Cosmochemical study of lunar surface materials observed
from REE abundances and Pb isotopic compositions

 Shinnosuke SHIBATA

Eleven samples taken from three Apollo sites, A-12, A-15 and A-17, wrere analyzed in this study.  Sample 77017.182 is a small fragment of A-17 granulitic breccia.  Three A-12 samples(12025, 12028.40, 12028.836) are lunar regolith, and seven A-15 samples (15001, 15002, 15003, 15004, 15005.14, 15005.79, 15006) are from different depth of the 2.4 m drill core of A-15 regolith.  An inductively coupled plasma mass spectrometer (ICP-MS) was used for whole rock analyses of REE abundances in all samples.  Moreover a Sensitive High Resolution Ion Micro-Probe (SHRIMP) was used for in situ analyses for REE abundances and Pb isotopic compositions in some silicate minerals of 77017.182.  These analyses suggest that the lunar surface samples have the following features.
(1) All REE patterns of the regolith samples reveal that the lunar regolith at A-12 and A-15 sites is chemically homogeneous.  These patterns are similar to those of some lunar KREEP (K, REE and P-rich) basalts.  This implies that the original basaltic rock was smashed by intense meteorite bombardment.
(2) The whole rock REE pattern of 77017.182 is an overall flat pattern with a slight positive Eu anomaly.  The REE patterns of glass materials in 77017.182 are quite similar to that of the whole rock.
(3) The Pb-Pb isochron of glass in 77017.182 gives 4.437}.003 Ga formation age.  This suggests that the original granulitic rock was one partly melted by some impact events at 4.44 Ga.

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