Neutron capture effects in lunar gadolinium and the irradiation histories of some lunar rocks

The Gd isotopic composition in 19 lunar rock and soil samples from three Apollo sites is reported. The analytical techniques and the high precision mass spectrometric measurements are discussed. Enrichments in¹⁵⁸GdO/¹⁵⁷GdO due to neutron capture range up to 0.75%. Integrated ‘thermal’ neutron fluxes derived from the isotopic anomalies of Gd are compared with spallation Kr data from aliquot samples to construct a model which gives both average cosmic-ray irradiation depths and effective neutron exposure ages (T_n) for some rocks. In the case of rock 12053, this yields an average sample location of ∼300 g/cm² below the lunar surface and an effective irradiation age of ∼230 my, compared to 99 my obtained by the⁸¹Kr-Kr method. Rock 14310 is the first lunar sample where Kr anomalies due to resonance neutron capture in Br are observed. A⁸¹Kr-Kr exposure age of 262 ± 7 my is calculated for this rock.