Pervasive impact modification of pristine lunar clasts

Abstract

Lunar rock fragments, particularly those deemed pristine, have long been considered vital records of the Moon’s formation and magmatic evolution. These fragments were thought to have largely escaped the Moon’s intense impact history, offering a window into the early lunar crust. However, the concept of “pristine” is increasingly debated, as traditional criteria for identifying pristine samples—based on texture, mineral content, and siderophile element abundances—may overlook the extensive effects of impact reworking. In this study, we apply a novel high-resolution geochemical and experimental approach, linking zircon Al content to parent melt composition, to critically assess lunar samples. Our findings reveal that clast zircons, assumed to preserve magmatic history, and matrix zircons, considered the last igneous remnants in brecciated samples, are not in chemical equilibrium with their surrounding glass. This disequilibrium, coupled with heterogeneous zircon ages, provides compelling evidence for pervasive impact reworking, challenging the assumption that these samples represent primary igneous lithologies. These results underscore the need for a serious re-evaluation of lunar materials. New analytical tools, tailored to each critical lunar lithology, will be essential for this reassessment—such as the Al-in-zircon method employed here for zircon-bearing samples.