Zircons Survived to Tell the Tale of the Ancient Lunar Crust

Pierazzo crater. Image credit: NASA/GSFC/Arizona State University

The lunar surface is littered with relicts of impacts large and small that have obfuscated the ancient geologic record of the Moon. Zircon (SiO4) is a mineral often used to look back into geologic history, thanks to its resilience to post-formation heating and shock metamorphism. This means that zircons often record original igneous processes. However, basin-forming impacts can also create zircons due to the large amounts of melt produced in such impacts, which can complicate the interpretation of zircon formation histories. In a new study led by Dustin Trail from the University of Rochester, the zircon chemistry and ages of zircons in samples returned by the Apollo 14 mission are used to understand what early magma compositions were like on the Moon.

The early lunar crust formed through extreme differentiation. However, only small amounts of material remain intact from this early time period. Using geochronology, trace-element chemistry, and thermodynamic models, the work of Trail and coworkers shows that at least two melt compositions were already present on the Moon before 4.2 billion years ago, indicating that the Moon was geologically diverse by then. In contrast, younger zircons (<4.0 billion years old) show evidence of only one magma composition, which falls in-between compositions of the two older melts. This suggests that older rock compositions were re-melted and mixed to form the melt from which the vast majority of younger zircons crystallized. These results demonstrate that the upper approximately 15 kilometers of the lunar crust has been thoroughly mixed and re-melted by basin-forming impacts during the Moonโ€™s early history. READ MORE