Single‐stage exposure history of lunar highland breccias 60018, 67435, and 67455

Abstract

Lunar breccias 60018, 67435, and 67455 experienced a single‐stage exposure to cosmic rays. These materials had been shielded from cosmic rays when 60018 was excavated by the South Ray cratering event about 2 m.y. ago and rocks 67435 and 67455 by the North Ray impact about 50 m.y. ago. After excavation, the shielding depth of the investigated samples was less than 40 g/cm2, in accord with the location at the time of collection. The gas retention ages, as calculated from 136Xe produced by the spontaneous fission of 238U and 244Pu, show that the breccia minerals solidified and cooled 3.6–4.1 b.y. ago. The breccias contain measurable amounts of trapped noble gases, which, however, are two to four orders of magnitude lower than in typical lunar soil.