Inert gases in a terra sample: measurements in six grain-size fractions and two single particles from Luna 20

Abstract

The inert gases have been measured in six size fractions covering the range below 500 μm, in a single feldspathic fragment weighing 523 μg, and in an agglutinate particle weighing 465 μg. The two size fractions between 125 and 250 μm as well as 250 and 500 μm were separated into magnetic and non-magnetic portions, which were measured separately. Like the Apollo and Luna 16 fines, the terra fines represented by Luna 20 are very rich in trapped solar-wind gases, but they contain relatively less He 4 and Ne 20, which is revealed by their average He 4 Ne 20 ratio of 35 and Ne 20 Ar 36 ratio of 2.9. Obviously the terra materials are less retentive for solar-wind He and Ne than typical mare fines such as 10084. Whether this is due to the relatively small TiO 2 or the relatively large plagioclase content of the former is not resolved. ( Ar 36 Kr 84 ) trapped and ( Ar 36 Xe 132 ) trapped ratios are relatively large; the average values are 2800 and 14400, respectively. The apparent Ne 21 radiation ages of all the size fractions are in the range 209–286 × 10 6 yr; the average is 260 × 10 6 yr. This is in the range of values known for the Apollo and Luna 16 fines. The feldspathic fragment has a much greater apparent Ne c 21 age of 780 × 10 6 yr. The Ar 40-Ar 36 systematic reveals the presence of two Ar 40 components, because Ar 40 = (1.41 ± 0.076) Ar 36 + (0.490 ± 0.130) × 10 −4 (cm 3 STP/g). The Ar 40 Ar 36 slope of 1.41 is not inconsistent with an origin of the sample from a relatively old terra region.