Precompaction exposure of chondrules and implications

Citation
E. Polnau et al., Precompaction exposure of chondrules and implications, GEOCH COS A, 65(11), 2001, pp. 1849-1866
Citations number
27
Categorie Soggetti
Earth Sciences
Journal title
GEOCHIMICA ET COSMOCHIMICA ACTA
ISSN journal
00167037 → ACNP
Volume
65
Issue
11
Year of publication
2001
Pages
1849 - 1866
Database
ISI
SICI code
0016-7037(200106)65:11<1849:PEOCAI>2.0.ZU;2-U
Abstract
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.