Meteorites 277
nor mass-independent. Two alternative explanations ex-
ist. Enstatite meteorites may derive from a single parent
body, partitioned in refractories but not volatiles during
primary accretion, which lost these volatiles by postac-
cretionary thermal metamorphism. Alternatively, Mother
Nature may have been particularly perverse in providing
samples of two parent bodies (EH and EL) with similar
oxygen isotopic compositions and volatile element distri-
butions, with primitive material coming mainly from the
former and evolved portions mainly from the latter.
For meteorites of less common types, meteorites from
hot deserts and, particularly Antarctica, doubtless provide a
broader sampling of extraterrestrial materials than do con-
temporary falls (Section 3.2). Systematic and reproducible
differences involving moderately to highly volatile ele-
ments suggest this may extend to ordinary chondrites.
This suggestion, which remains highly controversial, re-
ceives some support by observed asteroid streams, comet
stream formation by differential tidal disruption of Comet
Shoemaker–Levy 9, failures of alternatives to explain
Antarctic meteorite/fall compositional differences, identi-
fication of population differences of unambiguous preter-
restrial origin, and so on. However, members of putative
streams differ in cosmic ray exposure history. If differences
between falls and desert meteorites exist, they reflect vari-
ations in the near-Earth meteoroid flux with time.
6. Meteorite Chronometry
How old are meteorites? An “age” is a time interval be-
tween two events marked by specific chronometers. An ac-
curate chronometer must involve a mechanism operating on
a predictable, but not necessarily constant rate. The “clock”
starts by an event beginning the time interval and its end
must be clearly and sharply recorded. Chronometers used
in modern geo- and cosmochronology usually involve long-
lived, naturally occurring radioactive isotopes such as the U-
isotopes,^87 Rb, or^40 K. Radioactive decay allows calculation
of an age if the concentrations of both parent and daughter
nuclide are known, the time interval beginning is defined,
and the system is not disturbed (i.e., it is a “closed system”)
during the time interval. Some meteorite ages involve pro-
duction of particular stable or radioactive nuclides, or decay
of the latter. Typically, the chronometer half-life should be
comparable with the time interval being measured.
Meteorites yield a variety of ages, each reflecting a spe-
cific episode in its history. Some of these are shown in Fig. 2:
the end of nucleosynthesis in a star, the first formation of
solids in the solar system, melt crystallization in parent bod-
ies, excavation of meteoroids from these bodies, and the
meteorite’s fall to Earth. Other events, like volcanism or
metamorphism on parent objects can be established as can
formation intervals (based on extinct radionuclides) mea-
suring the time between the last production of new nu-
cleosynthetic material and mineral formation in early solar
system materials. CRE ages date the exposure of a mete-
oroid as a small body (<1 m) in interplanetary space, where
the meteorite’s terrestrial age is the time elapsed since it
landed on the Earth’s surface. In the following sections we
discuss some of these.
6.1 Terrestrial Ages
Terrestrial ages are determined from amounts of cosmo-
genic radionuclides found in meteorite falls and finds. The
principles of the method are depicted in Fig. 4 with^14 C
(t 1 / 2 = 5 .73 ka),^81 Kr (t 1 / 2 =200 ka),^36 Cl (t 1 / 2 =301 ka),
and^26 Al (t 1 / 2 =730 ka) being the nuclides most frequently
employed. In Section 3.2, we summarized the most im-
portant conclusions associated with meteorites’ terrestrial
ages. A meteorite’s survival time during terrestrial residence
is determined by the weathering conditions where the me-
teorite resides. Survival times (hence, terrestrial ages) for
meteorites are much lower for warm and/or wet areas than
for cold, arid ones.
Stony meteorites in Antarctica have terrestrial ages up to
2 Ma (Fig. 17), and age distributions depend on their loca-
tions, presumably reflecting ice sheet dynamic differences.
Meteorites from the Allan Hills average∼300 ka, whereas
those from Queen Maud Land have much younger ages
(<300 ka), averaging 100 ka. Meteorites from other parts
of western Antarctica have ages up to 400 ka, also averaging
100 ka.
Meteorites from hot deserts generally have terrestrial
ages up to 50 ka but a few have maximum ages up to 150 ka.
Age distributions tend to vary in the four sites, and although
most maximum ages are 40–50 ka, those for lunar and
0
0
10
20
No. of Cases
30
40
50
200 400
Terrestrial Age [Ka]
600 800 1000
ALH88019:
2000 ka
Allan Hills
FIGURE 17 Terrestrial age distributions of meteorites from the
Allan Hills region of Victoria Land, (west Antarctica), which, on
average, are older than meteorites from any other part of
Antarctica.