280 Encyclopedia of the Solar System
01110222K-Ar Age [Ga]33344455H Chondrites
(508)L Chondrites
U-Th-He Age [Ga](380)FIGURE 19 Gas retention ages of 508 H and 380 L chondrites.
Data from the U, Th–He and K–Ar methods are plotted against
each other. These data assume cosmogenic (^4 He/^3 He)=5, K
concentrations of 800 and 900 ppm for H and L, respectively, U
concentrations of 13 and 15 ppb, respectively and (Th/U)=3.6.
The 45◦line represents concordant ages. The two major
chondrite types exhibit strong thermal history differences. The
dominant concordant long ages of H chondrites suggest that
their parent(s) generally remained thermally unaltered since
formation 4–4.5 Ga ago. The concentration of data defining
concordant short ages of L chondrites suggests strong
shock-heating in a major collision(s) 0.1–1.0 Ga ago. Nearly all
discordant meteorites lie below the 45◦lines because radiogenic
(^4) He is lost far more easily than is radiogenic (^40) Ar.
shergottites, which are heavily shocked, have gas reten-
tion ages probably indicating partial degassing of their par-
ent material≤250 Ma ago, consistent with Rb–Sr internal
isochrons for shergottites at 180 Ma as discussed in the next
section.
6.4 Solidification Age
Solidification ages establish the time elapsed since the last
homogenization of parent and daughter nuclides, normally
by crystallization of a rock or mineral. Nuclides used to
establish solidification ages are isotopes of nongaseous el-
ements insensitive to events that might have affected gas
retention. Some techniques, such as the Pb/Pb method,
which involves the ultimate decay products of^235 U,^238 U,
and^232 Th (^207 Pb,^206 Pb and^208 Pb, respectively) involve rel-
atively mobile Pb that should be more easily redistributed
than would be the^147 Sm–^143 Nd dating pair. Hence, in prin-
ciple, a sample dated by several techniques might yield
somewhat different ages depending upon its postformation
thermal history.
Common techniques found to yield useful solidification
ages include: the Pb–Pb method mentioned previously;
(^147) Sm (t
1 / 2 =106 Ga)–
(^143) Nd; (^87) Rb (t
1 / 2 =48 Ga)–
(^87) Sr; and
(^187) Re (t 1 / 2 =41 Ga)– (^187) Os. Generally, methods used to de-
termine solidification ages depend upon data depicted in
isochron diagrams, in which, for example, enrichment of
radiogenic^87 Sr is proportional to the amount of^87 Rb, and
(^86) Sr is taken for normalization. The slope of such a line
yields an “internal isochron” for a meteorite or a single in-
clusion of a meteorite, if minerals having various^87 Rb/^86 Sr
ratios are measured. They-intercept provides the initial
(^87) Sr/ (^86) Sr ratio—a relative measure of the time that nucle-
osynthetic products were present in the system prior to so-
lidification (i.e., how “primitive” the system is). Clearly, the
lower the^87 Sr/^86 Sr ratio is, the less radiogenic (or evolved)
was the source material. For some time, basaltic achondrites
(e.g., HED meteorites) and the angrite, Angra dos Reis,
competed as the source containing the most primitive (least
radiogenic) Sr, but, more recently, Rb-poor CAI inclusions
in the C3V chondrite, Allende, have become “champions”
in this category, with^87 Sr/^86 Sr= 0. 69877 ±2.
Solidification ages for most meteoritic samples are “old”
(i.e., close to 4.56 to 4.57 Ga; Fig. 20). The results obtained
by different methods agree quite well, although some “fine-
structure” can be detected. A large number of chondrites
FIGURE 20 U–Pb ages of phosphates from ordinary
chondrites. Numbers on the Concordia line are in Ga. The oldest
solidification age (for H chondrites) is∼4.563 Ga ago and
thermal metamorphism occupied the next 60–70 Ma.