Lunar polymict breccia 14321: a compositional study of its principal components

Abstract

Forty-nine sub-samples of the polymict breccia 14321,184 have been excavated from the rock and analysed by instrumental activation analysis techniques, specially modified to allow examination of small samples. Two distinct types of microbreccia clasts were analysed. A mare-type basalt of unusual composition is present as several discrete clasts. Mixing models for the clastic components of the breccia illustrate that at least three stages of assembly may be distinguished on compositional grounds. The first was at a site such that KREEP-rich materials dominated the clastic rocks formed, although a variety of lithic fragments were apparently present. The second assembly stage may have been primarily that of comminution and mixing of the more primitive materials, with addition of mare-type basalt elasts. The third stage saw addition of clasts of 14321-type basalt. In the final assembly stage the light matrix was apparently formed entirely by mutual abrasion of the pre-existing clasts, resulting in little or no change in bulk composition