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

Abstract

Abstract— The laser 40Ar‐39Ar dating technique has been applied to the Dar al Gani (DaG) 262 lunar meteorite, a polymict highland regolith breccia, to determine the crystallisation age and timing of shock events experienced by this meteorite. Laser stepped‐heating analyses of three dominantly feldspathic fragments (DaG‐1, DaG‐2, and DaG‐3) revealed the presence of trapped Ar, mostly released at intermediate and high temperatures, with an 40Ar/36Ar value of ∼2.8. Trapped Ar is most likely released from melt glass present as small veins within the fragments. The 40Ar‐39Ar ages determined for the three fragments are ∼3.0 Ga for DaG‐1 and DaG‐2 and 2.0 Ga for DaG‐3 and probably relate to major impact events. Laser spot analyses were performed on a feldspathic clast, an impact crystalline melt basalt (ICMB), and the matrix in a polished section of DaG 262. The feldspathic and ICMB 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 similar 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 ICMB shows two age clusters at 3.37 and 3.07 Ga, where the older age may be that of the impact event that formed the impact melt. Several cosmic‐ray exposure (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, respectively. One of the feldspathic fragments, DaG‐2, shows a range between 200–400 Ma. These CRE ages, which are similar to those determined for returned samples of the lunar regolith, indicate that the different components of DaG 262 experienced preexposure prior to assemblage of the meteorite.