Jl. Mokos et al., Cosmogenic radionuclides and noble gases in Antarctic H chondrites with high and normal natural thermoluminescence levels, METEORIT PL, 35(4), 2000, pp. 713-721
We report noble gas data for 37 H chondrites collected from the Allan Hills
by EUROMET in the 1988-1989 field season. Among these are 16 specimens wit
h high levels (>100 krad) of natural thermoluminescence (NTL), originally i
nterpreted as signaling their derivation from a single meteoroid with an or
bit that became Earth-crossing similar to 100 ka ago. One of these 16 is an
H3 chondrite with a cosmic-ray exposure age of similar to 33 Ma and clearl
y represents a separate fall. The other 15 H4-6 chondrites derive from thre
e separate meteoroids, each of which is represented by a five or six member
group. These groups have mean exposure ages of 3.7, 4.1, and 6.6 Ma: the m
iddle-group members all contain solar Ne. The two younger groups also seem
to each include a few H chondrites with normal NTL levels.
Measurements of cosmogenic Be-10 (1.5 Ma), Al-26 (710 ka), and Cl-36 (301 k
a) in 14 of the high-NTL chondrites indicate that all reflect a simple irra
diation history. In contrast, many of a different (38 member) randomly sele
cted suite of Antarctic H chondrites seem to have different cosmic-ray irra
diation histories. The 3.7 and 6.6 Ma groups from the 37 member Allan Hills
suite come from about 5-30 and about 5-10 cm depths in 80-125 and 60-125 c
m radius meteoroids, respectively.