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ID 20656
file
creator
Yoneda, Shigekazu
NDC
Earth sciences. Geology
abstract
The isotopic compositions of Sm and Gd in lunar regolith samples from the Apollo 16 and 17 deep drill stems showed clear isotopic shifts in 150Sm /149Sm (ε= +124 to +191 for A-16, and +37 to +111 for A-17) and 158Gd/157Gd (ε= +107 to +169 for A-16, and +31 to +84 for A-17) corresponding to neutron fluences of (5.68 to 9.03)x1016 ncm-2 for A-16 and (1.85 to 5.04)x1016 ncm-2 for A-17. The depth profiles of neutron fluences suggest that the regoliths at both sites were due to incomplete mixing of three different slabs which experienced individual two-stage irradiation before and after deposition of the upper slabs. The variations in REE compositions provide chemical evidence for incompletely vertical mixing of regoliths especially at upper layers of the two sites. The thermal neutron energy index estimated from the combination of Sm and Gd isotopic shifts, defined as εSm/εGd, shows a small variation (0.61-0.64) in the A- 16 core except for the surface layer. On the other hand, a large variation in εSm/εGd = 0.67 to 0.83 in the A-17 core may result from complicated history such as two-stage irradiation and incomplete mixing during the gardening processes. Isotopic enrichments of 152Gd and 154Gd correlated with Eu/Gd elemental abundances and neutron fluences were also observed in almost all of fifteen samples, showing evidence of neutron capture from 151Eu and 153Eu, respectively.
journal title
Geochimica et Cosmochimica Acta
volume
Volume 71
issue
Issue 4
start page
1074
end page
1086
date of issued
2007-02-15
publisher
Pergamon-Elsevier Science Ltd.
issn
0016-7037
ncid
publisher doi
language
eng
nii type
Journal Article
HU type
Journal Articles
DCMI type
text
format
application/pdf
text version
author
rights
Copyright (c) 2006 Elsevier Inc.
relation url
department
Graduate School of Science