Ar-40/Ar-36 variations in Apollo 15 and 16 regolith

Values of Ar-40/Ar-36 have been determined for a large number of Apollo 15 and Apollo 16 fines, including samples from a range of depths in the Apollo 15 and 16 deep drill cores. Most of the Ar-40 in these samples is 'excess Ar-40' in that it has not been produced by in situ decay of K. Values of Ar-40/Ar-36 range from 0.74 to 4.35 and demonstrate that the regolith is not well mixed over depths of several cm or over surface distances greater than about 100 m. However, fines collected over surface distances of less than a few tens of meters tend to possess similar ratios. Values of Ar-40/Ar-36 in the Apollo 15 drill core increase with depth by nearly a factor of four (0.74 to 2.86) in the 0 to 80 cm depth range, then decrease for greater depths. Values of Ar-40/Ar-36 in the Apollo 16 drill core are nearly constant at about 1.3, except for the deepest sample analyzed (about 2.7). Concentrations of glass-bonded agglutinate particles in the fines tend to correlate with lower Ar-40/Ar-36, and may indicate that dynamic regolith processes also affect the trapped Ar-40/Ar-36.