Sm and Gd isotopic shifts of Apollo 16 and 17 drill stem samples and their implications for regolith history

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 (5.68-9.03) × 10¹⁶ n cm⁻² for A-16 and (1.85-5.04) × 10¹⁶ n cm⁻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 ε_Sm/ε_Gd, 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 ε_Sm/ε_Gd = 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 15 samples, showing evidence of neutron-capture from ¹⁵¹Eu and ¹⁵³Eu, respectively.