Meteorites 281
have been studied by the Pb–Pb, Rb–Sr, and Nd–Sm tech-
niques, and results for them are consistent with an age of
about 4.56 Ga. The U/Pb method used to date phosphates
from ordinary chondrites (Fig. 20) produce ages for H6
chondrites exhibiting small, but significant, differences in
Pb–Pb ages from H4 and H5 chondrites. These data sug-
gest 4.563 Ga as the oldest ordinary chondrite solidification
age with metamorphism requiring 60–70 Ma. The results
are consistent with a stratified (“onion-shell”) model for the
H chondrite parent body and suggestive of a simple, pro-
gressive metamorphic alteration with increasing depth in
it.
Most meteorites have solidification ages around 4.56 Ga;
however, there is clear evidence of more recent distur-
bances of chronometric systems—particularly Pb–Pb and
Rb–Sr—in many meteorites. For example, Rb–Sr internal
isochrons for E chondrites (believed by some to have expe-
rienced open-system thermal metamorphism as discussed
in Section 5.3) were disturbed 4.3–4.45 Ga ago. Of course,
chronometers in heavily shocked L chondrites show clear
evidence for late disturbance.
Four techniques (^40 Ar–^39 Ar, Rb–Sr, Pb–Pb, and Sm–Nd)
yield an age for nakhlites of 1.3 Ga, implying their deriva-
tion from a large planet, Mars (Section 4.2). The heavily
shocked shergottites seem to have derived from several
magma reservoirs and Rb–Sb internal isochrons suggest a
major shock-induced disturbance 180 Ma ago, before the
martian meteoroids were ejected from their parent planet.
6.5 Extinct Radioactivities
Measurements of decay products of an extinct radionu-
clide do not provide absolute dates in the sense discussed
in earlier sections, but they do permit relative chronolo-
gies on timescales comparable with the half-life of the ra-
dionuclide (Table 6). Thus far, clear positive evidence has
been found in meteorites or their constituent minerals for
the presence in the early solar system of the following
nuclides:^41 Ca (t 1 / 2 =110 ka),^26 Al (t 1 / 2 =730 ka),^60 Fe
(t 1 / 2 = 1 .5 Ma),^53 Mn (t 1 / 2 = 3 .7 Ma),^107 Pd (t 1 / 2 = 6. 5
Ma),^129 I(t 1 / 2 = 15 .7 Ma),^244 Pu (t 1 / 2 =82 Ma), and^146 Sm
(t 1 / 2 =103 Ma). In most cases, relative ages are calculated
from three-isotope plots involving decay products of the
extinct radionuclide. However, in some cases, the relative
chronologic information can be combined with data for ab-
solute ages, allowing small time differences in the early so-
lar system to be established. For example, combining the
(^53) Mn/ (^55) Mn ratio measured in the Omolon pallasite with
the absolute Pb–Pb age of the LEW 86010 angrite yields
an absolute age of 4557.8±0.4 Ga for Omolon.
In recent years, much effort has gone into this area so
that we should focus upon only one set of results in conclud-
ing this chapter. The oldest technique used is that of I–Xe
dating, which depends upon the decay of^129 I into^129 Xe.
In this technique, a meteorite on Earth is bombarded with
neutrons in a nuclear reactor as in^40 Ar–^39 Ar dating (see
Section 5.2) to convert some stable^127 I into short-lived
(^128) I(t 1 / 2 =25 m), which decays into stable (^128) Xe. Step-
wise heating releases Xe: a linear array with slope>0ona
three-isotope plot of^129 Xe/^132 Xevs.^128 Xe/^132 Xe indicates
an iodine-correlated^129 Xe release, whose slope is propor-
tional to^129 I/^127 I at the last time^129 I and^129 Xe were in
equilibrium. This ratio is a measure of the formation in-
terval. Absolute age values, however, can only be obtained
if the ratio^129 I/^127 I at the time of the closure of the solar
nebula is known. Because this number is not available, only
relative ages can be given.
The I–Xe clock proves to be remarkably resistant to re-
setting by heating: the principal effect is to degrade the lin-
earity, but not to destroy it completely. Shock seems quite
effective in resetting this clock, and hydrolysis, which af-
fected C1 and C2 chondrites, is even more effective.
Data for 79 chondrites, aubrites, and silicate inclu-
sions in iron meteorites, relative to the Bjurb ̈ole L4 chon-
drite, give highly reproducible I–Xe intervals; therefore,
the Bjurb ̈ole L4 chondrite is arbitrarily assumed to have
an age of zero. Each meteorite class spans an I–Xe inter-
val>10 Ma, whereas all meteoritic materials possessing
isochrons span∼55 Ma. Apparently, the only systematic
variation of the I–Xe formation interval with chondritic pet-
rographic type involves E chondrites: EH chondrite par-
ent material formed earlier than did EL. Clearly, while
the nuclide^129 I was still alive (i.e., during or shortly after
nucleosynthesis), primitive nebular matter condensed and
evolved into essentially the materials that we now receive as
meteorites. The conclusion is supported by other isotopic
and charged-particle track evidence (see Sections 4.1, 4.2,
5.3, and 5.4).
As we have seen from the foregoing summary, the mete-
oritic record can be read best in an interdisciplinary light.
Results of one type of study—say, trace element chemi-
cal analysis—provide insight to another—orbital dynamics,
for example. Early experience gained from meteorite stud-
ies, provided guidance for proper handling, preservation,
and analysis of Apollo lunar samples. Studies of these sam-
ples, in turn, led to the development of extremely sensi-
tive techniques now being used to analyze meteorites and
microgram-sized interplanetary dust particles of probable
cometary origin collected in Antarctica (Fig. 21) and just
successfully brought to Earth by theGenesisspacecraft,
despite its hard landing. Undoubtedly, this experience will
prove invaluable as samples from other planets, their satel-
lites, and small solar system bodies are brought to Earth for
study.
Previous studies of meteorites have provided an enor-
mous amount of knowledge about the solar system, and
there is no indication that the scientific growth curve in this
area is beginning to level off. Indeed, work on the present
version began late in 2004, and we were amazed to see how
much had been learned about meteorites since 1998 when