Inert gases in twelve particles and one “dust” sample from Luna 16

Abstract

The inert gases were measured mass-spectrometrically in 12 fragments and 1 “dust” sample from Luna 16. The fragments were classified petrologically by microscopic inspection. The major petrologic types were breccias and basalts. The former were much richer in trapped gases than the latter, and were apparently formed by the welding of local fines. However, there was no clear-cut difference in gas content of either breccias or basalts between zone A (top) and zone G (bottom). The 4He/ 20Ne ratio of the breccias (average 49) was systematically smaller than that of the basalts (average 78), probably because of He-Ne fractionation during or after the formation of the breccias. We suggest that the 4He/ 20Ne ratios of bulk fines in general may reflect the proportions of basaltic and breccia (plus cindery glasses) fragments in the fines. Substantial variations of 4He/ 3He were found, which could not be explained with the presence of variable proportions of cosmogenic 3He c. Either the solar-wind value has changed in time, or the fragments with the small ratios were exposed to solar flares rich in 3He and/or 4He. Exposure ages of four fragments are several hundred million years. The 40Ar/ 36Ar slopes of breccias and basalts are identical: 0.65.