Petrology, chemistry, and isotopic compositions of the lunar highland regolith breccia Dar al Gani 262

Lunar meteorite Dar al Gani 262 (DG 262)-found in the Libyan part of the Sa hara-is a mature, anorthositic regolith breccia with highland affinities. T he origin from the Moon is undoubtedly indicated by its bulk chemical compo sition; radionuclide concentrations; noble gas, N, and O isotopic compositi ons; and petrographic features. Dar al Gani 262 is a typical anorthositic h ighland breccia similar in mineralogy and chemical composition to Queen Ale xandra Range (QUE) 93069. About 52 vol% of the studied thin sections of Dar al Gani 262 consist of fi ne-grained (less than or similar to 100 mu m) constituents, and 48 vol% is mineral and lithic clasts and impact-melt veins. The most abundant clast ty pes are feldspathic fine-grained to microporphyritic crystalline melt brecc ias (50.2 vol%; includes recrystallized melt breccias), whereas mafic cryst alline melt breccias are extremely rare (1.4 vol%). Granulitic lithologies are 12.8 vol%, intragranularly recrystallized anorthosites and cataclastic anorthosites are 8.8 and 8.2 vol%, respectively, and (devitrified) glasses are 2.7 vol%. Impact-melt veins (5.5 vol% of the whole thin sections) cutti ng across the entire thin section were probably formed subsequent to the li thification process of the bulk rock at pressures below 20 GPa, because the bulk rock never experienced a higher peak shock pressure. Mafic crystalline melt breccias are very rare in Dar al Gani 262 and are si milar in abundance to those in QUE 93069. The extremely low abundance of ma fic components and the bulk composition may constrain possible areas of the Moon from which the breccia was derived. The source area of Dar al Gani 26 2 must be a highland terrain lacking significant mafic impact melts or mare components. On the basis of radionuclide activities, an irradiation position of DG 262 on the Moon at a depth of 55-85 g/cm(3) and a maximum transit time to Earth <0.15 Ma is suggested. Dar al Gani 262 contains high concentrations of sol ar-wind-implanted noble gases. The isotopic abundance ratio Ar-40/(36)A, < 3 is characteristic of lunar soils. The terrestrial weathering of DG 262 is reflected by the occurrence of frac tures filled with calcite and by high concentrations of Ca, Ba, Cs, Br, and As. There is also a large amount of terrestrial C and some N in the sample , which was released at low temperatures during stepped heating. High conce ntrations of Ni, Co, and Ir indicate a significant meteoritic component in the lunar surface regolith from which DG 262 was derived.