Laser argon-40-argon-39 age studies of Dar al Gani 262 lunar meteorite

The laser Ar-40-Ar-39 dating technique has been applied to the Dar al Gani (DaG) 262 lunar meteorite, a polymict highland regolith breccia, to determi ne the crystallisation age and timing of shock events experienced by this m eteorite. Laser stepped-heating analyses of three dominantly feldspathic fr agments (DaG-1, DaG-2, and DaG-3) revealed the presence of trapped Ar, most ly released at intermediate and high temperatures, with an Ar-40/Ar-36 valu e of similar to2.8. Trapped Ar is most likely released from melt glass pres ent as small veins within the fragments. The Ar-40-Ar-39 ages determined fo r the three fragments are similar to3.0 Ga for DaG-1 and DaG-2 and 2.0 Ga f or DaG-3 and probably relate to major impact events. Laser spot analyses we re performed on a feldspathic clast, an impact crystalline melt basalt (ICM B), and the matrix in a polished section of DaG 262. The feldspathic and IC MB clasts have low contents of trapped Ar compared with that in the matrix. The feldspathic clast shows a wide range of ages from 3.0 to 1.7 Ga simila r to those obtained by stepped heating. The younger age is interpreted as a minimum age for the last major event that assembled this meteorite. The IC MB shows two age clusters at 3.37 and 3.07 Ga, where the older age may be t hat of the impact event that formed the impact melt. Several cosmic-ray exp osure (CRE) ages were obtained as expected for a polymict regolith breccia. The CRE ages are 106 and 141 Ma for the feldspathic clast and the ICMB, re spectively. One of the feldspathic fragments, DaG-2, shows a range between 200-400 Ma. These CRE ages, which are similar to those determined for retur ned samples of the lunar regolith, indicate that the different components o f DaG 262 experienced preexposure prior to assemblage of the meteorite.