Pervasive impact modification of pristine lunar clasts.

Area of Science:

• Geochemistry
• Planetary Science
• Mineralogy

Background:

• Lunar rock fragments, especially pristine ones, are key to understanding Moon's formation and evolution.
• Traditionally, pristine samples were believed to have escaped significant impact history, preserving early lunar crustal information.
• The definition and identification of "pristine" lunar samples are increasingly questioned due to potential impact alteration.

Purpose of the Study:

• To critically assess lunar samples using a novel geochemical and experimental approach.
• To investigate the chemical equilibrium of zircons within lunar samples and their parent melt composition.
• To challenge the assumption that certain lunar samples represent primary igneous lithologies.

Main Methods:

• Application of a novel high-resolution geochemical and experimental approach.
• Linking aluminum (Al) content in zircon to parent melt composition.
• Analysis of clast and matrix zircons in brecciated lunar samples.

Main Results:

• Zircons from lunar samples (both clast and matrix) were found to be in chemical disequilibrium with their surrounding glass.
• Heterogeneous zircon ages further support evidence of significant impact reworking.
• The findings challenge the notion that these samples preserve pristine magmatic histories.

Conclusions:

• Lunar samples, even those considered pristine, show evidence of pervasive impact reworking.
• Traditional criteria for identifying pristine lunar samples may be insufficient.
• New analytical methods, like the Al-in-zircon technique, are essential for re-evaluating lunar materials and their formation history.