History of the Apollo 15 yellow impact glass and sample 15426 and 15427

Abstract

Individual pieces of the Apollo 15 yellow impact glass from 15426 and 15427 were studied using the laser microprobe to determine their 39Ar‐40Ar age and their 38Ar‐37Ar exposure age. Except for the extractions from one fragment, the age determinations required significant correction for trapped 40Ar. Data from multiple extractions on each fragment were used to make individual 40Ar/36Arsw versus 39ArK/36Arsw plots from which the apparent age and trapped component for each fragment was determined. An age of 3.35±0.05 AE was determined for the age of the impact event that formed these glasses. This age cannot be reconciled with the age prediction of Delano et al. [1982]. Since these glasses were probably produced by an impact into a target of moderate‐TiO2 basalts, and the only known moderate‐TiO2 basaltic flows are younger than the impact event [Boyce and Johnson, 1978], the target for these glasses must be covered up by later basaltic flows. The average 38Ar‐37Ar exposure age for these glasses was 274±74 m.y. Other glasses from clods 15426 and 15427 give similar exposure ages [Spangler et al. 1984]. Two possible reasons for this similarity is that (1) the clods formed at greater than 300 m.y. ago so that all glasses in 15426 and 15427 shared a common exposure history, and (2) the gardening at Station 7 produced a soil with an average exposure age of ∼300 m.y.