Ar-40-Ar-39 microanalysis of single 74220 glass balls and 72435 breccia clasts

Ar-40-Ar-39 age measurements on single orange glass balls from the Apollo 17 soil 74220 and individual clasts from the Apollo 17 highland breccia 72435 are reported. The measurements required the use of newly established microanalytical techniques to obtain high quality analyses on about 0.5 mg particles with only a few hundred ppm K. An age of 3.60 plus or minus 0.04 b.y. is determined for the orange glass. No corrections for a trapped Ar-40 component were required. The glass forming event occurred at the very end of or after the extrusion of the mare basalts at the Apollo 17 site. An extremely well defined age plateau at 3.86 plus or minus 0.04 b.y. was determined for a 72435 plagioclase clast with attached matrix. A second large plagioclase crystal yielded significantly older ages over the last 60% of Ar release at high temperatures and is a relict clast incompletely degassed at the time of breccia formation. 72435 also contains plagioclase clasts with primitive Sr and a 4.55 AE old dunite clast. The Ar results provide additional evidence for the association of chemically unequilibrated, relict clasts with both primitive Sr and older K/Ar ages.