Encyclopedia of the Solar System 2nd ed

278 Encyclopedia of the Solar System

martian meteorites and one H4 are∼150 ka. Their terres-
trial age distribution may depend on the hot desert recovery
site.

6.2 CRE Ages

In principle, we need to determine both a cosmogenic ra-
dionuclide (or its decay product) and a stable nuclide to
establish a CRE age. In practice, however, production rates
of stable cosmogenic noble gas nuclides in stony meteorites
are well-known, and it usually suffices to measure just their
concentrations. Absent contrary evidence, we generally as-
sume that irradiation by cosmic rays of solar and galactic ori-
gin is simple, that is, the meteoroid was completely shielded
(buried in a parent body) until an impact ejected it as a
meter-sized object that remained essentially undisturbed
until collision with Earth. Some stones (e.g., the H4 chon-
drite Jilin) and irons (e.g., Canyon Diablo) exhibit complex
irradiation histories involving preirradiation on the parent
body surface, or secondary collisions in space that fractured
the meteoroid and exposed new surfaces to CRE. In such
cases (complex irradiation history), different samples of a
meteorite exhibit different CRE ages. As noted in Section

1.2, meteoroids approaching the Sun to within 0.5 AU are

warmed, causing diffusive loss of gases, especially^3 H(a

contributor to^3 He production) from iron. Such cases are

recognized by low isotopic ratios, particularly of^3 He/^4 He

or^3 He/^21 Ne, or low natural TL.

The data for ordinary chondrites in Fig. 18 show that all

groups have CRE ages ranging up to 90 Ma, but the distribu-

tions differ markedly. For H chondrites, there is at least one

major peak,∼7 Ma and a smaller one at 33 Ma. For L chon-

drites, major peaks are not obvious, but clusters occur at 20–

30 and 40 Ma, with a smaller one at 5 Ma. For LL chondrites,

the major peak at 15 Ma includes∼30% of all measured

samples, and another is at 30 Ma. Major peaks correspond

to major collisional breakups on and of chondrite parent

bodies. Contrary to the H-chondrite situation, nearly two

thirds of the L chondrites have CRE ages>10 Ma.

Current data suggest that major CRE peaks vary with

petrographic grade for each ordinary chondrite group but

poor statistics may cloud the situation in some cases. For

example, among L chondrites, the 40-Ma event mainly pro-

duced L5 and L6 types. The 7-Ma CRE age peak is particu-

larly evident among H4 and H5 chondrites, and a cluster at

4 Ma is evident for H5 and H6. A possible 10 Ma peak for LL

60

50

40

30

20

10

10

10

5

0

1

Cosmic-Ray Exposure Age [Ma]

10 100

40

30

20

20

15

No. of Cases

LL

L

H

FIGURE 18 Cosmic ray exposure ages

for ordinary H, L, and LL chondrites.

Peaks in the histograms indicate major

collisional events on parent bodies that

generated many meter-sized fragments

(see Section 6.2).