Following the discovery of an apparent precompaction irradiation of a chond
rule, we studied the isotopic abundances of noble gases He, Ne, and Ar in s
eparated chondrule and matrix samples of eight ordinary chondrites. The cho
ndrules show systematically larger concentrations of cosmogenic He, Ne, and
with one exception, Ar when compared to the corresponding matrix samples.
Different exposure times were calculated based on production rates, which t
ake into account the abundances of relevant target elements in each chondru
le and matrix sample. The Kr-81-Kr method was used for samples of Bjurbole
to verify that different exposure times are not due to either a bias in pro
duction rates or to systematic losses of noble gases from matrix. This meth
od self-corrects for variable shielding, is independent of calculated produ
ction rates, and insensitive to gas loss. The results show that differences
in exposure ages are due to different precompaction exposures of chondrule
s, or components thereof, before final assembly of the meteorite. For the S
ena chondrite we studied various chondrule and matrix constituents. For thi
s meteorite we found chondrule preexposures relative to the matrix material
, with magnetic chondrules showing the largest effect. Copyright (C) 2001 E
lsevier Science Ltd.