Origin of lunar meteorite ALHA 81005: Clues from the presence of Terrae clasts and a very low-titanium mare basalt clast


  • Allan H. Treiman,

  • Michael J. Drake


Petrographic data and pyroxene compositions indicate that meteorite ALHA 81005 is a breccia from the terrae of the Earth's Moon. Thin section ALHA 81005,9 includes ferroan anorthositic clasts, a lone clast of Mg-suite plutonic composition, fragments which are intermediate in composition between ferroan anorthosite and Mg-suite plutonics, and a clast of very low-titanium mare basalt. Fragments of norite and harzburgite have mineral compositions like ferroan anorthosite, and a clast of ferroan anorthosite has pyroxenes with lower molar Mg/(Mg+Fe) than in known pristine rocks. The Mg-suite protoliths for the clasts of intermediate composition are inferred to be magnesian troctolites, spinel troctolites and feldspathic lherzolites. Although clasts of these lithologies are not present in ALHA 81005,9, mineral fragments from them are present. A single basaltic clast is of mare origin, based on the molar Mg/(Mg+Fe) ratio and Cr content of its pyroxenes. The composition of its plagioclase and the molar Ti/(Ti+Cr) ratios of its pyroxenes indicate that the clast is a fragment of very low-titanium (VLT) mare basalt. If this basalt is significantly younger than the last basin-forming impact event (≃ 3.9 × 109 years ago), its presence probably constrains the source crater for ALHA 81005 to be within a hundred kilometers of a VLT mare basalt flow.