Depositional history of Luna 24 drill core soil samples

Six soil samples from various depths of the Luna 24 drill core column have been analyzed for their particle track records and light noble gas compositions. The observed particle track records indicate higher degree of maturity for the upper zone (approximately 1 m) of this regolith column as compared to the soils in the lower zone (approximately 0.4 m). The cosmogenic 21Ne concentrations decrease rapidly with depth to 1 m after which the concentrations level off or increase slightly. These data suggest a multi-stage depositional history for this drill core soil column consisting of: (1) rapid deposition of regolith material, (2) a cratering event about 400 m.y. B.P. leading to excavation to a depth of approximately 1 m from the present regolith surface, (3) a relatively rapid fill up of the crater with near-surface irradiated material, and (4) in-situ irradiation during the last about 250-300 m.y. Such a depositional sequence can also explain the observed lack of correlation between different surface exposure-correlated maturity indices in these drill core soil samples. (Auth.)