Thermal and irradiation history of lunar meteorite Dhofar 280

• Published in: Meteoritics & Planetary Science
• Main Authors: Korochantseva, Ekaterina V., Buikin, Alexei I., Hopp, Jens, Lorenz, Cyrill A., Korochantsev, Alexander V., Ott, Ulrich, Trieloff, Mario
• Other Authors: Russian Foundation for Basic Research
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: Space and Planetary Science; Geophysics; Aitken; South Pole; South pole
• Online Access: http://dx.doi.org/10.1111/maps.12732; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fmaps.12732; https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12732

Abstract Dhofar 280 recorded a complex history on the Moon revealed by high‐resolution 40 Ar‐ 39 Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic 38 Ar, resulting in a (low‐temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low‐temperature) 40 Ar‐ 39 Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP ‐rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low 40 Ar/ 36 Ar ratio. It is apparently a mixture of two components, one endmember with 40 Ar/ 36 Ar = 17.5 ± 0.2 and a second less well‐constrained endmember with 40 Ar/ 36 Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the ( 40 Ar/ 36 Ar) trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.