Lunar neutron stratigraphy

Abstract

Variations in the isotopic ratios ¹⁵⁸Gd/¹⁵⁷Gd and ¹⁵⁰Sm/¹⁹⁴Sm in the Apollo 15 deep drill stem show that the neutron fluence is a smoothly varying function of depth with a relatively symmetric peak at a depth of 190 g/cm². The peak fluence is about 60% greater than the surface value. The observed peak is at a depth comparable to the theoretically calculated peak in the neutron flux. These data may be quantitatively explained by a model in which a blanket of pre-irradiated material was deposited rapidly 450 × 10⁶ yr ago which has remained essentially undisturbed since, i.e., the material was deposited during terrestrial Cambro-Ordovician time. The upper several cm of the drill stem represents soil which has been mixed and transported to the drill site by shallow impacts. Other models quantitatively compatible with the data are those for a regolith continuously accreting for a time as long as 400 × 10⁶ yr and subsequently irradiated for ∼ 500 × 10⁶ yr as an unmixed section. This would allow the base of the drill stem to be as old as 900 × 10⁶ yr. These results show that at this site the turnover time to 2.5 m depth is significantly greater than about 500 my. No positive evidence of ray material from Aristillus or Autolycus was observed in the fluence data. Studies of a variety of cosmic-ray nuclei from deep cores from several sites should permit the determination of detailed depositional histories. If deeper core samples could be obtained, it should be possible to study over a billion years of lunar stratigraphy. However, the determination of absolute stratigraphic ages will require that neutron capture rates be determined in situ by accurate methods.