39AR‐40Ar ages for the Apollo 15 green and yellow volcanic glasses

Abstract

A technique using the laser microprobe to extract noble gases from individual small (100–750 μm in diameter), low potassium (≤0.09 wt % K2O), glassy samples was developed. This technique has an advantage in that an individual sample can be chemically characterized by an electron microprobe and then analyzed for its argon age, thus eliminating any ambiguities in the classification of the sample and the need for large quantities of samples. This technique was then used to study the argon isotopes and apparent 39Ar‐40Ar ages of three varieties of volcanic glass from Apollo 15: yellow volcanic, green volcanic group A, and green volcanic group D. The apparent solidification age for the glasses are 3.62±0.07, 3.41±0.12, and 3.35±0.18 aeons, respectively. The ages for group A and D green glasses agree well with previously reported ages for aliquots of ‘bulk’ green glass. A value of 40Ar/36Arsw = 0.5 ± 0.5 was used to correct the data since a 40Ar/36Arsw versus 39ArK/36Arsw plot failed to identify a well‐defined trapped component. No significant difference in the solidification age (>0.2 aeons) was found between the A and D groups of the volcanic green glasses. These ages for volcanic glasses are consistent with the prediction of Delano and Livi [1981] that Array II volcanic glasses are generally older (≥3.6 aeons) than Array I glasses. The average exposure ages for the Apollo 15 yellow volcanic and green volcanic (groups A and D) glasses from 15426 and 15427 were indistinguishable, with ages between 300 and 275 m.y.