A. A. Chodos

Ages, Irradiation History, and Chemical Composition of Lunar Rocks from the Sea of Tranquillity

The 87Rb-87Sr internal isochrons for five rocks yield an age of 3.65 �0.05 x 109 years which presumably dates the formation of the Sea of Tranquillity. Potassium-argon ages are consistent with this result. The soil has a model age of 4.5 x109 years, which is best regarded as the time of initial differentiation of the lunar crust. A peculiar rock fragment from the soil gave a model age of 4.44 x 109 years. Relative abundances of alkalis do not suggest differential volatilization. The irradiation history of lunar rocks is inferred from isotopic measurements of gadolinium, vanadium, and cosmogenic rare gases. Spallation xenon spectra exhibit a high and variable 131Xe/126Xe ratio. No evidence for 129I was found. The isotopic composition of solar-wind xenon is distinct from that of the atmosphere and of the average for carbonaceous chondrites, but the krypton composition appears similar to average carbonaceous chondrite krypton.

Mineralogy, petrology, and chemistry of a Luna 16 basaltic fragment, sample B-1

Abstract Luna 16 sample B-1 was the largest fragment (62 mg) obtained in the sample exchange with the USSR. Petrologic, mineralogic, and chemical investigations have been made on this fragment in conjunction with Rb-Sr and40Ar/39Ar investigations by our colleagues. Sample B-1 is a fine-grained ophitic basalt but is distinguished from the Apollo samples by containing a single pyroxene, predominantly pigeonitic, an ilmenite content (7%) intermediate to that of the Apollo 11 and 12 samples, and subequal amounts of pyroxene (50%) and plagioclase (40%). Chemically it is distinguished by a high Sr content (437 ppm) and a high K/U value (4700). The K-content (1396 ppm) is higher than that of Luna 16 soil sample A-2.