest though least
accurate among them makes use of the integrated form of
the radioactive decay law (equation 9.1):
(
)
Ln
AT
A
1
(
)
=
AT Ae T
−λ
T
;
= −
Terr
,
(9.2)
Terr
Terr
Fall
Terr
λ
Fall
where
A
(
T
Terr
) denotes the activity of the radionuclide
measured in the laboratory (on a recorded date), and
A
Fall
the activity at the time of fall. By convention, the present
is taken as
T
present
= 0 and the terrestrial age as positive so
that we have
T
Terr
= −
T
Fall
.
