Meteorites 279
chondrites includes mainly LL6. The fall frequency of most
ordinary chondrites (except H5) has long been known to be
twice as great between noon and midnight (i.e.,p.m.falls) as
between midnight and noon (a.m.falls). This cannot reflect
some social cause so the difference must reflect meteoroids’
orbits. Meteoroids with perihelia∼1 AU will be predom-
inantlyp.m.falls, whereas those having aphelia of∼1AU
will bea.m.falls. Thesea.m.falls result from the Earth’s
overtaking meteoroids or involve meteoroids that narrowly
miss Earth, and subsequently are gravitationally perturbed,
causing their landing on the Earth’s forward hemisphere.
Fall frequencies for H5 chondrites differ significantly, with
a.m.andp.m.falls being about equal. Clearly, a fundamen-
tal difference exists between the orbital elements of H5 and
other ordinary chondrites.
There are too few CRE ages for carbonaceous and en-
statite chondrites to exhibit significant peaks. Carbonaceous
chondrites tend to have short CRE ages (<20 Ma). For
martian meteorites, exposure ages range from 0.5 to 16 Ma,
with some clustering being apparent.
Clustering of exposure ages is also observed for HED
meteorites with two diogenite clusters (at about 22 and
39 Ma) coinciding with those in the eucrite and howardite
CRE distributions. As discussed in Section 2.2.4, Vesta or
its daughters provided these three different achondritic
classes.
Attempts to develop a reliable CRE age method for iron
meteorites yielded unsatisfactory results except in one case,
a difficult, tedious, and no longer practiced technique in-
volving long-lived^40 K and stable^39 K and^41 K. About 70
iron meteorites were dated by the^40 K/^41 K method and the
resulting ages range from 100 Ma to 1.2 Ga. That CRE
ages for iron meteorites greatly exceed those of stones
is attributed to the greater resistance of iron meteoroids
to preterrestrial destructive collisions (so-called space ero-
sion). CRE exposure age peaks are evident for a few chem-
ical groups. For group IIIAB, 13 of 14 meteorites have a
CRE age of 650±60 Ma; this age is also exhibited by 3
of 4 measured IIICD meteorites, suggesting a major col-
lisional event involving the parent of the chemical group
III iron meteorites. Otherwise, only the IVA irons exhibit
a CRE age peak: 7 of the 9 dated samples have an age
of 400±60 Ma. From Section 2.4.5, recall that these iron
meteorite groups are the two most numerous ones and con-
tain the highest proportions of strongly shocked members.
Either the parent asteroids of these groups were unusu-
ally large (requiring unusually large and violent breakup
events) and/or the Earth preferentially sampled collisional
fragments that had been strongly shocked (thus acquiring
a significant shock-induced impulse).
6.3 Gas Retention age
As discussed in Section 5.1, the decay series initiated by the
long-lived^232 Th,^235 U, and^238 U yield six, seven, and eight
α-particles, respectively, whereas long-lived^40 K produces(^40) Ar. Thus, from measurements of U, Th, and radiogenic
(^4) He or of (^40) K and radiogenic (^40) Ar, one can calculate a gas
retention age or the time elapsed since a meteorite sample
cooled sufficiently low to retain these noble gases, if the sys-
tem was closed during this period. This radiogenic age could
record primary formation of the meteorite’s parent mate-
rial, but, in most cases, subsequent episodes (metamorphic
and/or shock) were accompanied by substantial heating that
partially or completely degassed the primary material. A
variant of the K/Ar age, the^40 Ar–^39 Ar method involves con-
version of some stable^39 Kto^39 Ar by fast-neutron bombard-
ment, that is,^39 K(n,p)^39 Ar, followed by stepwise heating
and mass-spectrometric analysis. From the^39 Ar/^40 Ar ratio
in each temperature step, it is possible to correct for later
gas loss. This variant even permits analysis of small, inho-
mogeneous samples with a pulsed laser heat source.
Gas retention ages of many chondrites, achondrites, and
even silicate inclusions in iron meteorites range up to about
4.6 Ga. Many meteorites, particularly L chondrites, have
young gas retention ages,∼500 Ma, while H chondrites
cluster at higher ages (Fig. 19). Meteorites with young gas
retention ages generally exhibit petrographic evidence for
strong shock-loading, implying diffusive gas loss from ma-
terial having quite high residual temperatures generated
in major destructive collisions. Almost always, meteorites
having young K/Ar or^40 Ar–^39 Ar ages have lower U, Th–He
ages. This occurs because He is more easily lost from most
minerals than is Ar. Diffusive loss of^40 Ar, incidentally, is
much more facile than is loss of trapped^36 Ar or^38 Ar, en-
hancing its value as a chronometer. Preferential^40 Ar loss
occurs because most of it is sited in feldspars and in min-
erals where K is and is associated with radiation damage
that provides a ready diffusive escape path. Highly mobile
trace elements are lost more readily than is even^40 Ar so
that L chondrites with young gas retention ages have lower
contents of such elements than do those with old ages. The
similarity in the CRE age of group III iron meteorites and
the gas retention age of L chondrites may be coincidental
or, perhaps, may reflect a particularly massive collision of
their parent(s).
The number of fossil meteorites discovered in ordinary
limestone beds in Swedish quarries implies that the me-
teorite flux on Earth∼480 Ma ago was 100×higher than
the contemporary flux. A recent study found that chromite
grains (highly resistant to weathering) from fossil L chon-
drites have CRE ages of 0.1–1.2 Ma. This implies that these
L chondrites arrived on Earth within 100–200 ka after the
major collisional breakup that produced contemporary L
chondrite falls.
As discussed in Section 4.2, solidification ages for most
martian meteorites are∼1.3 Ga (that of ALH 84001 be-
ing∼3.7 Ga), implying the existence of parent magmas
as recently as 1.3 Ga ago. The^40 Ar–^39 Ar ages of essen-
tially unshocked nakhlites accord with the 1.3 Ga age, but